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upon it is admitted, it does not follow that it is exerted ina manner similar to that of the heart. Still, I think, if theyare admitted as assistants, they can only be so by acting inaccordance with the efforts made by the organ itself.

If the heart is made to exert its force ù tergo upon the venousblood, the lateral pressure of the muscles, when in action, mustfall at right angles upon it, a combination of pressure which,if synchronous, must, I apprehend, be rather antagonistic.Were there no valves, it might be required to prevent regurgi-tation.Then with regard to these valves, which are so plentiful in

the veins, and absent in the arteries,-has their function hithertoDeen properly understood ? That they are placed there to assistthe heart in the circulation of the venous blood, no one candoubt; but it is questionable whether they can render muchassistance to that organ, if we confine its moving power to avis a tergo. The veins are extremely dilatable, and would, inmy opinion, readily expand under a force, sufficiently great forits object, so applied. The effect of that expansion wouldcause such a separation of their coats as to render the valvularapparatus inoperative, if not impedimental ; for, being toowidely separated to prevent regurgitation or to support theascending column of blood, their presence in the midst of itmust be obstructive to its onward movement, rather thanotherwise; yet, arguing on this theory, physiologists assertthat the uses of the valves are, like the tongue of a windlass,.to secure the ground gained by each pulsation of the heart, byopposing the reflnent propensity of the blood. But if this beso, how comes it that we find them in the jugular veins, whereno regurgitation from gravity is to be apprehended ? Yetthere they are, and placed in relation to the heart in a pre-cisely similar manner to those in the ascending vessels. That

they are there for the sake of uniformity cannot be admitted;and yet, if we accept the theory of a vis a tergo, it is difficultto explain their utility. But if the theory of lateral or externalpressure is admitted, and that the blood receives its impetusfrom without instead of from within, all these objections dis-appear ; for the action of external pressure would be, to bringthe sides of the vessel closer together, and by so doing to ob-tain the complete approximation of the valves-thus giving tothe blood an onward, whilst it secures it against a refluent, ’Icourse. If this be so, if it is to this external pressure that thecurrent in the veins is referable, the presence of the valves inthe jugulars then becomes as necessary, as points of resistanceto reflux action, as in those vessels that convey the blood fromthe lower extremities.

In my former letter I laid particular stress upon the lateralexpansion of the arteries, believing it to be a mode by whichthe heart exercises its influence on the circulation of the venousblood; but in so doing I did not mean to ignore the co-opera-tion of other agents. My object was to place it, if not in oppo-sition, side by side with the accepted theory of a vis a tergoexercised by that organ through the capillaries. But when weexamine the mode by which those agents produce their effects,we find that it is precisely similar to that which I have givento the heart-viz., by a force applied externally to the venouscoats. It is in this way, I take it, that respiration acts uponit both above and below the diaphragm. Nor are the veins bytheir structure suited to any or much other application of force.To have made them subject to that which operates in thearteries, their coats, to make that force effective, would haverequired a greater thickness and more elasticity to protectthem against dilatation ; for dilatation without resiliency isdestructive to progress depending on a vis à tergo. There wouldhave been a greater structural approximation to the arterialtube had they been intended to respond to similar influences.The circumstance also of the artery remaining patulous afterdivision, whilst the vein so divided collapses, is suggestive of adifference in the application of the moving power; the resist-ance of the one being in its coats, whilst the other resists byits contents. The constant emptiness, too, of the one and thefulness of the other, after death, point, in my opinion, to asimilar conclusion. The last contraction of the heart was suffi-cient to empty the arterial tubes; it had forced the blood

through the capillaries to the veins, but no further. Thereanother power was required, but that had ceased to exist-viz., the lateral pressure.

I know I shall be told that a line must be drawn somewhere- that the blood must have some point of arrestation ; but ifboth systems of vessels depended for the performance of theirfunctions on a similar application of the same power, whyshould the one have so completely discharged its obligations,whilst the other remains gorged beyond its living condition ?No satisfactorv exnlanation has. T think. bpfn hithertn vPn

of this difference of condition; but if you allow that the heartacts upon the venous circulation-not so much by a direct asby a lateral application of its power-I think it will go someway to clear up the mystery. For to exert that influence, itis necessary that that power should be most complete, which itcannot be unless the heart is in the full vigour of life. Whenthat declines, it is natural that the borrowed power (for so, inone sense, that given to lateral pressure is) should be the firstto cease.The heart, some time before dissolution sets in, begins to

lose its contractile force ; its strokes are more frequent andfeeble. The dilatation of the arteries is, consequently, muchlessened, and the pressure of and on the surrounding structuresis gone. The blood then accumulates in those vessels that owe,in my opinion, their circulation to this pressure, till, at last, thequantity passing from them to the heart is so small, that itslast systolic action takes place upon an almost empty chamber.The quantity of blood, thus reduced, finds no impediment inthe arterial channels; the slightest impulse suffices to carry itthrough these patulous tubes to the capillaries, and thence tothe veins, that are now but passive recipients.

Valentin says, when speaking of the circulation in the veins," In the smallest arterial branches and capillaries, the pressureexerted by the left ventricle of the heart is for the most partlost. There remains but a proportionally small residue, whichtends to impel the venous blood from the capillaries towardsthe heart, or in the central direction, and which is called thevis A tergo of the venous circulation." Speaking of the veinsthemselves, he says, "Their great extensibility is therefore in-timately connected with their small elasticity." Still, he says,"it cannot be doubted that the walls of the veins possess acertain degree of contractility ; yet no vermicular movementsoccur during life, nor is there any rapid, instantaneous, orrhythmical alteration of their diameter." Well then, accordingto this, the vis à tergo scarcely reaches the veins, and conse-quently must be very inoperative, as a motive power, on theircontents. The great extensibility and small elasticity given tothe veins themselves by this physiologist seem to prove theiragency in this matter to be of the smallest, and their contrac-tility cannot be allowed much efficacy; for whilst he says itcannot be doubted, it gives us no perceptible evidence of itspresence; yet, if we take this away, to what other agency canwe refer the venous circulation ? There is none that I knowof; hence these suggestions.

T am Sir mire: nharlienllc

J. NICHOLS, F.R.C.S.Savile-row, November, 1859.

THE QUARANTINE LAWS.To the Editor of THE LANCET.

SiR,—It appears, by THE LANCET of Oct. 29th, p. 447, thata committee, or commission, or something of the sort, has beenconstituted, or has constituted itself, for the purpose of in-quiring into the subject of Quarantine. I, a short time ago,saw, somewhere, the names of the gentlemen constituting thisbody, and I was struck with the number of them who havean interest in the continuance of quarantine regulations. Istrongly advise you to keep a sharp look out on their proceed-ings. Seeing the wideness apart of their respective homes, Ishould be glad to learn where their common habitation, orusual place of meeting, is situated; and, in fact, I should liketo have another look at their names, if I only knew where tofind the list.One statement is made which much astonishes me-viz.,

Application was likewise made to the medical officers of theRoyal Mail Packet Steamers which leave Southampton for theMediterranean, the West Indies, and South America; but asyet no replies had been received." If such application hadreally been made, it is very strange that I should have heardnothing of it. Now, these very gentlemen are precisely thosewho are in a position to give the most valuable information;and it would have been but a befitting act of courtesy, not tosay of justice, to invite one or more of them to join the com-mittee, or whatever it may be. By this omission, they laythemselves open to the suspicion that the views entertainedby those who have had an ample opportunity of seeing andjudging might not be acceptable to those " Gentlemen of Eng·land who sit at home at ease."

I am, Sir, your obedient servant,November, 1859. VIATOR.P. S.-I enclose my card for your satisfaction ; but will not

fail to come forward if wanted.