OF THE CIRCULATION IN GENERAL. 293 



the Liver, in which organ its newly-absorbed materials undergo assimilation, 

 whilst its excrementitious matters are separated by the secreting process. 

 The Vena Portre, which is formed by the convergence of the gastric, intestinal, 

 splenic, and pancreatic veins, subdivides again like an artery, so as to form 

 a capillary plexus which extends through the whole substance of 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 f/ciicrnl circulation, in precisely the same mode as the respiratory circu- 

 lation is grafted upon it in Mollusca and Crustacea; and if the "sinus" of 

 the Vena Porta 1 had possessed contractile muscular walls, it would have 

 ranked as the proper heart of the portal system. The really arterial char- 

 acter of the Vena Portie 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 gen- 

 erally. 



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

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

 dependent 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 capillaries will immediately cease almost entirely to deliver the blood 

 into the veins, and the venous circulation will be consequently 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. 1 But there are cer- 

 tain "residual phenomena" even in Man, which clearly indicate that this is 

 not the whole truth ; for not only is the general current of blood greatly 

 modified in its passage through the circulating system, but there are many 

 variations in its movement, which, being very limited in their extent, cannot 

 be attributed to" any central disturbance, and must therefore be dependent 

 on causes purely local. Hence we are led to perceive that forces existing in 

 the Bloodvessels themselves must have a considerable influence, in producing 

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

 There are also indications of the existence of influences in which the blood- 

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

 blood and the tissues, that constitute the processes of Nutrition, Secretion, 

 etc. Of the nature of these influences, and of the degree 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. Thus we find that in Plants and the low- 

 est animals, as in the earliest embryonic state of the highest, a movement 

 of nutritious fluid takes place through a system of minute passages or chan- 

 nels excavated in the tissues (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 peri- 

 staltic movement, 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 reg- 

 ular movements of contraction and dilatation ; and this tendency to concen- 



1 See Dr. Williams's Principles of Medicine, 2d edit., p. 185, note. 



