376 OF THE CIRCULATION OF BLOOD. 



499. Even in Vertebrated animals, we find indications of the same defi- 

 ciency of central power, over the peripheral circulation. When we look at 

 the simple, thin-walled heart of Fishes, for example, it seems impossible that 

 it should have rnuch power over the current of blood flowing back to it by the 

 veins ; for of this blood, a considerable portion has to pass through three sets 

 of capillaries, between its ejection from the heart and its return to it. It is 

 first transmitted through the respiratory capillaries, for the purpose of aera- 

 tion ; the confluent vessels, which collect the arterial blood from these, termi- 

 nate in the general systemic trunk or Aorta, in which, as in the veins of Man, 

 there is an absence of pulsation, and by these it is distributed to the systemic 

 capillaries ; and the blood which, after passing through these, returns from 

 the posterior part of the body, and from the viscera, passes through ariother 

 set of capillaries, those of the liver and kidneys, before it returns to the heart. 

 Even in the warm-blooded Vertebrata, in which the respiratory circulation is 

 separately performed, the blood, which is returned from the intestines, passes 

 into a trunk, the Vena Portse, which again subdivides into capillary ramifi- 

 cations, being transmitted over the plexus of biliary ducts, of which the liver 

 is chiefly composed ; and thus the Vena Portse, as Hunter justly observed, 

 should be considered rather in the light of an artery,* resembling as it does 

 the aorta of Fishes. Considering the small amount of pressure which is 

 exerted by the blood, upon the sides of the vessels that are formed by the 

 reunion of capillaries, it seems impossible to imagine that the vis a tergo, 

 derived from the impulsive action of the Heart, can be alone sufficient to 

 maintain the portal circulation. 



500. We have next to consider the influence of the Arterial tubes, on the 

 flow of blood through them. This influence is exerted by the middle or fibrous 

 coat, which alone is possessed of contractile properties. We find in this coat, 

 a layer of annular fibres, possessing no small resemblance to that of which the 

 muscular coat of the alimentary canal is composed. On the outside of this, 

 is a layer of yellow elastic tissue, which is much thicker in the larger arteries, 

 in proportion to their size, than in the smaller. To this last tissue is due the 

 simple elasticity of the arterial walls, which is a physical property that per- 

 sists after death, until a serious change takes place in their composition : 

 whilst to the one first mentioned, we are to attribute the property which they 

 unquestionably possess in common with proper muscular tissue, of con- 

 tracting on the application of a stimulus, so long as their vitality remains. 

 These two endowments exist, in various proportional degrees, in the different 

 parts of the Arterial system. Thus it was justly remarked by Hunter, that 

 elasticity, being the property by which the interrupted force of the Heart is 

 made equable and continuous, is most seen in the large vessels more imme- 

 diately connected with that organ. On the other hand, the contractility is 

 most observable in the smaller vessels, where it is more required for regulating 

 the flow of blood towards particular organs. 



501. It is easily shown that the action of the Elasticity of the Arterial tubes 

 is one of a purely physical character ; and that its purpose is to convert the 

 intermitting impulses, which the fluid receives from the heart, into a continu- 

 ous current. The former are very evident in the larger trunks; but they 

 diminish with the subdivision of these, until they entirely disappear in the 

 capillaries, in which the stream is usually equable or nearly so. We may 

 imagine a powerful forcing-pump injecting water, by successive strokes, into 

 a system of tubes with unyielding walls ; the flow of fluid at the farther 



* That it conveys venous blood, is no reason to the contrary; since this is the case 

 with the pulmonary artery. The character of an artery is derived from the division of 

 its current into several diverging streams. 



