The body structure, or anatomy, of the frog is very similar to the anatomy of man. Both man and the frog have the same kinds of organs and systems of organs. The frog's anatomy, however, is much simpler.

General Body Features

As in other higher vertebrates, the frog body may be divided into a head, a short neck, and a trunk (see Vertebrates). The flat head contains the brain, mouth, eyes, ears, and nose. A short, almost rigid neck permits only limited head movement. The stubby trunk forms walls for a single body cavity, the coelom.

Man's internal organs are housed in one of three distinct hollow cavities--the chest, the abdomen, and the pelvis. The human chest is separated from the abdomen by a powerful muscular partition, the diaphragm (see Diaphragm). There is no such partition in the frog's coelom. All the frog's internal organs--including the heart, the lungs, and all organs of digestion--are held in this single hollow space.

The Skeleton and Muscles

The frog's body is supported and protected by a bony framework called the skeleton (see Skeleton).

The skull is flat, except for an expanded area that encases the small brain. Only nine vertebrae make up the frog's backbone, or vertebral column. The human backbone has 24 vertebrae. The frog has no ribs.

The frog does not have a tail. Only a spikelike bone, the urostyle, remains as evidence that primitive frogs probably had tails. The urostyle, or "tail pillar," is a downward extension of the vertebral column.

The shoulders and front legs of the frog are somewhat similar to man's shoulders and arms. The frog has one "forearm" bone, the radio-ulna. Man has two forearm bones, the radius and the ulna. Both frog and man have one "upper arm" bone, the humerus.

The hind legs of the frog are highly specialized for leaping. The single "shinbone" is the tibiofibula. Man has two lower leg bones, the tibia and the fibula. In man and in the frog, the femur is the single upper leg (thigh) bone. A third division of the frog's leg consists of two elongated anklebones, or tarsals. These are the astragalus and the calcaneus. The astragalus corresponds to the human talus. The calcaneus in the human skeleton is the heel bone.

As in other vertebrates, the frog skeleton is moved by muscles (see Muscles). Skeleton-moving muscles are made of skeletal, or "striated," muscle. Internal organs contain smooth muscle tissue.

The Circulatory System

The frog heart is the only organ contained within the coelom which has its own protective covering. This is the pericardium (see Heart). There are two upper chambers of the heart, the right atrium and the left atrium. The frog heart, however, has only one lower chamber, a single ventricle. In man, the lower heart chamber is divided into two compartments, the right ventricle and the left ventricle.

Oxygen-laden blood and oxygen-poor blood containing waste gases are present together in the frog ventricle at all times. The oxygen-laden and oxygen-poor bloods, however, do not mix. Such mixing is prevented by a unique arrangement of the frog's heart. Instead of "perching" on top of the ventricle, the right atrium dips downward into the ventricle. This causes oxygen-poor blood entering the right atrium to pass all the way down to the bottom of the ventricle.

Meanwhile, oxygen-laden blood is received by the left atrium and enters the same single ventricle. The pool of oxygen-poor blood at the bottom of the ventricle holds up the oxygen-laden blood and prevents it from sinking to the bottom. When the oxygen-poor blood flows from the ventricle into vessels leading to the lungs, the oxygen-laden blood tries to "follow" it. The lung vessels, however, are filled with oxygen-poor blood, blocking the oxygen-laden blood and forcing oxygen-laden blood to detour into the arteries. These carry the oxygen-laden blood to the tissues.

Frog blood has both a solid and a liquid portion. The liquid plasma carries solid elements such as red blood cells and white blood cells. (See also Blood.)

The Skin and Respiratory System

The frog is covered by a soft, thin, moist skin composed of two layers, an outer epidermis and an inner dermis (see Skin). The skin does not merely protect the frog but helps in respiration (see Respiratory System).

An extensive network of blood vessels runs throughout the frog's skin. Oxygen can pass through the membranous skin, thereby entering directly into the blood. When a frog submerges beneath

the water, all its respiration takes place through the skin. Oxygen is obtained directly from the water.

The frog does not breathe through its skin alone. Adult frogs have paired, simple, saclike lungs. As in man, air enters the body through two nostrils, passes through the windpipe, and is received by the lungs (see Lungs). The mechanism of breathing, however, is different in the frog from that in man. In humans breathing is aided by the ribs, the diaphragm, and the chest muscles. The frog has no ribs or diaphragm, and its chest muscles are not involved in breathing.

A frog may breathe by simply opening its mouth and letting air flow into the windpipe. However, it may also breathe with its mouth closed. The floor of the mouth is lowered, causing the frog's throat to "puff out." When the nostrils open, air enters the enlarged mouth. Then, with nostrils closed, the air in the mouth is forced into the lungs by contraction of the floor of the mouth.

The Digestive and Excretory Systems

The frog's mouth is where digestion begins. It is equipped with feeble, practically useless teeth. These are present only in the upper jaw. The frog's tongue is highly specialized. Normally, the tip of its tongue is folded backward toward the throat. From this position the frog can flick it out rapidly to grasp any passing prey. To better hold this prey, the tongue is sticky. (See also Tongue.)

Food passes from the frog's mouth into the stomach by way of the esophagus. From the stomach, the food moves into the small intestine, where most of the digestion occurs. Large digestive glands, the liver and the pancreas, are attached to the digestive system by ducts. A gall bladder is also present (see Digestive System).

Liquid wastes from the kidneys travel by way of the ureters to the urinary bladder. Solid wastes from the large intestine pass into the cloaca. Both liquid and solid waste material leave the body by way of the cloaca and the cloacal vent.

The Nervous System and Sense Organs

The frog has a highly developed nervous system. It consists of a brain, a spinal cord, and nerves. (See also Brain and Spinal Cord; Nervous System.)

The important parts of the frog brain correspond to comparable parts in the human brain. The medulla regulates automatic functions such as digestion and respiration. Body posture and muscular co-ordination are controlled by the cerebellum. The cerebrum is very small in the frog. By comparison the human cerebrum is very large. In man the cerebrum is involved in many important life processes.

Only 10 cranial nerves originate in the frog's brain. Man has 12. Similarly, the frog has only 10 pairs of spinal nerves. Man has 30 pairs.

Two simple holes make up the nostrils for the frog. There are complex valves but no long nasal passages as there are in man (see Nose). The frog's sense of smell is registered by olfactory lobes. These make up the forward portion of the brain.

The eye is crude. Its fixed lens cannot change its focus. Poorly developed eyelids do not move. To close its eye, the frog draws the organ into its socket (see Eye). A third eyelid, or nictitating membrane, may be drawn over the pulled-in eyeball.

There is no external ear (see Ear). Both eardrums, or tympanic membranes, are exposed. There is only one bone in the frog's middle ear. The human middle ear contains three bones (ossicles). As in man, semicircular canals help to maintain body balance.

Muscle FunctionThe main framework of the body (skeleton) is covered by muscles, whose function is to permit movement and maintain posture. Sensory receptors in the muscles monitor the tension and length of the muscles and provide the nervous system with crucial information about the position of the body parts, thereby enabling posture to be maintained. Muscle tissue has four main properties: Excitability (ability to respond to stimuli), Contractibility (is ability to contract), Extensibility (the ability of a muscle to be stretched without tearing) and Elasticity (the ability to return to its normal shape).

Movement Definitions

Each of the movements of the muscles for the various parts of the body is described by various terms.Abductor - moves a limb away from the midlineAdductor - moves a limb towards the midlineExtensor - increase the angle at a joint - extends a limbFlexor - decreases the angle at a joint - flexes a limbPronator - turns a limb to face downwardsSupinator - turns a limb to face upwardsRotator - rotates a limbSphincter - closes an orifice of opening

Muscle actions

Muscles which move the shoulder and their actionLevator scapulae - Raises shoulder bladePectoralis minor - Lowers shoulder bladeTrapezius - Lifts clavicle. Adducts, elevates and rotates scapular outwards. Extends headRhomboideus major - Adducts scapular and rotates it inwards

Serratus anterior - Stabilises scapula when hand exerts pressure on an object

Muscles which move the arm and their actionPectoralis major - Flexes, adducts and rotates arm mediallyLatissimus dorsi - Extends, adducts and rotates arm medially. Moves arm downward and backwardsDeltoid - Abducts, flexes, extends and medially and laterally rotates armTeres major - Extends arm, assists in adduction and medial rotation of arm

Muscles which move the forearm and wrist and their actionBiceps brachii - Flexes and supinates forearm. Flexes armBrachialis - Flexes the forearmBrachoradialis - Flexes, semi-supinates and semi-pronates the forearmTriceps brachii - Extends forearm. Extends armPronator teres - Pronates and flexes forearmPronator quadratus - Pronates the forearm and handSupinator - Supinates forearm and handMuscles which move the abdominal wall and their actionRectus abdominis - Compresses abdomen and flexes vertebral columnExternal obliques - Bends vertebral column laterally and rotates vertebral columnTransversus abdominis - Compresses abdomenQuadratus lumborum - Side flexion

Muscles which move the vertebral column and their actionIliocostalis lumborum - Extends lumbar regionIliocostalis thoracis - Maintains the spine's erect positionIliocostalis cervicis - Extends cervical regionLongissimus thoracis - Extends thoracis regionLongissimus cervicis - Extends cervical regionLongissimus capitis - Extends the head and rotates it to opposite sideSpinalis thoracis - Extends vertebral columnSpinalis cervicis - Extends vertebral columnSpinalis capitis - Extends vertebral columnMuscles which move the thigh and their actionPsoas major - Flexes and rotates thigh medially and flexes vertebral columnIliacus - Flexes and rotates thigh medially and flexes vertebral columnGluteus maximus - Extends and rotates thigh laterally.Adductor longus - Adducts, medially rotates and flexes the thighAdductor brevis - Adducts, laterally rotates and flexes the thighAdductor magnus - Adducts, flexes, laterally rotates and extends the thigh.Muscles which act on the leg and their actionRectus femoris - Extends knee and flexes hipVastus lateralis - Extends kneeVastus medialis - Extends knee

Vastus intermedius - Extends kneeSartorius - Flexes knee. Flexes hip and rotates femur laterallyBiceps femoris - Flexes leg and extends thighSemitendinosus - Flexes leg and extends thighSemimembranosus - Flexes leg and extends thighMuscles which move the foot and their actionTibialis anterior - Dorsiflexes and inverts footPeroneus tertius - Dorsiflexes and everts footGastrocnemius - Plantar flexes foot and flexes kneeSoleus - Plantar flexes footPlantaris - Plantar flexes footTibialis posterior - Plantar flexes and inverts the footPeroneus longus - Plantar flexes and everts the footPeroneus brevis - Plantar flexes and everts the foot

Internal Organ PicturesBelow are some pictures of various ACF internal organs.  The pictures were taken by the Perry's.  Thank you for the pictures.

BEWARE: Below are graphic pictures of internal organs, some pictures contain blood.

Full Body with Internal Organs

Brain (from underside)

The 3 chambered Heart.

Heart, Stomach and Intestines.

Intestines

Large Intestine (Centre) and Ovaries.

Liver

One of the Lungs

Stomach

Pictures

The frog's mouth houses it's extremely elastic tongue which it uses to capture prey. The mouth also is home to the nares and teeth.

The frog's skin encases the protection of it's vital organs. Once removed the powerful muscles are exposed.

Once the muscles are removed, the most predominate organs in the frog's coelem are the stomach and liver.

FEMALE A female frog has ovaries which are easily identifiable if engorged with eggs.

MALE The male's testes are a bright orange color.

External featuresThe frog is generally considered as a typical vertebrate animal. It is an amphibious animal capable of living both in water and on land. However, it spends most of the time in its aquatic habitat such as ponds, pools and ditches.

Frog has a body which can be distinguished into a head and a trunk. The body is covered by a moist, slimy skin which is loosely attached. The skin has a dark blackish green colour on the dorsal surface and pale white on the ventral surface. It helps in respiration.

The head is roughly triangular in shape. Anteriorly it has a cavernous mouth that can ingest large animals, since the gape of the mouth is large. A pair of nostrils are situated at the anterior tip, dorsally. Two large, retractible eyes are present. Each eye has an upper eyelid, a lower eyelid which is immovable, and a transparent movable third eye lid, called nictitating membrane. A pair of large, flattened tympanic membranes are present posteriorly one on each side, representing the ear drums.

External Features of a Frog

Respiratory systemIt consists of a pair of lungs, a pair of bronchi and a trachea. The lungs contain numerous microscopic functional units called alveoli.

These structures are involved in pulmonary respiration. In addition, there is buccal respiration, through the buccal cavity and cutaneous respiration through the moist skin.

Respiratory System

Excretory systemIt is represented by a pair of kidneys, a pair of ureters and a urinary bladder. Kidney contain numerous units called nephrons which filter blood, remove urea and convert it into urine. It is collected in the urinary bladder through ureters (kidney ducts), temporarily stored and then periodically eliminated.

The excretory system become closely associated with the reproductive system, particularly in the males. The two systems together represent the urinogenital system.

Excretory System

Nervous systemIt is represented by a brain, a spinal cord, cranial nerves, spinal nerves and the autonomic (sympathetic and parasympathetic) nerves.

Brain and spinal cord together form the central nervous system. Brain consists of a pair of olfactory lobes, a pair of cerebral hemispheres, a pineal body, a pair of optic lobes a cerebellum and a medulla oblongata. The spinal cord extends from medulla oblongata up to the tip of the trunk.

Ten pairs of cranial nerves arise from the brain and innervate the different parts of the body. Ten pairs of spinal nerves arise from the spinal cord and supply to different parts of the body. The autonomic nerves are formed by two strands of nerves one on either side and parallel to the spinal cord.

Reproduction systemIn frog sexes are separate. Male and female frogs can be morphologically distinguished. The male frog have vocal cords which enable them to make a croaking noise to attract the females during the rainy, breeding season.

The male reproductive system consists of a pair of testis that are attached to the kidneys. Each testis opens into the kidneys with the help of numerous fine ducts called vasa efferentia. Sperms produced by the testis are carried through the ureters and hence, in the male, ureters are called urinogenital ducts.

The female reproductive system consists of a pair of ovaries and a pair of oviducts. Ovaries are attached to the kidneys. They carry numerous ova which are released to outside through the oviducts.

In the breeding season, the male and the female frogs enter into a process of temporary union called amplexus. The male holds the female, presses her trunk and thereby forces the female to release the eggs, in a cluster called spawn. This process is called spawning. The male frog releases sperms simultaneously to bring about external fertilization.

Development and metamorphosisDevelopment is indirect. It involves the occurrence of a larva called tadpole. It is a small, fish-like creature which hatches out from the egg after completing the development. The tadpole swims freely in water feeding on planktonic food. Initially the tadpoles have external gills which become replaced by internal gills. Limbs appear gradually and tail shortens. Along with these, a few other significant changes take place, transforming the tadpole into a young adult. These changes together constitute metamorphosis.