G68 



CIRCULATION. 



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. Magendie 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. He 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 Hales 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- 

 vessels 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 foetuses, in 

 all of which the proper muscular heart seems 

 to be wanting; for although the distribution of 

 the vessels in these foetuses 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 foetuses 

 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 foetus; and even were 



