294 THE CIRCULATION. 



the pressure of the muscles, may readily find a passage through 

 others, which communicate by cross branches with the first. (Figs. 

 96 and 97.) 



3. The force of the capillary circulation. This last cause of the 

 motion of the blood through the veins is the most important of all, 

 as it is the only one which is constantly and universally active. In 

 fish, for example, respiration is performed altogether by gills ; and 

 in reptiles the air is forced down into the lungs by a kind of deglu- 

 tition, instead of being drawn in by the expansion of the chest. In 

 neither of these classes, therefore, can the movements of respiration 

 assist mechanically in the circulation of the blood. In the splanch- 

 nic cavities, again, of all the vertebrate animals, the veins coming 

 from the internal organs, as, for example, the cerebral, pulmonary, 

 portal, hepatic, and renal veins, are unprovided with valves; and 

 the passage of the blood through them cannot therefore be effected 

 by any lateral pressure. The circulation, however, constantly going 

 on in the capillaries, everywhere tends to crowd the radicles of the 

 veins with blood ; and this vis a tergo, or pressure from behind, fills 

 the whole venous system by a constant and steady accumulation. 

 So long, therefore, as the veins are relieved of blood at their cardiac 

 extremity by the regular pulsations of the heart, there is no back- 

 ward pressure to oppose the impulse derived from the capillary cir- 

 culation ; and the movement of the blood through the veins continues 

 in a steady and uniform course. 



With regard to the rapidity of the venous circulation, no direct 

 results have been obtained by experiment. Owing to the flaccidity 

 of the venous parietes, and the readiness with which the flow of 

 blood through them is disturbed, it is not possible to determine this 

 point for the veins, in the same manner as it has been determined 

 for the arteries. The only calculation which has been made in this 

 respect is based upon a comparison of the total capacity of the 

 arterial and venous systems. As the same blood which passes out- 

 ward through the arteries, passes inward again through the veins, 

 the rapidity of its flow in each must be in inverse proportion to the 

 capacity of the two sets of vessels. That is to say, a quantity of 

 blood which would pass in a given time, with a velocity of x, 

 through an opening equal to one square inch, would pass during 

 the same time through an opening equal to two square inches, with 

 a velocity of J ; and would require, on the other hand, a velocity 

 of 2 x, to pass in the same time through an opening equal to one- 

 half a square inch. Now the capacity of the entire venous system, 



