RELATIVE CAPACITY OF THE VESSELS. 283 



fering but little from those of the arteries, The middle coat, how- 

 ever, in the large veins is distinguished from that of the large 

 arteries by being much thinner, owing to the paucity of yellow 

 elastic tissue. It is also characterized by its relative richness in 

 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. 



FIG. 128. 



Diagram intended to give an idea of the aggregate sectional area of the 

 different parts of the vascular system. A. Aorta c. Capillaries, v. Veins. 

 The transverse measurement of the shaded part may be taken as the width 

 of the various kinds of vessels supposing them fused together. 



The veins are capable of considerable distension, 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 

 coat exist which prevent the backward flow of blood to the ca- 

 pillaries, and insure its passage towards the heart. These valves 

 resemble in their general plan the pocket valves that protect the 



