THE CIRCULATION. 297 



the action of the valves, urge the blood from the periphery toward the 

 heart. 



III. A third cause, contributing to the movement of the venous 

 blood, is the force of aspiration exerted by the thorax. The expan- 

 sion of the chest in inspiration diminishes the pressure upon its con- 

 tents, and consequently tends to draw into the thorax any fluids which 

 can gain access to it. The expanded cavity is principally filled by the 

 entrance of atmospheric air through the trachea and bronchi. But the 

 blood in the neighboring veins is solicited at the same time in a simi- 

 lar direction. The influence of this force extends indirectly through- 

 out the venous system, each expansion of the chest diminishing the 

 resistance at the centre of the circulation, and thus causing an in- 

 creased flow toward the intra-thoracic veins, while the remainder are 

 filled from behind as they are emptied in front. 



Rapidity of the Venous Current. With regard to the rapidity of 

 the venous current, no results have been obtained by direct experi- 

 ment. 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 for the arteries. But 

 a calculation has been made, based on the comparative capacity of the 

 arterial and venous systems. As the blood which passes outward 

 through the arteries returns through the veins, the rapidity of its 

 flow in each direction must be in inverse ratio to the capacity of the 

 vessels. The entire venous system, when distended by injection, con- 

 tains about twice as much fluid as the arteries. During life, however, 

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

 arteries ; and, allowing 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 "blood in the veins, 

 as compared with that in the arteries, 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 veins as about 20 centimetres per second. 

 This estimate, however, is only approximative ; since the venous circu- 

 lation varies, according to many circumstances, in different parts of the 

 body. It may nevertheless be considered as expressing with sufficient 

 accuracy the general relative velo.city of the arterial and venous currents 

 in corresponding parts of their course. 



The Capillary Circulation. 



The capillary blood-vessels are minute inosculating tubes, which per- 

 meate the vascular organs, and bring the blood into close proximity 

 with their tissues. They are continuous, on the one hand, with the 

 terminal ramifications of the arteries, and, on the other, with the com- 

 mencing rootlets of the veins. They vary somewhat in size in differ- 

 ent tissues, their average diameter in man being about 10 mmm., or 

 j-J-Q of a millimetre. According to Kolliker, the largest capillaries are 



