In the first place, Haller, Spallanzani, Thomson, and many others have observed in the transparent parts of animals that the im pulse of the heart is transmitted to the very ends of the small arteries, which may be less than th part of an inch in diameter, and that in some states of the circulation the impulse of the heart is continued on through the capillary vessels and into the commencements of the veins. The fact that this generally occurs when the action of the heart is weakened, and when the vessels are consequently not sufficiently distended by its impulse to react by their elasticity and convert the remitting into a uniform force, is a distinct proof that in the natural state of the circulation a greater pro portion of the force of the heart must be trans mitted through the blood to the capillaries, and must act through them upon the column of blood returning in the veins.
From the same experiments it has appeared that in general the instant any obstruction pre vents the action of the heart from being pro pagated onwards in the arteries, the progressive current of the blood in the small vessels be comes slower and soon ceases, any motion which goes on afterwards being quite of a dif ferent kind from that occurring in the natural circulation.
An experiment performed by M. Magendie, and formerly referred to, also affords a very satisfactory proof that the heart's force acts in propelling the blood through the whole vascular system. M. IVIagendie dissected the femoral artery and vein separate from the neighbouring parts, and passing a ligature under them tight ened it round the whole limb, excepting the two principal bloodvessels, through which the blood was allowed to flow freely. Ile was thus enabled to shew that the flow of blood from an orifice in the vein was immediately dependent on the force of the heart acting through the artery, as it was suddenly diminished and soon completely ceased the instant that the latter vessel was obstructed, and became more or less rapid according as it was more or less com pressed. We would further remark that the experiments of Bales and Poiseuille, more par ticularly the latter, have shewn that there is little if any difference in the force of the blood in arteries of very different size.
On the other hand, it appears to us suffi ciently clear that the occurrence of any general contraction of the coats of the arteries would have the effect of opposing an obstacle to rather than of assisting the progress of the blood in the arteries just in proportion to the degree of the force of? the heart, which would necessarily be expended in dilating them to the required size, in order to allow of the free transmission of the blood by them; and as, according to the commonly received opinion, the contractile powers are greater in the smaller than in the larger arteries, the operation of this contraction would be much the same as the diminution of the aperture through which blood flows from an inorganic tube, and would thus cause a still greater obstruction to the flow of blood than a general contraction. It is only on the supposition that the arteries undergo an undu latory or vermicular contraction, proceeding from the larger to the smaller branches, that this contractile force can be believed to con tribute to the progressive motion of the blood, because then it might be conceived to assist the elasticity of the arterial parietes in propa gating the force of the heart along the column of contained blood, and even augment this force by an additional power. But we would remark that no such vermicular action has been ascertained to occur by any observations or experiments with which we are acquainted ; that in artificial injection of fluids into the large arteries of dead animals a force of a few pounds-is found to be sufficient to propel these fluids, when not of an irritating kind, from the arteries into the veins; and that it follows from the direct experiments of many, more particu larly those of Hales, Poiseuille, and Magendie, that the action of the heart, transmitted by the elastic arteries, is the only cause operating in the progressive propulsion of the blood in arteries of such a size as to admit of the force of the blood being measured in them.
In asserting, however, that a general con traction of this kind, if it occurred in the vas cular system, would upon the whole obstruct or retard rather than assist the progressive motion of the blood in the arteries, we would not be supposed to deny that the vital powers of the arteries may modify very considerably the dis tribution of blood to different parts, for it is manifest that an increased action occurring in one part of an artery may hinder the blood from being transmitted in its usual quantity into a neighbouring part, while a dilated state of an artery or its branches, or, if we-please to call it so, a diminished action or greater weak ness of resistance of the coats of the artery considered relatively to the powers of propul sion operating through it, may occasion the flow of a greater quantity of blood to a part, as occurs in local inflammations. Among the many indirect arguments adduced on both sides of this question may be mentioned the follow ing. In the first place, the fact that in the lowest classes of animals, as in Vermes and Insects, which have no proper heart, the blood yessels propel the blood by their contractile power, and that in some of the higher animals, particularly Reptiles and Fishes, parts of the vascular system, as the bulb of the aorta, a considerable portion of this vessel, parts of the veins, and so on, are distinctly contractile, and assist the powers of the heart, are adduced as proofs from analogy that the arteries in warm blooded animals may have the same power and perform the same function. Now it may be answered to this, that the circumstance of the lowest classes of animals having no proper heart is the final cause of or an obvious reason for the greater contractility of these vessels ; and in the second place, that no rythmic con traction is observed to occur in the arteries of warm-blooded animals of the same nature as that observed by Haller, Spallanzani, M. Hall, and others in the bulb of the aorta and other parts of the vascular system of cold-blooded Vertebrata. For similar reasons we are not inclined to attach much importance to the ar gument in favour of the independent powers of the arteries deduced from the alleged occur rence of circulation in acephalous fcetuses, in all of which the proper muscular heart seems to be wanting; for although the distribution of the vessels in these fcetuses has been suffi ciently accurately determined, the nature of the circulation which occurs in them is a sub ject involved in the greatest obscurity. There seems good reason to doubt that such fcetuses have ever existed alone in the uterus, in which case their vessels may, as is known in many of them to have occurred, have been connected with those of a perfect fcetus; and even were this not the case, the absence of the heart might be attended in these nialformed productions with an unusnal development of muscular power,in parts of the vascular system.* In conclusion, we may remark that the argu ment drawn from the occurrence of circula tion apparently little impaired through arteries which have been completely ossified for a con siderable time, seems to be very much in favour of the viel,v we have taken that the heart alone is the cause of the progressive flow of blood through the arterial tubes.