THE CIRCULATION IN GENERAL. 467 



the liver; and the Hepatic vein, collecting the blood from this plexus, conveys 

 it into the Vena Cava. Thus the portal circulation is grafted (so to speak) upon 

 the general circulation, in precisely the same mode as the respiratory circulation 

 is grafted upon it in Mollusca and Crustacea ; and if the* il sinus" of the vena 

 portse had possessed contractile muscular walls, it would have ranked as the 

 proper heart of the portal system. The really arterial character of the Vena 

 portae is well shown by comparing it with the Aorta of Fishes ; which is formed 

 by the convergence of the branchial veins, and then distributes the blood which 

 it has received from them to the body generally. 1 



493. That the movement of the Blood through the arterial trunks and the 

 capillary tubes, is, in Man, and in other warm-blooded animals, chiefly depend- 

 ent upon the action of the Heart, there can be no doubt whatever. It can be 

 easily shown by experiment, that if the arterial current be checked, the capil- 

 laries will immediately cease almost entirely to deliver the blood into the veins, 

 and the venous circulation will be instantaneously arrested. And it has also 

 been proved, that the usual force of the Heart is sufficient to propel the blood, 

 not only through the arterial tubes, but through the capillaries, into the veins; 

 since even a less force will serve to propel warm water through the vessels of an 

 animal recently dead. 3 But there are certain " residual phenomena" even in 

 Man, which clearly indicate that this is not the whole truth; and that forces 

 existing in the Bloodvessels themselves have a considerable influence, in pro- 

 ducing both local and general modifications of the effects of the Heart's action. 

 There are also indications of the existence of an influence in which the blood- 

 vessels do not partake, arising from those changes occurring between the blood 

 and the tissues, that constitute the process of Nutrition, Secretion, &c. Such, 

 for instance, would appear to be the interpretation of the fact, that whilst any 

 variations in the action of the Heart affect the whole system alike, there are 

 many variations in the Circulation, which, being very limited in their extent, 

 cannot be attributed to such central disturbances, and must therefore be depend- 

 ent on causes purely local. Of the nature of these influences, and of the mode 

 of their operation, the most correct idea may be obtained by examining the 

 phenomena of the Circulation in those beings, in which the moving power is 

 less concentrated than it is in the higher Animals; for we find that in Plants 

 and the lowest Animals, as in the earliest embryonic state of the highest, a 

 movement of nutritious fluid takes place through a system of minute passages 

 or channels excavated in the tissue (representing a capillary plexus), without 

 any vis a tergo derived from an impelling organ. Ascending a little higher in 

 the series, we meet with a system of vascular trunks, distributing the blood to 

 these plexuses, and collecting it again from them ; and the walls of these trunks 

 are so far endowed with contractility, as to assist, by a sort of peristaltic move- 

 ment, in the maintenance of the current through them. Still passing upwards, 

 we find this contractility manifesting itself especially in some limited portion or 

 portions of the vascular system, which execute regular movements of contrac- 

 tion and dilatation; and this tendency to concentration is observed to increase, 

 until the whole movement is subordinated to the action of a principal propelling 

 organ, the Heart. 3 We shall now examine what agency in the Human Circu- 

 lation may be attributed to the Heart, the Arteries, and the Veins respectively ; 

 and what other forces may be fairly presumed to operate in the Capillary circu- 

 lation. 



1 For an account of the principal modifications of tjie Circulating apparatus in the 

 Animal Kingdom, see "Princ. of Phys., Gen. and Comp.," pp. 677-710, Am. Ed. 



2 See Dr. Williams's "Principles of Medicine," 3d Am. Ed., p. 154, note. 



3 See the Author's " Princ. of Phys., Gen. and Comp.," CHAP, xn., Am. Ed. 



