3 oo TEXT-BOOK OF PHYSIOLOGY. 



These properties come into play and are of value in furthering the 

 movement of the blood toward the heart, especially after a temporary 

 obstruction. 



Veins are distinguished by the presence of valves throughout their 

 course. These are arranged in pairs and formed by a reduplication 

 of the internal coat, strengthened by fibrous tissue. They are 

 always directed toward the heart and in close relation to the walls 

 of the veins, so long as the blood is flowing forward (Fig. 137). An 

 obstruction to the flow causes the valves to turn backward until they 

 meet in the middle line, when they act as a barrier to regurgitation. 

 Under these circumstances the elastic tissue permits the veins to 

 distend and accommodate the blood. With the removal of the 

 obstruction the recoil of the elastic tissue, and perhaps the contrac- 

 tion of the muscle-tissue, forces the blood quickly onward. 



The Stream-bed. The stream-bed, the path along which the 

 blood flows, varies widely in its total sectional area in different parts 

 of its course, being greatest in the capillaries, least in the aorta and 



Capillaries. 



Arteries. 



Veins. 



FIG. 138. SCHEME or THE CIRCULATORY APPARATUS. 



venae cavae. In passing from the base of the aorta toward the capil- 

 laries the sectional area of individual arteries, in consequence of 

 repeated branching, diminishes, though their total sectional area 

 increases and in direct proportion to their distance from the heart. 

 In the capillary system the sectional area of an individual capillary at- 

 tains its minimal value, though the total sectional area attains its maxi- 

 mal value. Comparing one with the other, it has been estimated that 

 the total sectional area of the aortic bed is to the total sectional area 

 of the capillary bed as i is to 600 or 800. In passing from the capil- 

 lary into the venous system the sectional area of individual veins in- 

 creases, though the total sectional area decreases and in direct pro- 

 portion to their distance from the capillaries. 



The stream-bed in the aorta is relatively narrow, but widens 

 gradually as it approaches the capillaries, where it attains its maxi- 

 mum width; it again narrows gradually as it passes into the veins, 

 until in the venae cavae it becomes almost as narrow as in the aorta. As 

 the combined sectional areas of the venae cavae are greater than the sec- 



