290 MANUAL OF PHYSIOLOGY. 



extremities, in which the blood current has to move against 

 the force of gravity. 



The general aggregate diameter of the different parts of the 

 vascular system varies greatly. The combined calibre of the 

 branches of an artery exceeds that of the parent trunk, so that 

 the aggregate sectional area of the arterial tree increases as one 

 proceeds from the aorta toward the capillaries. After the mus- 

 cular arterioles are passed the general diameter of the vascular 

 system suddenly increases enormously, and in the capillaries it 

 reaches its maximum, which is said to be about eight hundred 

 times as great as the diameter of the aorta. 



The aggregate sectional area of the veins also diminishes 

 as the tributaries unite to form main trunks, and reaches 

 its minimum at the entrance of the vena cava into the right 

 auricle. 



The capacity of the veins is, however, everywhere much greater 

 than that of the corresponding arteries, the least difference being 

 near the heart, where, however, the calibre of the veins is more 

 than twice that of the aorta. 



After this brief anatomical sketch, the most important proper- 

 ties of each different part of the vascular system may be sum- 

 marized thus : 



1. The structure of the walls of the large arteries shows 



them to be capable of sustaining considerable pressure, 

 and of exerting powerful elastic recoil. 



2. In the small arteries, as well as this elasticity, great varia- 



bility in their calibre, dependent on the contraction of 

 their muscular coat, occurs. 



3. In the capillaries we find extreme thinness, elasticity, and 



permeability of their wall, which presents an enormous 

 surface, so as to allow free interchange between the 

 blood and the surrounding textures. 



4. The veins have yielding and distensible coats, great gen- 



eral capacity to accommodate a large quantity of blood, 

 and valves to prevent its backward flow upon the 

 capillaries. 



