THE CAPILLARY CIRCULATION. 343 



repetition, which, combined with the action of the valves, urge the blood 

 from the periphery toward the heart. 



III. A third cause, which is more or less active in promoting the 

 movement of the venous blood, is the force 'of aspiration exerted by 

 the thorax. When the chest expands by the lifting of the ribs and the 

 descent of the diaphragm, this movement tends to diminish the pressure 

 upon its contents, and consequently to draw into the thoracic cavity any 

 fluids which can gain access to it. The expanded cavity is principally 

 filled by the atmospheric air, which passes in through the trachea to fill 

 the bronchial tubes and the pulmonary vesicles. But the blood in the 

 neighboring parts of the venous system is solicited at the same time, 

 though to a less degree, in a similar direction. This force of aspiration, 

 like the respiratory movements themselves, is gentle and uniform in 

 character. Its influence extends indirectly throughout the venous sys- 

 tem, each expansion of the chest causing an increased flow of blood from 

 the extra- to the intra- thoracic veins, while the former are filled up from 

 behind as fast as they are emptied in front. 



Eapidity of the Venous Circulation With regard to the velocity of 

 the venous current, no direct results have been obtained by experiment. 

 Owing to the flaccidity of the veins, and the readiness with which the 

 flow of blood through them is disturbed, it is not possible to determine 

 this point, 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 outward through the arteries returns 

 inward through the veins, the rapidity of its flow in each direction must 

 be in inverse proportion to the capacity of the two systems. The ca- 

 pacity of the entire venous system, when distended by injection, is about 

 twice as great as that of the entire arterial system. During life, how- 

 ever, the venous system is at no time so completely filled with blood as 

 is the case with the arteries; and, making allowance for this difference, 

 it may be estimated that the entire quantity of venous blood is to the 

 entire quantity of arterial blood nearly as three to two. The velocity 

 of the venous blood, as compared with that of the arterial, is therefore 

 as two to three ; and if we regard the average rapidity of the arterial 

 current, according to Yolkmann's experiments, as 30 centimetres per 

 second, this would give the movement of blood in the large veins as 

 about 20 centimetres per second. This calculation, however, is alto- 

 gether an approximative one ; since the venous circulation varies, 

 according to many different circumstances, in different parts of the 

 body. 'It may nevertheless be considered as expressing with sufficient 

 accuracy the general relative velocity of the arterial and venous currents 

 in corresponding parts of their course. 



The Capillary Circulation, 



The capillary bloodvessels are minute inosculating tubes, which per- 

 meate the vascular organs in various directions, and bring the blood into 



