RELATIVE CAPACITY OF THE VESSELS. 



289 



muscle fibre, while the structure of the middle coat of the small 

 veins can only be distinguished from that of the arterioles by the 

 comparative sparseness of the muscle cells running around the 

 tubes. 



The veins are capable of considerable distention, but, though 

 possessed of a certain degree of elasticity, they are much inferior 

 to the arteries in resiliency. 



In a large proportion of veins, valve-like folds of their lining 



Fio. 128. 



Diagram intended to O ive an idea ot' thj aggregate sectional area of the different parts 

 of the vascular system. A. Aorta, c. Capillaries, v. Veins. The transverse measure- 

 ment of the shaded part may be taken as the width of the various kinds of vessels, sup- 

 posing them fused together. 



coat exist which prevent the backward flow of blood to the 

 capillaries, and insure its passage toward the heart. These 

 valves resemble in their general plan the pocket valves that 

 protect the great arterial orifices of the heart. They vary 

 much in arrangement, there being commonly but two or some- 

 times one flap or pocket entering into the formation of the 

 valve. They are most closely set in the long veins of the 

 25 



