76 PHYSIOLOGY. 



consequently driven from it by the superior attraction then possessed 

 by the tissue for another portion of the fluid containing the required 

 ingredients ; this in its turn is rejected for a fresh supply. 



But the fluid, thus repelled from one part, may still be attracted 

 towards another ; because that portion of its contents which the latter 

 requires may not yet have been abstracted ; and thus the flow of the 

 blood is maintained through the capillary network until it is alto- 

 gether exhausted of its nutritive matter. 



Although the circulation is not dependent upon nervous power, its 

 influence is nevertheless manifest, as is occasionally seen in the 

 functions of nutrition and secretion, by the control it exerts over the 

 diameter of the blood-vessels. The phenomena of blushing, pallor, 

 and the erection of certain tissues, are also examples of a modified 

 condition of the blood-vessels through the agency of this system. 



The venovs system arises in the small trunks formed by the union 

 of the capillaries ; and it returns the blood to the heart. These ves- 

 sels are possessed of the same number of coats and the same-proper- 

 ties as the arteries, only not so strongly developed. The internal 

 membrane is thrown into valves or folds to prevent the reflux of the 

 blood. The capacity of the venous system is said to be consider- 

 ably greater than the arterial, holding two or three times as much 

 blood as the latter. 



The venous circulation is mainly due to vis a tergo ; it is assisted, 

 however, by a vis a fronte, in the suction pcnver of the heart. The 

 inspiration of xenons blood is also said to assist it. By this is meant 

 the rush of blood towards the chest, in order to supply the vacuum 

 created there by the descent of the diaphragm. That it has some 

 influence is seen in the partial emptying of the veins in inspiration, 

 and their turgidity in expiration. But that it can have no general 

 influence over the venous circulation, will perhaps be seen from the 

 fact that the pulmonary circulation, being entirely within the chest, 

 cannot be affected by atmospheric pressure, and also that the circu- 

 lation of the fcEtus in utero cannot be affected by any such agency. 



Muscular movements are among the most important adjuvants to 

 the venous circulation, every contraction being accompanied by a 

 pressure upon the veins of the part ; and as the blood is prevented 

 by the valves from flowing back into the small vessels, it must be 

 driven on towards the heart. This is familiarly seen in bloodletting, 

 every movement of the hand increasing the flow of blood from the 

 orifice. 



A few of the peculiarities of the circulation may be mentioned in 

 conclusion. These are first, in the pulmonary circulation^ in which 

 venous blood is sent from the heart, and through a tube which is arte- 

 rial in its structure ; whilst arterial blood is delivered to the heart 

 through several venous trunks. 



The portal circulation is also peculiar in its arrangements, the 



