1. 1

2. THIS BOOK ISDUE ON THE DATEINDICATED BELOW AND IS SUB-JECT TO AN OVERDUE FINE ASPOSTED AT THE CIRCULATIONDESK.iAUG 21 1974 m6198?.>"0JUN30 1!76 NOV 2 3198 MAR 2 7 1991 4i9??Hm 2^ lis 3 1i7^m 5 f ? 3. THE ANATOMYOF THE DOMESTIC ANIMALSSEPTIMUS SISSON, S.B., V.S.PROFESSOR OF COMPARATIVE ANATOMY IN THE OHIO STATE UNIVERSITY, COLUMBUS, OHIOMEMBER OF THE AMERICAN ASSOCIATION OF ANATOMISTS FELLOW OF THE AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE WITH 725 ILLUSTRATIONS MANY IN COLORSSECOND EDITION, ENTIRELY RESET PHILADELPHIA AND LONDONW.B. SAUNDERS COMPANY 4. Copyright, 1910, by W. B. Saunders Company Reprinted June, 1911 Revised, Entirely Reset, ReprintedAND ReCOPYRIGHTED SEPTEMBER, 1914Copyright. 1914, by W. B. Saunders CompanyReprinted September, 1917 Reprinted March, 1921Reprinted October, 1927 Reprinted September, 1930 PRESS OF W. B. SAUNDERS COMPANY PHILADELPHIA 5. TO MY WIFE KATHERINE OLDHAM SISSONIN GRATEFUL RECOGNITION OF CONSTANT INSPIRATIONAND ENCOURAGEMENT 6. PREFACE This book supersedes the authors Text-book of Veterinary Anatomy. Acomparison of the two will show the new title to be justified by the extent andcharacter of the changes which have been made. The number of illustrations has been increased from 588 to 725. More thanthree hundred new and original figures have been prepared.Nearly all of theseare reproductions of photographs, most of which were taken by the author. Thepreparation of the prints for reproduction has been executed by Mr. W. J. Norriswith unusual care and skill. Continued observations of well-hardened material and frozen sections have ledto a considerable number of modifications of statement. It is scarcely necessaryto say that the recent literature, so far as available, has been utilized. Many changes in nomenclature have been made. Most of the synonyms havebeen dropped or relegated to foot-notes. Exceedingly few new names have beenintroduced.Nearly all eponyms have been eliminated, on the ground that theyare not designative and are usually incorrect historically.The changes made inthis respect are in conformity with the report of the Committee on Revision ofAnatomical Nomenclature which was adopted by the American Veterinary Med-ical Association two years ago. Progress in the direction of a simplified and uni-form nomenclature is much impeded by the archaic terminology which persists toa large extent in clinical literature and instruction. The author is under special obligation to Professors EUenberger, Baum, andSchmaltz, and to their publishers, for permission to use or to copy figures fromtheir excellent works.Their generosity in this matter has been of great value tothose who are unable to use the German literature. A few illustrations have beentaken from other sources and credit has been given in each case. For helpful suggestions and for assistance in the reading of the proof-sheetsthe author is much indebted to his colleague, Dr. F. A. Lambert. Great credit is due the publishers for their determination to spare neitherpains nor expense in attaining a high degree of typographical excellence. Septimus SissonThe Ohio State University, Columbus, Ohio. 7. PREFACE TO THE FIRST EDITIONThe lack of a modern and well-illustratedbook on the structure of the princi-pal domestic animals hasbeen acutelya long time by teachers, students, felt forand practitionersof veterinary medicine.The work here offered is the expressionof a desire to close this gap in our literature.The studyand of material which has been hardened by intra- of frozen sectionsvascular injection of formalin has profoundly modified our views concerning thenatural shape of many of the viscera and has rendered possible much greater pre-cision in topographic statements. The experience of the author during the lastten years, in which almost all of the material used for dissection and for frozensections in the anatomical laboratory of this University hasbeen hardened withformalin, has demonstrated that many of the current descriptions of the organs inanimals contain the same sort of errors as those which prevailed in regard to similarstructures in man previous to the adoption of modern methods of preparation. While the method of treatment of the subject is essentially systematic, topog-raphy is not by any means neglected either in text or illustrations; it is hoped thatthis will render the book of value to the student in his chnical courses and tothe practitioner. Embryological and histological data have been almost entirelyexcluded, since it was desired to offer a text-book of convenient size for the studentand a work of ready reference for the practitioner. It is believed that the use ofblack type for the names of important structures and of small print for certaindetails or matter of secondary importance will prove useful in this respect. Veterinary anatomical nomenclature is at present quite chaotic in EngUsh-speaking countries. In this work an attempt is made to eliminate some termswhich do not appear to the author to fulfil any useful purpose, and others which areclearly erroneous or otherwise undesirable. In many cases the terms agreed uponby the Congresses at Baden and Stuttgart are adopted either in the original Latinor in anglicized form; otherwise these terms are added in parenthesis.Theauthor favors the substantial adoption of this terminology, but considered itdesirable to offer a sort of transitional stage at present. The original illustrations are chiefly reproductions of photographs, many ofwhich were taken by Mr. F. H. Haskett. The preparation of the pictures forreproduction was carried out by Messrs. J. V. Alteneder and W. J. Norris. Theauthor takes pleasure in expressing his appreciation of the care and skill exercisedby these gentlemen in this often difficult task. The author is under great obligation to Professors Ellenberger and Baum inDresden, to Professor Schmaltz in Berlin, and to their pubUshers for permission touse or to copy figures from their most excellent works.Their generosity in thismatter has made it possible to supply this text with a larger number of high-classillustrations than is to be found in any other. A few figures have been taken fromother sources, and proper credit has been given in each case. For checking over certain data and for assistance in the correction of the proofsthe author is much indebtedto his associate, Dr. F. B. Hadley.The authordesires to express his high appreciation of the determinationandconstant effort of the publishers to do all in their power to render the book worthyof favorable reception by the profession for whom it is intended.Ohio State University, Columbus, OhioSeptimus Sisson. 8. CONTENTSINTRODUCTION OSTEOLOGYp^obThe Skeleton 20Structure of Bones 21Development and Growth of Bone 23Composition and Physical Properties of Bone24Descriptive Terms25The Vertebral Column 25The Ribs and Costal Cartilages 27Costal Cartilages27The Sterjium 28The Thorax 28The Skull28Bonesof the Thoracic Limb29BonesLimb of the Pelvic 30Skeleton of the Horse32Vertebral Column 33Ribs 45Sternum48Bones of the Skull 49Cranium49Face 63The Skull as a Whole 73The Cranial Cavity 81The Nasal Cavity 82The Paranasal Sinuses84Bonesof the Thoracic Limb86Bonesof the Pelvic Limb 105Skeleton of the Ox125Vertebral Column125Ribs130Sternum 131Bones of the Skull131The Skull as a Whole140Bones of the Thoracic Limb145Bones of the Pelvic Limb151Skeleton of the Sheep 156Vertebral Column156Ribs156Sternum 157Skull 157Bonesof the Thoracic Limb 160Bonesof the Pelvic Limb 160Skeleton of the Pig 161Vertebral Column161Ribs165Sternum 166Bones of the Skull166TheSkull as a Whole 17311 9. 12CONTENTSPAGE Bones of the Thoracic Limb176 Bones of the Pelvic Limb 180Skeleton of the Dog 184Vertebral Column184 Ribs 188 Sternum188 Bones of the Skull 188 The Skull as a Whole 195 Bones of the Thoracic Limb 197 Bones of the Pelvic Limb 202 ARTHROLOGYSynarthroses207DiARTHROSES 208Amphiarthroses210Articulations of the Horse211Joints and Ligaments of the Vertebrae 211 Atlanto-occipital Articulation 214 Costo-vertebral Articulations215 Costo-chondral Articulations 216 Chondro-sternal Ailiculations216Sternal Joints and Ligaments217 Articulations of the Skull 217Limb Articulations of the Thoracic218 Limb Articulations of the Pelvic229Articulations of the Ox, Pig, and Dog 244 MYOLOGYThe Muscles and Accessory Structures252Fascl and Muscles of the Horse 254Fasciae and Muscles of the Head 255Fascia? and Muscles of the Neck 266Fascite and Muscles of the Back and Loins 276Fasciae and Muscles of the Tail 279Muscles of the Thorax 281Abdominal Fasciae and Muscles 287Muscles of the Thoracic Limb293 Fasciae and Muscles of the Pelvic Limb 317Muscles of the Ox 343Muscles of the Pig359Muscles of the Dog368SPLANCHNOLOGYDigestive System of the Horse 385 The Mouth385 The Tongue 390 The Teeth394 The SaHvary Glands 404 The Pharynx405 The CEsophagus 409 The Abdominal Cavity 410 The Peritoneum 411 The Pelvic Cavity412 The Stomach415 The Small Intestine419 The Large Intestine422 The Pancreas 432 The Liver434 10. CONTENTS13 PAGEThe Spleen 439The Peritoneum 441Digestive System of the Ox 444Digestive System of the Sheep470Digestive System of the Pig477Digestive System of the Dog491THE RESPIRATORY SYSTEMRespiratory System of theHorse 508The Nasal Cavity 508The Larynx 514The Trachea523The Bronchi525The Thoracic Cavity525The Pleuraj526The Lungs530The Thyroid Gland of the IIorse535The Thymus of the Horse536Respiratory System of theOx537Respiratory System of thePig 545Respiratory System of theDog 548 THE UROGENITAL SYSTEMUrinary Organs of theHorse554The Kidneys 554The Ureters 561The Urinary Bladder 561The Adrenal Bodies563LTrinary Organs of the Ox 564Urinary Organs of thePig567Urinary Organs of theDog569 THE MALE GENITAL ORGANSGenital Organs of the Stallion571The Testicles 571The Scrotum 574The Ductus Deferens 575The Spermatic Cord575The Tunica VaginaUs 576Descent of the Testicles577The Vesiculse Seminales 578The Prostate578The Uterus MascuUnus579The Bulbo-urethral Glands 579The Penis 580The Prepuce 582Genital Organs of the Bull586Genital Organs of the Boar591Genital Organs of the Dog 593 THE FEMALE GENITAL ORGANSGenital Organs of the Mare596The Ovaries 596The Uterine or Fallopian Tubes599The Uterus599The Vagina602 11. 14CONTENTS PAGBThe Vulva 603The Urethra 604The Mammary Glands604Genital Organs of the Cow 605Genital Organs of the Ewe 609Genital Organs of the Sow 610Genital Organs of the Bitch 612 ANGIOLOGYGeneral Considerations614The Organs of Circulation 614Blood-vasctlar System of the Horse 617The Pericardium 617The Heart 617The Pulmonary Artery629The Systemic Ai-teries629The Coronary Arteries 630CommonBrachiocephalic Trunk 631Arteries of the Thoracic Limb 650Branches of the Thoracic Aorta660Branches of the Abdominal Aorta 661Arteries of the Pelvic Limb 673The Veins 681The Pulmonary Veins 681Cardiac Veins 681The Anterior Vena Cava andits Tributaries 682 The Posterior Vena Cava and its Tributaries692The Lymphatic System697 The Thoracic Duct697 The Right Lymphatic Duct 698 The Lymph Glands and Vessels of the Head and Neck698 The Lymph Glands and Vessels of the Thorax 700 The Lymph Glands and Vessels of the Abdomen and Pelvis 701 The Lymph Glands and Vessels of the Thoracic Limb703 The Lymph Glands and Vessels of the Pelvic Limb703ThePcetal Circulation704Blood-vascular System op the Ox 705The Pericardium and Heart 705The Arteries706The Veins 719Lymphatic System of the Ox and Sheep722Circulatory System of the Pig 734The Pericardium and Heart 734The Arteries736The Veins 740Lymphatic System740Circulatory System of the Dog 742The Pericardium and Heart 742The Arteries744The Veins 754Lymphatic System756NEUROLOGY.THE NERVOUS SYSTEMGeneral Considerations760Nervous System of the Horse 764 The Spinal Cord764 The Brain768 12. CONTENTS15PAGEThe Cranial Nerves793The Spinal Nerves 810Sympathetic Nervous System of the Horse 829Nervous System of the Ox834Nervous System of the Pig 843Nervous System of the Dog 847 ^STHESIOLOGYThe Sense Organs and Common Integument of the Horse 857 The Eye857 The Ear870 The Common Integument884 The Ergot and Chestnut 895 The OKactory and Gustatory Apparatus 895The Sense Organs and Common Integument of the Ox896The Sense Organs and Common Integument of the Pig 900The Sense Organs and Common Integument of the Dog 902Index 907 13. THE ANATOMY OF THE DOMESTIC ANIMALSINTRODUCTION Anatomythe branch of biological science which deals with the form and isstructure of organisms, both animal and vegetal. It is therefore in close correlationwith physiology, which treats of the functions of the body. Etymologically the word "anatomy" signifies the cutting apart or disassociat-ing of parts of the body. In the earlier phases of its development anatomy wasnecessarily a purely descriptive science, based on such observations as were possible with the unaided eye and simple dissecting instruments the scalpel, forceps, andthe like. At this time, therefore, the term adequately expressed the nature of thesubject. But as the scope of the science extended and the body of anatomicalknowledge grew, subdivisions became necessary and new terms were introduced todesignate special fields and methods of work. With the introduction of the mi-croscope and its accessories it became possible to study the finer details of structureand minute organisms hitherto unknown, and this field of inquiry rapidly developedinto the science of microscopic anatomy or histology as conventionally distinguishedfrom macroscopic or gross anatomy. In the same way the study of the changeswhich organisms undergo during their development soon attained sufficient im-portance to be regarded on practical grounds as a separate branch known asembryology.^ Comparative anatomy is the description and comparison of the structure ofanimals, and forms the basis for their classification.By this means includingextinct forms in the scope of inquiry it has been possible to show the geneticrelationship of various groups of animals and to elucidate the significance of manyfacts of structure which are otherwise quite obscure. The deductions concerningthe general laws of form and structure derived from comparative anatomicalstudies constitute the science of morphology or philosophical anatomy. Themorphologist, however, deals only with such anatomical data as are necessary toform a basis for his generalizations. The anatomical knowledge required in thepractice of medicine and surgery is evidently of a different character and mustinclude many details which are of no particular interest to the morphologist. Special anatomy is the description of the structure of a single type or species,e. g., anthropotomy, hippotomy. Veterinary anatomy is the branch which deals with the form and structure ofthe principal domesticated animals. It is usually pursued with regard to pro-fessional requirements, and is therefore largely descriptive in character.As amatter of convenience, the horse is generally selected as the type to be studied indetail and to form a basis for comparison of the more essential differential charactersin the other animals. Two chief methods of study are employed the systematic and the topo-graphic. In the former the body is regarded as consisting of systems of organs or 1 Tliis term is usually limited in its application to the earlier phases of development duringwhich the tissues and organs are formed. The term ontogeny is used to designate the entiredevelopment of the individual. The ancestral history or phylogeny of the species is constitutedby the evolutionary changes which it has undergone as disclosed by the geological record. 217 14. : :X8 INTRODUCTIONapparatus which are similar in origin and structure and are associated in the per-formance of certain functions. The divisions of systematic anatomy are 1. Osteology, the description of the Skeleton. 2. Arthrology, the description of the Joints. 3. Myology, the description of the Muscles and accessory structures. 4. Splanchnology, the description of the Viscera. This includes the followingsubdivisions(1) Digestive System(2) Respiratory System(3) Urogenital System(a) Urinary OrgansGenital Organs(h) 5. Angiology, the description of the Organs of Circulation 6. Neurology, the description of the Nervous System 7. ^sthesiology, the description of the Sense Organs and CommonIntegu- ment The term topographic anatomydesignates the methods by which the relativepositions of the various parts of the body are accurately determined. It presup-poses a fair working knowledge of systematic anatomy. Descriptive Terms. In order to indicate precisely the position and directionof parts of the body, certain descriptive terms are employed, and must be under-stood at the outset. In the explanation of these terms it is assumed here thatthey apply to a quadruped such as the horse in the ordinary standing position.The surface directed toward the plane of support (the ground) is termed ventral(or inferior), and the opposite surface is dorsal (or superior) the relations of parts in ;this direction are named accordingly. The longitudinal median plane divides thebody into similar halves. A structure or surface which is nearer than another to themeclian plane is medial (or internal) to it, and an object or surface which is furtherthan another from the median plane is lateral (or external) to it. Planes parallelto the median plane are sagittal.Transverse or segmental planes cut the long axisof the body perpendicular to the median plane, or an organ or limb at right anglesto its long axis. A frontal plane is perpendicular to the median and transverseplanes. The term is also used with reference to parts of the limbs or various organsin a similar sense. The head end of the body is termed anterior or cranial; and thetail end posterior or caudal ; relations of structures with regard to the longitudinalaxis of the body are designated accordingly.With respect to parts of the head,the corresponding terms are oral and aboral. Certain terms are used in a specialsense as applied to the limbs. Proximal and distal express relative distances ofparts from the long axis of the body. The anterior face of the thoracic limb fromthe elbow downward is termed dorsal, and the opposite face volar. In the corre-sponding part of the pelvic limb the terms are dorsal and plantar respectively. Inthe same regions radial and ulnar (thoracic limb), tibial and fibular (pelvic limb),may be used to designate that side of the extremity on which the correspondingbone is situated; they are therefore equivalent respectively to medial and lateralin the animals with which we are concerned. The terras superficial (superficialis)and deep (profundus) are useful to indicate relative distances of i);irts from thesurface of the body. It is evidently advantageous to employ terms which are as far as possible independent ofthe position of the body in space and capable of general application, e. g., dorsal, ventral, proximal,etc. It is also desirable that the terms internal and external be reserved to indicate relations ofdepth in cavities or organs, and medial and lateral to designate relations to the median plane.Such terms are coming into more extensive use in human and veterinary anatomy, but the oldernomenclature is very firmly established and cannot well be discarded at once and entirely. Tofacilitate the transition, a table of the older and more recent terms is given below; the recentterms are in the first column and the older equivalents in the second. 15. INTRODUCTION 19A. Relating to Head, Neck, and Trunk:SuperiorDorsalisInferiorVentralisMediaUs I?tf^ExternalLateraUsCranialis 1 AnteriorOralis/Caudalis 1PosteriorAboralis JB. Relating to Limbs:Superior Proximalis Distalis Mf"?^Anterior DorsaUs Volaris , . Posterior Plantaris f Radialis ] Internal Tibialisf Ulnaris External Fibularis f 16. OSTEOLOGY THE SKELETON The term skeleton applied to the framework of hard structures which sup- isports and protects the soft tissues of animals. In the descriptive anatomy of thehigher animals it is usually restricted to the bones and cartilages, although theligaments Avhich bind these together might well be included.In zoology the term is used in a much more comprehensive sense, and includes all the hardersupporting and protecting structures. When the latter are situated externally, they form anexoskeleton, derived from the ectoderm. Examples of this are the shells and cliitinous coveringsof many invertebrates, the scales of fishes, the shields of turtles, and the feathers, hair, and hoofsof the higher vertebrates. The endoskeleton (with which we have to deal at present) is embeddedin the soft tissues.It is derived from the mesoderm, but includes the notochord or primitiveaxial skeleton, which is of entodermal origin. The skeleton may be divided primarily into three parts:(1) axial; (2)appen-dicular; (3) splanchnic. The axial skeleton comprises the vertebral column, ribs, sternum,andskull. The appendicular skeleton includes the bones of the limbs. The splanchnic or visceral skeleton consists of certain bones developedin thesubstance of some of the viscera or soft organs, e. g., the os penis of the dog and theOS cordis of the ox. The number of the bones of the skeleton of an animal varies with age, owingto the fusion during growth of skeletal elements which are separate in the fostusor the young subject. Even in adults of the same species numerical variationsoccur, e. g., the tarsus of the horse may consist of six or seven bones, and the carpusof seven or eight; in all the domestic mammals the number of coccygeal vertebraevaries considerably. The bones are commonly dividedinto four classes according to their shapeandfunction.^Long bones (Ossa longa) are typically of elongated cylindrical form with (1)enlarged extremities. They occur in the limbs, where they act as supportingcolumns and as levers. The cylindrical part, termed the shaft or body (Corpus),isand incloses the medullary cavity, which contains the medulla or marrow. tubular, (2)Flat bones (Ossa plana) are expanded in two directions.They furnishsufficient area for the attachment of muscles and afford protection to the organswhich they cover.(3) Short bones (Ossa brevia), such as thoseof the carpus and tarsus, presentsomewhatsimilar dimensions in length, breadth,and thickness. Their chief func-tion appears to be that of diffusingconcussion.Sesamoid bones, which are de-veloped in the capsules of some joints or in tendons, may be included in this group.They diminish friction or change the direction of tendons. (4) Irregular bones (Ossa irregularia) This group would include bones of.irregular shape, such as the vertebrae and the bones of the cranial base; they aremedian and unpaired. Their functions are various and not so clearly specializedas those of the preceding classes. ^ This classification is not entirely satisfactory; some bones, e. g., the ribs, are not clearlyprovided for, and others might be variously placed. 20 17. .STRUCTURE OF BONES21STRUCTURE OF BONES i Bones consist chiefly of bone tissue, but considered as organs they presentalso an enveloping membrane, termed the periosteum, the marrow, vessels, andnerves. The architecture of bone can be studied best by means of longitudinal andtransverse sections of specimens which have been macerated so as to remove mostof the organic matter.These show that the boneconsists of an externaldense compact sub- shell ofstance, within whichthe more loosely arrangedisspongy substance. In tyjDical long bones the shaftis hollowed to form the medullary cavity (Cavummedullare) The compact substance (Substantia compacta)differs greatly in thickness in various situations, inconformity with the stresses and strains to whichthe bone is subjected. In the long bones it isthickest in or near the middle part of the shaft andthins out toward the extremities.On the latter thelayer is very thin, and is especially dense and smoothon joint surfaces. The spongy substance (Substantia spongiosa)consists of delicatebony plates and spicules whichrun in various directions and intercross. These aredefinitely arranged with regard to mechanical re-quirements, so that systems of pressure and tensionplates can be recognized, in conformity with thelines of pressure and the pull of tendons and liga-ments respectively.The intervals between theplates are occupied by marrow, and are termedmarrow spaces (Cellulse meduUares). The spongysubstance forms the bulk of short bones and of theextremities of long bones; in the latter it is not con-fined to the ends, but extends a variable distancealong the shaft also. Some bones contain air-spaceswithin the compact substance instead of spongybone and marrow, and hence are called pneimiaticbones (Ossa pneumatica) These cavities are termed .sinuses, and are lined with mucous membrane;they communicate indirectly with the external air.Fig. 1. Sagittal Section op LargeIn certain situations the two compact layers of flat Metatarsal Bone of Horsebones are not separated by spongy bone, but fuse (Right).with each other; in some cases of this kind the bone S.c, Compact substance; S.s.,is so thin as to be translucent, or may undergospongy substance; Cm., medullarycavity; F.n., nutrient foramen. Noteabsorption, producing an actual deficiency. the greater thickness of the compact The flat bones of the cranial vault and sidessubstance of the anterior part of theshaft.are composed of an outer layer of orchnary compactsubstance, the lamina externa, an inner layer ofvery dense bone, the lamina interna or tabula vitrea, and between these a variableamount of spongy bone, here termed diploe.The periostetun is the membrane which invests the outer surface of bone,except whereit is covered with cartilage. It consists of an outer protective fibrous 1 Only the gross structureis discussed here.For the microscopic structure reference is tobe made to histological works. 18. 22 OSTEOLOGYlayer,and an innercellular osteogenic layer.During active gro^vth the osteogeniclayer is well developed, but laterit becomes much reduced. The fibrous layervaries much in thickness, being in general thickest in exposed situations.Theadhesion of the periosteum to the bone also differs greatly in various places; itis usually very thin and easily detached where it is thickly covered with musculartissue which has little or no attachment.The degree of vascularity conforms tothe activit}^ of the periosteum.The endosteum is a thin fibrous membrane which lines the medullary cavityand the larger Haversian canals.The marrow (Medulla ossium) occupies the interstices of the spongj^ bone andthe medullary cavity of the long bones. There are two varieties in the adultred and yellow. In the young subject there is only red marrow (Medulla ossiummbra), but later this is replaced in the medullary cavity by yellow marrow (Medullaossium flava). The red marrow contains several types of characteristic cells andis a blood-forming suV)stance, Avhile the yellow is practically ordinary adipose tissue.^ Vessels and Nerves. It is customary to recognize two sets of arteries theperiosteal and the medullary. The former ramify in the periosteum and give offFig. 2. Cross-section of Proximal Third of ShaftFig.Cross-section of Distal Third of Shaft op OF Right Humerus of Horse. Left Humerus of Horse. Section passes through nutrient foramen and canal.innumerable small branches which enter minute openings (Volkmanns canals) onthe surface and reach the Haversian canals of the compact substance. Otherbranches enter the extremities of the long l^ones and supply the spongy bone and marrow in them. In the case of the larger bones and especially the long bonesthe large nutrient or medullary artery (Arteria nutricia) enters at the so-called nu-trient foramen (Foramen nutricium), passes in a canal (Canalis nutricius) throughthe compact substance, and ramifies in the marrow; its branches anastomose -withthe central branches of the periosteal set. The larger veins of the spongy bone donot, as a rule, accompany the arteries, but emerge chiefly near the articular surfaces.Within the bone thej^ are destitute of valves. The l5anph-vessels exist as peri-vascular channels in the periosteum and the Haversian canals of the compact sub-stance. They also form a fine subperiosteal network, froxn which the larger vesselsproceed, usually in company with veins.Lymph-spaces exist at the periphery ofthe marrow. The nerves appear to be distributed chiefly to the blood-vessels. Special nerve-1 Since yellow marrow is formed by regressive changes in red marrow, including fatty infiltra-tion and degeneration of the characteristic cells, wefind transitional forms or stages in the process.In aged or badly nourished subjects the marrowmay undergo gelatinous degeneration, resultingin the formation of gelatinous marrow. 19. DEVELOPMENT AND GROWTH OF BONE 23endings (Vater-Pacini corpuscles) in the periosteum are to be regarded as sensory,and probably are concerned in mediating the muscle sense (kinesthesia) DEVELOPMENT AND GROWTH OF BONEi The primitive embryonal skeleton consists of cartilage and fibrous tissue, inwhich the bones develop. The process is termed ossification or osteogenesis, andis by bone-producing cells, called osteoblasts. It is customary, effected essentiallytherefore, to designate asmembrane bones those which are developed in fibroustissue, and as cartilage bones those which are preformed in cartilage.The princi-pal membrane bones are those of the roof and sides of the cranium and most of thebones of the face. The cartilage bones comprise,therefore, most of the skeleton. Correspondinglywe distinguish intramembranous and endochondralossification. In intramembranous ossification the processbegins at a definite center of ossification (Punctumossificationis) where the osteoblasts surround them-,selves with a deposit of bone. The process extendsfrom this center to the periphery of the future bone,thus producing a network of bony trabeculae. Thetrabeculae rapidly thicken and coalesce, forming abony plate which is separated from the adjacentbones by persistent fibrous tissue. The superficialpart of the original tissue becomes periosteum, andon the deep face of this successive layers of perios-teal bone are formed by osteoblasts until the boneattainsits definitive thickness.Increase in circum-ference takes place by ossification of the surround-ing fibrous tissue, which continues to grow untilthe bone has reached its definitive size. In endochondral ossification the process isfundamentally the same, but not quite so simple.Osteoblasts emigrate from the deep face of the peri-Fig.4.Left Femur of Young Piq,chondrium or primitive periosteum into the cartilage Lateral View, to Show Di- VI8IOX OF A Long Bone intoand cause calcification of the matrix or ground- Shaft (s) and Extremities.substance of the latter. Vessels extend into the cal-Proximal extremity consistsofcifying area, the cartilage cells shrink and disappear, two parts, head (h) and trochantermajor (t.m.), which have separateforming primary marrow cavities which are occupiedcenters of ossification. Distal extrem-by processes of the osteogenic tissue. There is thus and condylesity consists of trochlea (0formed a sort of scaffolding of calcareous trabeculae (c); e.L, epiphyseal cartilages; s.f.,supracondyloid fossa.on which the bone is constructed by the osteoblasts.At the same time perichondral bone is formed bythe osteoblasts of the primitive periosteum. The calcified cartilage is broken downand absorbed through the agency of large cells called osteoclasts, and is replacedby bone deposited by the osteoblasts. The osteoclasts also cause absorption of theprimitive bone, producing the marrow cavities; thus in the case of the long bonesthe primitive central spongy bone is largely absorbed to form the medullary cavityof the shaft, and persists chiefly in the extremities.Destruction of the centralpart and formation of subperiosteal bone continue until the shaft of the bone hascompleted its growth. A typical long bone is developed from three primary centers of ossification, Only a brief general statement of osteogenesis can be 1made here; details must be soughtin embryological literature. 20. :24 OSTEOLOGYone, which appears first, for the diaphysis or shaft and one for each epiphysis oiextremity. Many bones have secondary centers from which processes or apophysesdevelop. The foregoing outhne accounts for the gro^Hh of bones except in regard tolength.Increase in length may be explained briefly as follows Provision for con-:tinued ossification at either end of the diaphysis is made by a layer of actively groAving cartilage the epiphyseal cartilage which intervenes between the diaph-ysis and the epiphysis. It is evident that so long as this cartilage persists andgrows, new bone may continue to be formed at its expense, and increase of lengthis possible. When the epiphyseal cartilage ceases to grow, it undergoes ossification,the bone is consolidated, and no further increase in length is possible. This fusiontakes place at fairly definite periods in the various bones, and it is of value to knowthe usual times at which it occurs in the larger bones of the limbs at least. In thecase of membrane bones, increase in circumference is provided for by the ossificationand new formation of the surrounding fibrous tissue.After the bones have reached their full size, the periosteum becomes relatively reduced andinactive so far as its osteogenic layer is concerned; the bone-forming function may be stimulatedby various causes, as is well seen in the heaUng of fractures and the occurrence of bony enlarge-ments. Profound changes occur in the skeleton after birth, and during the period of growth the bonesare much more plastic than might be supposed. In the new-])orn foal, for example, it is evidentthat the rnetacarpal and metatarsal bones are relatively long and the scapula and humerus short;also that in general the shafts of the long bones are slender in comparison with the extremities.The various prominences are much less pronounced than in the adult, and most of the minorsurface markings are absent, so that the bones have a relatively smooth appearance. The periodof gro^v-th may be regarded as terminating with the union of the extremities and shafts of the longbones and the fusion of the parts of other bones. During adult life the skeletal changes proceedmore slowly; they comprise accentuation of the larger prominences and depressions and the ap-pearance of smaller ones. These secondary markings are chiefly correlated with the attachmentsof muscles, tendons, and hgaments, or are produced by pressure exerted by various structures onthe bones. Later in hfe ossification invades more or less extensively the cartilages and the at-tachments of tendons and ligaments. Senile changes in the bones, consisting of decrease of theorganic matter and rarefaction of the bone tissue, render them brittle and liable to fracture.CHEMICAL COMPOSITION OF BONE Dried bone consists of organic and inorganic matter in the ratio of 1 2 ap-:proximately. The animal matter gives toughness and elasticity, the mineralmatter hardness, to the bone tissue. Removal of the organic matter by heat doesnot change the general form of a bone, but reduces the weight by about one-third,and makes it very fragile. Conversely, decalcification, while not affecting the formand size of the bone, renders it soft and pliable. The organic matter (ossein) whenboiled yields gelatin.The following table represents the composition in 100 partsof ox bone of average quality Gelatin33.30 Phosphate of Ume 57.35 Carbonate of lime 3.85 Phosphate of magnesia 2.05 Carbonate and chlorid of sodium 3.45 100.00 PHYSICAL PROPERTIES OF BONEFresh dead bone has a yellowish-white color; when macerated or boiled andbleached, it is white.The specific gravity of fresh compact bone is a little over1.93. It is very hard and resistant to pressure; a 5-millimeter cube of compactbone of the ox will resist pressure up to 852 pounds, if the pressure be applied inthe line of the lamellae (Rauber). Its tensile strength is estimated to be nearlytwice that of oak. 21. THE VERTEBRAL COLUMN25 DESCRIPTIVE TERMSThesurfaces of the bones present a great variety of eminences and depressions,as well as perforations.The prominences and cavities may be articular, or non-articular, furnishing attachment to muscles, tendons, ligaments, or fascia. Anumber of descriptive terms are used to designate these features, and the followingare some of those in generaluse:Process (Processus)is a general term for a prominence.Atuberosity (Tuber, Tuberositas) is a large, rounded projection; a tubercle(Tuberculum) is a smaller one. The term trochanter is applied to a few prominences, e. g., the trochanters ofthe femur.Aspine (Spina) or spinous process (Processus spinosus) is a pointed projection.Acrest (Crista) is a sharp ridge.Aline (Linea) is a very small ridge.Ahead (Caput) is a rounded articular enlargement at the end of a bone; itmaybe joined to the shaft by a constricted part, the neck (Collum). A condyle (Condylus) is an articular eminence which is somewhat cylindrical;a non-articular projection in connection with a condyle may be termed an epi-condyle (Epicondylus). A trochlea is a pulley-like articular mass. A glenoid cavity (Cavitas glenoidalis) is a shallow articular depression, and acotyloid cavity or acetabulum is a deeper one.The term facet is commonly applied to articular surfaces of small extent,especially when they are not strongly concave or convex.The terms fossa, fovea, groove or sulcus, and impression are applied to variousforms of depressions.A foramen is a perforation for the transmission of vessels, nerves, etc.A sinus is an air-cavity within a bone or bones; it is lined with mucousmem-brane and communicates with the exterior.Other terms, such as canal, fissure, notch, etc., require no explanation.^ THE VERTEBRAL COLUMNThe (Columna vertebralis) is the fundamental part of thevertebral coliramskeleton.Ita chain of median, unpaired, irregular bones whichconsists ofextends from the skull to the end of the tail. In the adult certain vertebrae havebecome fused to form a single bony mass with which the pelvic girdle articulates.Vertebrae so fused are termed fixed (or false") vertebrae (Vertebrae immobiles), asdistinguished from the movable (or Hrue") vertebrae (Vertebrae mobiles). The column is subdivided for description into five regions, which are namedaccording to the part of the body in which the vertebrae are situated. Thus thevertebrae are designated as cervical, thoracic, lumbar, sacral, coccygeal (Vertebraecervicales, thoracales, lumbales, sacrales, coccygeae).The number of vertebrae ina given species is fairly constant in each region except the last, so that the verte-bral formula may be expressed (for the horse, for example) as follows: C7Ti8L6S5Cyi5.21. The vertebrae in a given region have characters by which they may be dis-tinguished from those of other regions, and individual vertebrae have specialcharacters which are more or less clearly recognizable. All typical vertebrae have1 As might be expected from the history of anatomy, a good many of these terms are moreorless interchangeable; furthermore, a given skeletal feature may differ greatly in various species. 22. 26 OSTEOLOGYa common plan of structure, which must first be understood. The parts of whicha vertebra consists are the body, the arch, and the processes. The body (Corpus vertebrae) is the more or less cylindrical mass on which theother parts are constructed. The anterior and posterior extremities of the bodyare attached to the adjacent vertebrse by intervertebral fibro-cartilages, and areusually convex and concave respectively. The dorsal surface is flattened and entersinto the formation of the vertebral canal, while the ventral aspect is rounded later-ally, and is in relation to various muscles and viscera. In the thoracic region thebody presents two pairs of facets (Foveae costales) at the extremities for articula-tion with part of the heads of two pairs of ribs. The arch (Arcus vertebrae) is constructed on the dorsal aspect of the body.It consists originally of two lateral halves, each of which is considered to consistof a pedicle and a lamina. The pedicle (Radix arcus vertebrge) forms the lateralpart of the arch, and is cut into in front and behind by the vertebral notches (In-cisura vertebralis cranialis, caudalis).The notches of two adjacent vertebrae formintervertebral foramina (Foramina intervertebralia) for the passage of the spinal nerves and vessels; in some vertebrae, how- ever, there are complete foramina instead of notches.The laminae are plates which complete the arch dorsally, uniting with each other medially at the root of the spinous process. The body and the arch form a bony ringwhich incloses the vertebral foramen (Foramen vertebrale); the series of verte- bral rings, together with the ligaments which unite them,inclose the vertebral canal (Canalisvertebralis), Avhichcontains the spinal cord and its coverings and vessels. The articular processes,two anterior and two posterior (ProcessusarticularesCosraijkcef craniales,caudales), project from the bor- ders of the arch.Theypresent articular surfaces adapted to those of adjacent verte-Fig. 5. First Thoracic Vertebra of Horse.brae,and the remaining surface is roughened To illustrate plan of structure of vertebrae. for muscular and ligamentous attachment. The spinous process or spine (Processusspinosus)isand projects dorsally from the middle of the arch. It variessingle,greatly in form, size, and direction in different vertebrae. It furnishes attachmentto muscles and ligaments. The transverse processes (Processus transversi) are two in number and projectlaterally from the sides of the arch or from the junction of the arch and body.In the thoracic region each has a facet for articulation with the tubercle of a rib(Fovea costalis transversalis)They also give attachment to muscles and liga- .ments.Some vertebrae have also a ventral spine or a haemal arch.Mammillary processes (Processus mamillares) are found in most animals onthe last thoracic and anterior lumbar vertebrse, between the transverse and an-on the latter.terior articular processes or Accessory processes (Processus accessorii), when present, are situated betweenthe transverse and posterior articular processes. Development. The vertebrae are developed byossification in the cartilageAvhich surrounds the notochordand forms the sides of the neural canal. There arethree primary centers ofossification, one for the body and one for each side of the 23. .THE COSTAL CARTILAGES27arch.Secondary centers appear later for the summit of the spinous process(except in the cervical region), the extremities of the transverse processes, and thethin epiphyseal plates at the extremities of the body. Sometimes there are at first two centers for the body, which soon fuse. The process of ossifi-cation extends from the lateral centers to form, not only the corresponding part of the arch, butalso the processesand a part of the body next to the root of the arch (Radix arcus) In the horse.and ox the body and arch are fused at birth or unite very soon after, but the epiphyses do notfuse till growth is complete. In the pig, sheep, and dog the body and arch are united at l)irthby cartilage (neurocentral synchondrosis), but fuse in the first few months.THE RIBSTheribs (Costae) are elongated curved bones which form the skeleton of thelateral thoracic walls.They are arranged serially in pairs which correspond innumber to the thoracic vertebrae.Each articulates dorsally with two vertebraeand is continued ventrally by a costal cartilage. Those which articulate with thesternum by means of their cartilages are termed sternal ribs (Costae sternales);the remainder are asternal ribs (Costae asternales). Ribs at the end of the serieswhich have their ventral ends free in the abdominal wall are named floating ribs(Costae fiuctuantes). The intervals between the ribs are termed intercostal spaces(Spatia intercostalia)Atypical rib^ consists of a shaft and two extremities.The shaft (Corpuscostae) is band-like and varies much in length, breadth, and curvature in differentribs.In the case of some ribs the curvature is not uniform, but is accentuated at acertain point, termed the angle of the rib (Angulus costae) this occurs at a variable;distance from the vertebral end, and is usually marked by a rough ridge. Thedirection also varies; the first rib is usually almost vertical, while the remainderslope backward in increasing degree. The lateral surface (Facies lateralis) is con-vex, and the medial surface (Facies medialis) flattened from edge to edge; on thelatter, close to the posterior border, is the costal groove (Sulcus costae), which fadesout ventrally. It contains the intercostal vein. The anterior and posteriorborders (Margo cranialis, caudalis) are thin and sharp on some ribs, rounded onothers.Thevertebral extremity (Extremitas vertebralis) consists of the head, neck,and tubercle. The head (Capitulum costae) is the actual end of the rib, and isrounded and somewhat enlarged. It presents two facets (Facies articularis capitulicostae) for articulation with the bodies of two adjacent thoracic vertebrae; thesesurfaces are separated by a groove in which the conjugal ligament is attached.The neck (CoUum costae) joins the head to the shaft. It varies in length anddiameter.Its lateral surfaceis rough, its medial smooth. The tubercle (Tuber-culumcostae) projects backward at the junction of the neck and shaft. It has afacet (Facies articularis tuberculi costae) for articulation with the transverse processof the posterior vertebra of the two with which the head articulates. The tuberclegradually approaches the head in the posterior ribs, and eventually fuses with it. The sternal extremity (Extremitas sternalis) is commonly slightly enlarged,and is roughened at the junction with the costal cartilage. Development. The ribs are ossified in cartilage from three centers one eachfor the shaft (and sternal end), head, and tubercle; the third center is absent insome ribs at the terminal part of the series. THE COSTAL CARTILAGESThese (Cartilagines costales) are bars of hyaline cartilage which continue theribs. Those of the sternal ribs articulate with the sternum, while those of the1 The term is employedhere, as is usual in descriptive anatomy, to designate only the bonypart of the rib (Os costale); morphologically it includes the cartilaginous part also. 24. 28OSTEOLOGYasternal ribs overlap and are attached to each other to form the costal arch (Arcuscostalis).The cartilages of floating ribs are not attached to those adjacent. THE STERNUMThe stemxun(or breast-bone) is amedian segmental bone -which completes theskeleton of the thorax ventrally, and articulates Avith the cartilages of the sternalribs laterally. It consists of six to eight bony segments (Sternebra^) connected byintervening cartilage in the yoimg subject.Its form varies with that of the thoraxin general anilwith the development of the clavicles in animals in which these bonesare present.Its anterior extremity, the manubrium stemi or presternum, isspecially affected by the latter factor, being broad and strong when the claviclesare well ileveloped and articulate with it (as in man), relatively small and laterallycompressed Avhen they are absent (as in the horse) or rudimentary (as in the dog).The cartilages of the first pair of ribs articulate with it. The body or mesostemimi(Corpus sterni) presents laterally, at the junction of the segments, concave facets(Incisura^ costales) for articulation Avith the cartilages of the sternal ribs. Theposterior extremity or metastemiun presents the xiphoid cartilage (Processusxiphoideus);this is thin and wide, as in the horse and ox, or narroAv and short, asin the dog. Development. The cartilaginous sternmn is formed by the fusion mediallyof two lateral bars which unite the ventral ends of the first eight or nine costalcartilages, and is primitively imsegmented. The manubrium ossifies from a singlecenter,but the centers for the other segments appear to be primitively paired.The sternum never becomes completely ossified; details in regard to persistingcartilage will be given in the special descriptions. The layer of compact tissueisfor the greater partvery thin and the spongy substance is open-meshed and ver^vascular. THE THORAXThe skeleton of the thorax comprises the thoracic vertebnp dorsally. the ribsand costal cartilages laterally, and the sternmn ventrally. The thoracic cavity(CaAinn thoracis) resembles in shape an irregular truncated cone; it is compressedlaterally, especially in front, and the dorsal wall or roof is much longer than theventral wall or floor.The anterior apertvire or inlet (Apertura thoracis cranialis) isbc^unded by the first thoracic vertebra dorsally, the first pair of ribs and costalcartilages laterally, and the manubrium sterni ventrally.The posterior aperture(Apertura thoracis caudalis) is bounded by the last thoracic vertebra, the lastpair of ribs, the costal arches, and the anterior part of the xiphoid cartilage.It may be noted here that the diaphragm (which forms the partition hetween the thoracicand abdominal caA-ities) does not follow the costal arches in its posterior attachment, so that theposterior ribs enter also into the formation of the abdominal wall. THE SKULLThe term usually imderstood to includeskull is all of the bones of the head.The head consists of the cranium and the face, and it istherefore convenient todiAnde the bones into cranial antl facial groups.Thecranialbones (Ossa cranii) inclose the brainAA-ith its membranes andvesselsand the essential organs of hearing.They concur withthe facial bones informing the orbital and nasalcaAities, in AA-hich the peripheral organs of sight amiof smell are situated.Thefacial bones (Ossafaciei) form the skeleton of the oral and nasalcaAities,and also support the pharAnx, the larynx,and the root of the tongue. 25. THE BONES OF THE THORACIC LIMB 29 Mostmembrane; those of the bones of the skull are flat bones, developed inof the cranial base may be classed as irregular, and are developed in cartilage. Onlytwo form permanent movable joints with other parts of the skull. The mandible(or lower jaw-Vjone) forms diarthrodial joints with the temporal bones, and thehyoid bone is attached to the latter Vjy bars of cartilage. The other bones formimmovable joints, most of which disappear with age.In order to study the separate bones, skulls of young subjects are necessary, since later mostof the hnes of demarcation become effaced. The relations of each bone to its surroundings shouldbe specially noted, since the final object is to understand the skull as a whole. In the descriptionswhich follow the skull is considered with its long axis horizontal, and that of the horse will serveas a type. THE BONES OF THE THORACIC LIMB The thoracic limb (Extremitas thoracalis) consists of four chief segments, viz.,the shoulder girdle, the arm, the forearm, and the manus. The shoulder girdle (Cingulum extremitatis thoracalis) when fully developed, ,consists of three bones the scapula (or shoulder-blade), the coracoid, and theclavicle (or collar-bone).In the domesticated mammalsonly the scapula, a large,flat bone, is well developed,and the small coracoid element has fused with it, whilethe clavicle iseither absent or is a small rudiment embedded in the brachiocephali-cus muscle.There is therefore no articulation of the shoulder with the axialskeleton.The shoulder girdle is fully developed in birds and the lower mammals (monotremata). Inthe higher manmials the coracoid is reduced to the coracoid process of the scapula, and the develop-ment of the clavicle is in conformity with the function of the hmb. Thus in typical quadrupeds,such as the horse and ox, in which the foreUmbs are used only for support and locomotion, theclavicle is absent. Other animals which use these hmbs for grasping, burrowing, chmbing, etc.(e. g., man, apes, moles), have well-developed claicles which connect the scapula with the stemiun. The arm (Brachium) contains a single long bone, thehumerus (or arm bone). In the forearm (Antibrachium) are two bones, the radius and ulna. Thesevary in relative size and mobility. In the horse and ox the two bones are fu.sed,and the distal part of the limb is fixed in the position of pronation. The radius isplaced in front and supports the weight. The ulna is well developed only in itsproximal part, which forms a lever for the extensor muscles of the elbow. In thepig the ulna is the larger and longer of the two bones, but is closely attached tothe back of the radius. In the dog the ulna is also well developed and a smallamount of movement is possible between the two bones.The manus,! the homologue of the hand in man, consists of three subdivi-sions, viz., the carpus, metacarpus, and digit or digits. The carpus, popularly termed the "knee" in animals, and homologous withthe wrist of man, contains a group of short bones, the ossa carpi.^ These are typic-ally eight in number and are arranged in two transverse rows a proximal or anti-brachial, and a distal or metacarpal.The bones of the proximal row, named fromthe radial to the ulnar side {i. e., from within outward), are the radial, intermediate,ulnar, and accessory carpal bones.The bones of the distal row are designatednumerically, in the same direction, as first, second, third, and fourth carpal bones.This nomenclature, introduced by Gegenbaur, and now used largely by comparative anat-omists, seems decidedly preferable to the ^ariety of terms borrowed from human anatomy andbased on the form of the bones in man. The follo^vang table of synonyms in cornmon use is ap-pended for comparison. The Latin terms and abbreviated notations are given in parentheses.1 It is unfortunate that there is no popular name for this part of the limb. The term "fore-foot" is sometimes appUed to it, but this leads to confusion, since the word "foot" has long beenused in a different sense.2 The term "knee" as applied to this region is unfortunate, but the usage is very firmlyestablished and there is no other popular name. 26. 30 OSTEOLOGYThe central carpal bone (Os carpi centrale) is omitted, since it is not a separate element in theanimals under consideration here.Radial (Os carpi radiale, Cr) ScaphoidIntermediate (Os carpi intermedium, Ci) SemilunarUlnar (Os carpi ulnare, Cu) CuneiformAccessory (Os carpi accessorium, Ca)PisiformFirst carpal (Os carpale primum, CI)TrapeziumSecond carpal (Os carpale secundum, C2j TrapezoidThird carpal (Os carpale tertium, C3) Os magnumFourth carpal (Os carpale quartum, C4)Unciform The metacarpus contains typically five metacarpal bones (Ossa metacarpaliaI-V), one for each digit; they are long bones and are designated numerically fromthe radial to the ulnar side (i. e., from within outward). This arrangement occursin the dog, although the first metacarpal is much smaller than the others, and thesecond and fifth are somewhat reduced. Further reduction has taken place in theother animals, resulting in the perissodactyl and artiodactyl forms.In the horsethe first and fifth metacarpals are absent, the third is the large supporting meta-carpal bone and carries the single digit, while the second and fourth are much re-duced. In artiodactyls (e. g., ox, sheep, pig) the third and fourth are the chiefmetacarpals and carry the well developed digits; they are fused in the ox and sheep.The others are variously reduced or absent, as noted in the special descriptions tofollow.The fossil remains of the ancestors of the existing Equidse illustrate in a most complete man-ner the reduction which has occurred in this respect. The earliest known ancestor of the horse,Eohippus of the Lower Eocene, had four well developed metacarpal bones, each of which carrieda digit; the first metacarpal bone was small. Intermediate forms show the gradual evolution ofthe race from this primitive animal, which was about the size of the domestic cat. There is reasonto believe that earlier forms had five digits. The digits (Digiti manus) are homologous with the fingers of man, and aretypically five innumber. They are designated numerically from the radial tothe ulnar side, in correspondence with the metacarpus. The full number is presentin the dog.In the ox and pig the third and fourth are well developed and supportthe weight, while the second and fifth are reduced. The existing horse has a singledigit, the third of his pentadactyl ancestors. The skeleton of each fully developeddigit consists of three phalanges and certain sesamoid bones. The first phalanx(Phalanx prima) articulates with the corresponding metacarpal bone above andwith the second phalanx (Phalanx secunda) below. The third phalanx (Phalanxtertia) is inclosed in the hoof or claw, and is modified to conform to the latter. Thesesamoid bones (Ossa sesamoidea) are developed along the course of tendons or inthe joint capsules at points where there is increased pressure. Two proximalsesamoids (Ossa sesamoidea phalangis primse) occur at the flexor side of the meta-carpo-phalangeal joint and form a pulley for the flexor tendon. The distal sesa-moid (Os sesamoideum phalangis tertise) is similarly placed between the deepflexor tendon and the joint between the second and third phalanx; it is absent inthe dog, which has a small sesamoid on the extensor side of the metacarpo-phalan-geal joint, and often at the proximal interphalangeal joint also. Numerous cases are recorded of the occurrence of supernumerary digits (hyperdactylism)in the horseand other animals. In some pigs, on the other hand, the two chief digits are fused,,and the condition (syndactylism) appears to be inherited.THE BONES OF THE PELVIC LIMB The pelvic limb (Extremitas pelvina), like the thoracic, consists of four seg-ments, viz., the pelvic girdle, thigh, leg, and the pes; the last is subdivided intotarsus, metatarsus,and digits. The pelvic girdle (Cingulum extremitatispelvinse) consists of the os coxae (or 27. THE BONES OF THE PELVIC LIMB 31hip bone), which joins its fellow of the opposite side ventrally at the symphysispelvis, and articulates very finnlj^ with the sacrum dorsally. The two coxal bones,together with the sacrum and the first three or more coccygeal vertebrae, constitutethe bony pelvis. The os coxae consists originally of three flat bones, the ilium,ischium, and pubis, which meet at the acetabulum, a large cotyloid cavity withwhich the head of the femur articulates. These three parts are fused before growthis complete, but are considered separately for convenience of description.Theiliimi (Os ilium) is situated in the lateral wall of the pelvis, the pubis (Os pubis) inthe anterior part, and the ischium (Os ischii) in the posterior part of the ventralwall.The thigh (Femur), like the arm, contains a single long bone, the femur (orthigh bone) (Os femoris). This articulates with the acetabulum above and thetibia and patella below.The skeleton of the leg (Crus) comprises three bones (Ossa cruris), viz., thetibia, fibula, and patella. The tibia is a large, prismatic long bone which supportsthe weight, and articulates distally with the tibial tarsal bone. The fibula issituated along the lateral border of the tibia, from which it is separated by theinterosseous space of the leg.It is much more slender than the tibia and does notarticulate with the femur. In the pig and dog it has a complete shaft and twoextremities, but in the horse and ox it is much reduced and otherwise modified.The patella (or "knee-cap") is a short bone which articulates with the trochlea ofthe distal end of the femur; it is to be regarded as a large sesamoid bone intercalatedin the tendon of the quadriceps femoris muscle.The tarsus (or "hock") consists of a group of short bones, the ossa tarsi,numbering five to seven in the different animals. The proximal or crural row con-sists of two bones, the tibial and fibular tarsals ; the former is situated at the tibialside, and has a trochlea for articulation with the distal end of the tibia; the latter,situated at the fibular side, has a process, the tuber calcis, which projects upwardand backward and constitutes a lever for the muscles which extend the hock joint.The distal or metatarsal row consists of four bones when seven tarsal elements arepresent, as in the pig and dog. They are best designated numerically as first tarsal,secondtarsal, etc. The central tarsal is interposed between the rows. The preceding terms are anglicized abbreviations of those introduced by Gegenbaur intocomparative anatomy. The Latin names and synonyms are given in the following table: Tibial (Os tarsi tibiale, Tt.) Astragalus or Talus Fibular (Os tarsi fibulare, Tf.) Calcaneus or Os calcis Central (Os tarsi centrale, Tc.) Scaphoid or Navicular First Tarsal (Os tarsale primum, Tl) First or internal cuneiform Second Tarsal (Os tarsale secundum, T2)Second or middle cuneiform Third Tarsal (Os tarsale tertium, T3)Third or external cuneiform Fourth Tarsal (Os tarsale quartum, T4) CuboidThe metatarsal and digitalbones resemble in general those of the correspondingregions of the thoracic limb;the differential features will be noted in the specialdescriptions. 28. 32THE SKELETON OF THE HORSETHE SKELETON OF THE HORSE The skeleton of the horse consists of 205 bones, as shownin the following table: Vertebral column 54 Ribs 36 Sternum1 Skull (including auditory ossicles)34 Thoracic limbs 40 Pelvic limbs 40 205In this enumeration the average number of coccygeal vertebrae is taken to be 18, the tem-poral and OS coxae are not divided into parts, the usual number of carpal and tarsal elements istaken, and the sesamoids are included.tFig. 6.Skeleton of Horse, with Outune of Contour of Body. I.H., Atlas; 7.H., seventh cervical vertebra;I.R., first thoracic vertebra; 17. R.,seventeenth thoracic vertebra;I.L., first lumbar vertebra; O.L., sixthlumbar vertebra; K, sacrum;I.S., firstcoccygeal vertebra; 16.S., sixteenthcoccygeal vertebra;^.i?., sixth rib; e. A., costal cartilage; 18. R., lust rih;1, scapula;1, cartilage of scapula; 2, spineof scapula; 4,humerus;4 lateral epicondyle of humerus;5, lateral tuberosity ofhumerus; 6, deltoid tuberosity;7, shaft of ulna;8, olecranon; 9, radius; 10, carpus;accessory carpal bone; 12, metacarpus; 13, digit; 14, sternum, 11,14", xiphoid cartilage;15, ilium; 16, 16, angles of ilium; 17, ischium; 18, femur (shaft) 19, trochanter major; 20, ;patella;21, tibia (shaft); 21, lateral condyle of tibia; 2.3, fibula; 22, tarsus; 24, tuber calcis; 25, metatarsus;26,digit; 27, trochanter minor of femur; 28, trochanter tertius of femur. (After EUenberger-Baum, Anat. fiirKiinstler.) 29. THE VERTEBRAL COLUMN 33The Vertebral ColumnThevertebral formula of the horseis C7Ti8L6S5Cyi5-2i.Anterior artic- ular process of axisSpinous proc-ess of axis Transverseprocess of axis Fig. 7. Cervical Vertebrae of Horse; Dorsal View. Fig.8. Cervical Vertebr.eof Horse; Ventral View.a, Articular processes;b, transverseprocesses; 1, dorsal arch of atlas;2, w-ing of atlas; 3, intervertebrala. ventral tubercle of atlas; Transverse processes; 1, foramen of atlas; 6, foramen3,S, fossa atlantis;anterior articular cavities of atlas; foramen of atlas; 4, alar transversarium ofatlas; 6, dens of axis; 7, intervertebral4, alar foramen; 6, foramen transversarium; 6, ventral foramen of axis; 8, foramen transversarium of axis; 9, spinous processes.3 30. 34THE SKELETON OF THE HORSE THE CERVICAL VERTEBRA The cervical vertebrae (Vertebrae cervicales) are seven innumber. The first and second cervical vertebrae are highly modified in conformity with the special functions of support and movements of the head. The sixth and seventh present special characters, but do not differgreatly from the tjpe. With the exception of the first, they are quadrangular, mass- ive, and longer than the ver- tebrae ofother regions; they decreaseinlength from the second to thelast.The third, fourth,andfifth have thefol- lowing characters: 1. The body is long as compared with those of other vertebrae. The ventral surface median ventral presents a which becomes more spine, Headprominent as it is traced back-Fig. 9. Sixth Cervical Vertebra of Horse; Anterior View. ward, and is tuberculate at its posterior end it separates two; concave areas. The dorsal sur-face has a flat central area which is narrow in the middle of the vertebrae, and ^^deat either end it gives attachment to the dorsal longitudinal ligament. ; On eitherside of this area there is a groove which lodges the longitudinal spinal vein. Theselateral grooves are connected about the middle of the surface by a transverse furrow,in which there are several for-amina through which veinsemerge from the spongy sub-stance of the body. The an- Spinous processVertebralterior extremity or head (Ca-foramenArticularput vertebrae) has an oval process Iarticular surface which facesforward and downward; it isstrongly convex, and widerabove than below. The pos-terior extremity is larger andhas a nearly circular cotyloidPlBff^fcavity (Fossa vertebrae). 2. The arch is large andstrong.Itis perforatedon by a foramen whicheither sidecommunicates with the for-amen trans versarium. Thevertebral notches are large. 3. The articular processesare large.Their articular sur-faces are extensive, oval in outline,and concave; the anterior ones are slightl}^The remaining surface isdirected dorso-medially; the posterior, ventro-laterally.mainly roughened for ligamentous and muscular attachment. A crest connects 31. THE ATLAS35the articular processes of the same side on the fourth and fifth; on the third it doesnot reach the anterior process. 4. The transverse processes are large and plate-like. Each arises by tworoots, one from the arch and one from the body; between these is the foramentransversariiun, through which the vertebral vessels and a nerve pass. The aggre-gate of these foramina constitutes the canalis transversarius. The process divideslaterally into anterior and posterior branches, which are thickened and rough formuscular attachment. 5. The spinous process has the form of a low crest (Crista spinosa), whichwidens behind, and is connected by ridges with the posterior articular processes. The sixth cervical vertebra has the following distinctive features: It is shorterand wider than the fifth. The arch is large, especially posteriorly. The posteriorarticular processes are shorter, thicker, and further apart; each is connected withthe corresponding anterior one by a thick ridge. The spinous process is less rudi-mentary; it is half an inch or more (ca. 1.5 cm.) in height. The transverse proc-esses have three branches; the third part is a thick, almost sagittal plate, whichforms with its fellow and the body a wide ventral groove; the other branchescorrespond to those of the typical vertebrae, but are short and thicker.^ The fora-men transversarium is large; below its posterior end there is a fossa. Theventral spine is small and is less prominent posteriorly. The seventh cervical vertebra is readily distinguished by the following charac-ters It is shorter and wider than the others.: The body is flattened dorso-ventrallyand wide, especially behind; here it has a facet on each side for articulation withpart of the head of the first rib. The arch and its notches are large. The anteriorarticular processes are wider and longer than the posterior pair.The spinousprocess is an inch or more (ca. 3 cm.)in height.The transverse process isundivided, and has no foramen trans-versarium.2 The ventral crest is re-placed by a pair of tubercles.The AtlasThis vertebra is decidedly atypi-cal inThe bodyform and structure.and spinous process are absent. Ithas the form of a strong ring, fromwhich two curved plates, the wings,project laterally.Thering incloses avery large vertebral foramen, and con- Fig. Atlas of Horse, Dorsal, View after Removal OF Dorsal Arch.sists of two lateral masses connected1, Anterior articular cavities; 2, 2, posterior articularby dorsal and ventral arches.surfaces;3, articular surface of ventral arch for dens of The lateral masses (Massse later- axis; 4, transverse ridge; 5, 5, alar foramina; 6, 6, foram-ales) present two deep oval anterior ina transversaria.articular cavities (Foveas articularescraniales) which receive the occipital condyles; they are separated by a wide notchabove and a narrow one below. The lateral margin is also notched, and a trian-gular non-articular depression cuts into the medial part of each cavity. The pos-somewhat saddle-shaped;terior articular surfaces (Facies articulares caudales) arethey are confluent on the ventral arch, but are widely separated dorsally, and donot conform in shape to the corresponding surfaces of the axis.1The third branch of the transverse process and the fossa are sometimes absent or reducedon oneside.2 In some specimens a large foramen transversarium is present on one side or (rarely) onbothsides. 32. 36THE SKELETON OF THE HORSEdorsal arch (Arcus dorsalis) presents a median dorsal tubercle (Tuber-Theculumand is concave ventrally. It is perforated on either side near its dorsale)anterior margin by the intervertebral foramen (Foramen invertebrate). The an-terior border is deeply notched, and the posterior is thin and concave. The ventral arch (Arcus ventrahs) is thicker, narrower, and less curved thanthe dorsal. On its lower surface is the ventral tubercle (Tuberculum ventrale),into which the terminal tendon of the longus colli muscle is inserted. The upperface has posteriorly a transversely concave articular surface, the fovea dentis, onwhich the dens or odontoid process of the axis rests. In front of this is a transverserough excavation and a ridge for the attachment of the ligamentum dentis. The wings or alae are modified transverse processes. They are extensivecurved plates which project ventro-laterally and backward from the lateral masses.The dorsal surface is concave. Between the ventral aspect of the wing and thelateral mass is a cavity, the fossa atlantis; in this there is a foramen which opensinto the vertebral canal. The border is thick and rough; its position can be recog-nized in the living animal. Two foramina perforate each Aving. The anterior one,the foramen alare, is connected with the intervertebral foramen by a short groove.The posterior one is the foramen transversariimi. Development. The atlas ossifies from four centers, two for the ventral arch,and one on either side for each lateral mass, wing, and half of the dorsal arch. Atbirth the bone consists of three pieces the ventral arch and two lateral parts,which are separated by a layer of cartilage in the dorsal median line and by twoventro-lateral layers.These parts are usually fused at about six months. The AxisThe axis (Axis s. Epistropheus) is the longest of the vertebrae, and is character-izedby the presence of the dens or odontoid process, which projects from the an-terior part of the body. Theanterior extremity of the body presents centrally the dens or odontoid Spinous processPosterior articular processDensTrajisverse processAnterior articular process Body Fig. 12. Axis of Horse,Left View.1, Arch; 2, intervertebral foramen; 3, notch; 4. foramen transversarium.process (Dens axis)this has a convex articular surface ventrally for articulation ;with the ventral arch of the atlas, and two rough depressions for the attachment ofthe ligamentum dentis dorsally.Flanking this on either side are the modifiedanterioi articular processes, which have saddle-shaped articular surfaces confluent 33. THE THORACIC VERTEBRA 37ventrally with that of the dens. The posterior extremity has the usual cavity.The ventral spine resembles that of the typical vertebrae. The arch presents in the young subject a notch on each side of its anteriorborder; this is converted into a foramen by a ligament which ossifies later.Theposterior border has the usual notches. The posterior articular processes are typical. The transverse processes are small, single, and project backward.Theforamen transversariiun is small. The spinous process is very large and strong. Its free border is rough, thickensposteriorly,andcontinued to the articular processes by two ridges. The lateralissurfaces are concave and rough for muscular attachment. Development. The axis has six or seven centers of ossification. In additionto the usual five, one or two appear for the dens, which is regarded as the displacedbody of the atlas. A nucleus behind the dens, which remains distinct to three orfour years of age, is considered to be the head of the axis. THE THORACIC VERTEBRA The thoracic vertebrae (Vertebrae thoracales) are usually eighteen in number inthe horse, but there are sometimes nineteen, rarely seventeen. As regional char-acters we note the surfaces for articulation with the ribs and the length and form ofthe spinous processes. Those in themiddle of the series are the mosttypical and present the following fea-tures: 1. The bodies are short and con-stricted in the middle.The endsareexpanded and have articular surfaceswhich are not strongly curved; the an-terior surface is convex, the posterior Spinousconcave. On the upper part of each processside are anterior and posterior costalfacets (Fovea costalis cranialis, cau-dalis), which, with those of adjacentvertebrae and the intervening fibro-car-tilages, form sockets for the heads ofthe ribs. 2. The arches are small. Their Facet for tubercleof ribposterior notches are relatively largeand are often converted into foram-Facet forina.headof rib 3. The articularprocesses aresmall.The anterior processes are in Bodyfact represented onlyby two oval facetsFig. 13. Seventh Thoracic Vertebra of Horse, An-on the anterior part of the arch which terior View.face almost directly upward.The pos-terior processes spring from the base of the spinous process;their facets facealmost directly downward. 4. The transverse processes are short, thick, and tuberous at the free end.Each has a facet (Fovea transversaria) for articulation with the tubercle of therib which has the same serial number. 5. The spinous process is large, narrow, and slopes upward and backward.The anterior border is thin, the posterior wider and furrowed. The summit isexpanded and rough. 34. 38 THE SKELETON OF THE HORSEThe first thoracic vertebra has the following specific characters: The body iswide and flattened dorso-ventrally. In front it has a head like the cervical verte-Spinnus processArticular processesArchFacet for tubercle of first ribA^~-^__^ Transverse process Facet for head of second rib""^fl^^ BodyFig. 14.First Thoracic Vertebra of Horse, Posterior View.brae, and behind a cavity somewhat deeper than any other thoracic vertebra. Twolarge costal facets are found on either side, and a well-marked spine ventrallj".Fir., l.i. T,sT Three THORArir ^ERTEBR.E of Horse,[.eft iew.1, Body; 2, 2, facets for head of rib;S, facet for tubercle of rib; 4, J/ , articular processes; 5, intervertebral foramen; 6, mammillary process; 7, spinous process.The arch is large and strong, and has large notches. The articular processes aremuch larger than those of other thoracic vertebrae, and resemble a good deal thoseof the seventh cervical.The transverse processes are short and thick, and each 35. THE LUMBAR VERTEBRA39has onits ventral aspect a large concave facet for articulation with the tubercle ofthe first rib. The spinous process is curved backward and tapers to a point. Itslength is usually about three or four inches (ca. 8 to 10 cm.). This vertebra may bemistaken at first glance for the last cervical, but is promptly identified by the threecostal facets on each side and the length of the spine. The last thoracic vertebra is distinguished by the absence of the posterior pairof costal facets, and the confluence of the anterior pair with those on the transverseprocesses. The serial position of others may be determined at least approximately by thefollowing data: (1) The bodies graduallj^ diminish in length and width to themiddle of the region and then increase slightly. Their costal facets become smallerand less concave from first to last. Theventral crest is distinct on three or fourvertebrae at either end of the region. The transverse processes diminish in (2)size and are placed lower down as they are traced backward.Their costal facetsbecome smaller and lower in position; on the last (and sometimes on its predecessoralso) it fuses with the costal facet of the body. The upper non-articular part ofthe process gradually becomes more sharply defined, and in the last four or fiveforms a distinct mammillary process. (3) The spinous processes increase in lengthto the third and fourth, and then gradually diminish to the fifteenth, beyond whichthey have about the same length.The backwardinclination is most pronouncedin the second, the sixteenthis vertical, and the two are directed a little forward. lastThe longest spines {i. e., those of the withers) are the thickest and have expandedsummits which remain more or less cartilaginous; the others are more plate-like,and are surmounted by a thick lip. The second spine is more than twice as largeas the first. The summits of the fourth and fifth usually form the highest pointof the withers. Development. There are six or seven centers of ossification, three for thebody, two for the arch, and one for the spinous process; some of the latter have anadditional center for the summit.THE LUMBAR VERTEBRJE The lumbar vertebrae (Vertebrae lumbales) are usually six in number in thehorse. They are characterized by the size and form of their transverse processes. The bodies of the first three are semi-elliptical on cross-section, and present adistinct ventral crest.From the fourth backward they become wider and flatterand the ventral crest subsides. The arches of the first three are about equal in size and similar to that of thelast thoracic; behind this they increase in breadth and height. The posteriornotches are much deeper than the anterior ones. The anterior articular processes are fused with the mammillary processes, andpresent dorsally concave surfaces for articulation with the posterior pair of thepreceding vertebra. The posterior articular processes project distinctly from thearch at the base of the spinous process, and have ventrally convex articular surfaces,which fit into the concave surfaces of the anterior pair of the next vertebra.The transverse processes are elongated plates, flattened dorso- ventrally, whichproject outward and usually curve slightly downward; their length increases tothe third or fourth, and then diminishes to the last, which is the shortest. Thefirst one or two usually curve somewhat backward, the last two decidedly forward.Those of the fifth have an oval concave facet on the medial part of the posteriorborder for articulation with the sixth process; the latter has a corresponding con-vex facet on the anterior border, and a larger concave surface on the posterior borderfor articulation with the wing of the sacrum.Sometimes the fifth process has asmall surface for articulation with the fourth. The medial part of the sixth process 36. 40 THE SKELETON OF THE HORSE the lateral part thinner, narrower, and curved forward. The medial partis tliick,of the fifthis also somewhat thickened.Medial to the articular surfaces the edgesof the transverse processes are cut into by notches, which form foramina by apposi-tion with each other and the sacrum. The spinous processes resemble those of the last two thoracic vertebrae. Spinous processMammilhinj processArticular processr } Transverse process f Ventral spine Fig. 16. Second Lumbar Vektebra of Horse; Posterior View.They are usually about equal in height, but minor differences are common, and thewidth diminishes in the last three. Development. This is similar to that of the thoracic vertebrae. The extremi-ties of the transverse processes remain cartilaginous for some time after ossificationis otherwise complete. Thetransverse processes of this region are considered equivalent to the proper transverseprocess +the costal element, and hence the distinctive term processus lateralis (s. costarius)Spinous processVertebralforamen > Articular processes Transverse process Notch BodyArticular surface forwing of sacrum-Last Lumbar Vertebra op Horse; Posterior View.has been proposed. The occurrence of a rib in connection with the transverse process of the firstlumbar vertebra is common.Reduction of the number to five has been observed frequently, andmayor maynot be compensated by an additional thoracic vertebra. Very few cases are recordedof seven lumbar vertebra^ especially with the normal thoracic number.An anomalous vertebrawith mixed thoracic and lumbar characters sometimes occurs at the junction of the two regions. 37. THE SACRUM41THE SACRUM The sacrum (Os sacrum) is formed by the fusion offive vertebrae, and is con-veniently described as a single bone. It is triangular in form and is wedged inbetween the ilia, with which it articulates very firmly on each side. Its long axisis gently curved and slightly oblique, so that the posterior end is a little higherthan the anterior. It presents two surfaces, two borders, a base, and an apex. The dorsal surface (Facies dorsalis) presents centrally the five sacral spines(Processus spinosi), which are directed upward and backward, and have (with theexception of the first) tuberous summits which are sometimes bifid. WingTransverse process Fig. 18. Sacrum of Horse; Dorso-lateral View.I-V, Spinous processes; I-4, dorsal sacral foramina;5, 0, articular processes; 6, surfaces of wings for articulationwith transverse processes of last lumbar vertebra; 7, bodyof first sacral vertebra. Arrows point into sacralcanal. The first spine is relatively thin and narrow, and is not so high as the sacral angle of theilium. The second is the longest and highest, and the length and height diminish to the last.The bases of the spines are often fused in old subjects. On either side of the spines there is a groove, in which are the four dorsal sacralforamina (Foramina sacralia dorsalia); the dorsal branches of the sacral nervesemerge through them. The pelvic surface (Facies pelvina) is concave in its length, wide in front,narrow behind. The curvature is variable and is more pronounced in the marethan in the stallion. It is marked by four more or less distinct transverse lines(Lineae transversse) which indicate the demarcation of the bodies of the vertebrae. ,At the ends of these lines are the ventral sacral foramuia (Foramina sacralia ven- 38. 42 THE SKELETON OF THE HORSEtralia),which are larger than the dorsal series and diminish in size from first tolast; they transmit the ventral divisions of the sacral nerves. The dorsal and ventral foramina communicate with the sacral canal and aretogether equivalent to the usual intervertebral foramina. The lateral borders are rough, thick in front, thin behind. The base (Basis ossis sacri) is directed forward, and is relatively very wide.It presents centrally the body of the first sacral segment, which is wide transversely,flattened dorso-ventrally, and has a rounded surface which articulates with thelast lumbar vertebra through the medium of an intervertebral fibro-cartilage.The ventral margin projects slightly, forming the promontory (Promontorium) On . Fig. 19. Sacrum of Horse; Ventral View. J- F, Bodies of original five vertebrse, marked off by transverse lines; /-4, ventral sacral foramina; 5, articularsurface of body of first vertebra; 6, 6, notches; 7, 7, surfaces of wings for articulation with transverse processes of lastlumbar vertebra; S, S, wings; 9, auricular surface; 10, lateral border; 11, transverse process; 12, posterior orificeof sacral canal; 13, last spinous process. body there is a smooth notch, which, with one on the last lumbareither side of thevertebra, forms a large foramen for the passage of the ventral branch of the lastlumbar nerve. Above the body is the entrance to the sacral canal, flanked by apair of articular processes, which project upward and forward from the arch, andhave concave surfaces medially for articulation with those of the last Imnbarvertebra. Lateral to each of these is a smooth notch which is converted into aforamen by apposition with the last lumbar vertebra. The lateral parts of the base,the alae or wings (Alae sacrales), are strong prismatic masses with pointed ends.Each has in front a large, oval, slightly convex surface for articulation with thetransverse process of the last lumbar vertebra. Posteriorly there is an elongatedoval area which faces dorso-laterally; this is the auricular surface (Facies auricu- 39. THE COCCYGEAL VERTEBRA 43laris),which articulates with the ihum;it is slightlyconcave inits length, andsomewhat rough and irregular.The rest of the dorsal surface of thewingis rough-ened for ligamentous attachment, while the ventral surface is smooth. The apex (Apex ossis sacri) is the posterior aspect of the last sacral vertebraand is quite small. It presents the elliptical flattened surface of the body, abovewhich is the triangular posterior opening of the sacral canal, surmounted by thelast sacral spine. There is a pair of narrow notches between the arch and body,above which rudiments of articular processes may occur. The name sacral canal (Canalis sacralis) is applied to that part of the vertebralcanal which traverses the sacrum. Its anterior part is large and has the form ofa triangle with the angles rounded off; its width is nearly tAvice its height. Tracedbackward it is seen to diminish in size rapidly, and the posterior opening is smallandtriangular.The term lateral part (Pars lateralis) designates the portion lateral to theforamina, which results from the fusion of the transverse processes. Development. The several sacral vertebrae ossify in the typical manner.Separate centers for costal elements in the lateral parts have not yet been found inthe domesticated animals. Fusion begins in front, and is usually not completetill adult age.i The lateral parts unite before the bodies. It is rather curiousthat the epiphyseal plates of adjacent segments unite with each other before theyfuse with the main portion of the bodies.THE COCCYGEAL VERTEBRA The coccygeal vertebrae (Vertebrae coccygeae) vary considerably in number, buteighteen may be taken as an average. From first to last they become reducedin size and, with the exception of a few at the beginning of the series, consist ofSpinous process Spinous process ] crtebral foramen Vertebral Vertebraforamen ArchA rchTransverse processTransverse proc- Groove BodyBodyFig. 20. First CoccygEAL Vertebra of Horse; Fig. 21. Second Coccygeal Vertebra of Horse; View. Posterior View. Leader to arch points to rudimentary articularbodies only. Thefirstthree have bodies which are somewhat flattened dorso-ventrally, constricted in the middle,and have at the ends convex, elliptical, artic-ular surfaces.The ventral surface has a median groove (Sulcus vasculosus) forthe coccygeal artery. The arch is small and triangular; it is formed of two flatplates which are prolonged to form a short spinous process with a thickened andoften double summit.The anterior notches are absent. Functional articularprocesses are not present, but small rudiments of the anterior pair commonly occur.The transverse processes are relatively large plates which project horizontallyoutward. Further back the arch becomes incomplete dorsally, and soon disap-pears; the transverse processes gradually fade out, and the vertebrae are reducedto cylindrical rods of diminishing size.The last one has a pointed end. ^ It is not rare to find fusion of some of the bodies incomplete even in adult subjects. 40. 44THE SKELETON OF THE HORSEVariations. The number is said by good observers to vary between fourteen and twenty-one. In old age the first is often fused with the sacrum, and sometimes with the second. Thearch of the third may be open.THE VERTEBRAL COLUMN AS A WHOLE In the mid-dorsal line is the series of spinous processes, which are low ridgesin the cervical region with the exception of the second and seventh, reach theirmaximum height at the fourth and fifth thoracic vertebrae, and diminish to thefifteenth or sixteenth thoracic.Behind this they are about equal in height as faras the last lumbar and first sacral, which are somewhat lower. The second sacralspine is about as high as the middle luml^ar; behind this they diminish ratherrapidly in height and fade out about the third coccygeal.Their inclination back-ward is most decided at the second thoracic and diminishes from the sixth or seventhto the sixteenth thoracic,which is vertical and is termed the anticlinal or dia-phragmatic vertebra. Behind this they are inclined a little forward until thesacrum is reached; here there is an abrupt change to the backward inclination, sothat a considerable interspinous angle is formed. On either side of the spinous processes is a vertebral groove which contains thedeep muscles of the spine. The floor of the groove is formed by the arches andarticular processes. It is wide in the neck and narrows progressively in the back. Viewed from the side, the column presents a series of curves. When the headand neck are in the ordinary neutral position, the anterior part of the cervical spineforms a gentle curve, concave ventrally. The posterior cervical and first thoracicvertebrae form a more pronounced curve in the opposite direction. At the junctionof the cervical and thoracic regions there is a marked change of direction, forming aventral projection or angle. At the second thoracic vertebra a gentle curve, con-cave ventrally, begins. This is continued to the lumbo-sacral junction, wherethere is