982 



PHYSIOLOGY 



chiefly circular, lined by the endothelium. In the latter vessels a 

 contraction of their walls may result in an entire obliteration of 

 the lumen, so shutting off altogether the supply of blood to the 

 capillaries beyond. In the veins the same three coats can be dis- 

 tinguished as in the typical artery, but the wall of the vessel is much 

 thinner in proportion to the lumen. In the vein moreover there is a 

 preponderance of the fibrous tissue elements, the muscular and 

 elastic tissue being but little marked. On this account the vein 

 collapses unless it is distended by some internal pressure. The 

 histological difference between veins arid arteries is of considerable 

 importance for the understanding of the distribution of pressures 

 in the vascular system, since the distensibility and reaction to pressure 



Capacity in c.c. 



90 



FIG. 369. Curves of distensibility of an artery (thick line) and of a vein (thin line). 

 The figures at the left side of the diagram represent the capacity of a section ( f 

 the vessel when distended under a certain pressure, expressed by the figures 

 on the base line in mm. Hg. (Constructed from figures given by ROY.) 



of these vessels are conditioned by their structure. In Fig. 369 is 

 represented the extensibility, i.e. the increase in capacity of an artery 

 and a vein under gradually increasing internal pressure. It will be 

 seen that an artery which has a certain capacity at zero pressure 

 gradually distends with increasing pressure. The increase in capacity 

 is small at first, and becomes most rapid between 90 and 110 mm. Hg. 

 After this point every increment of pressure brings about a gradually 

 diminishing increment of capacity. Thus a change of internal pressure 

 causes the greatest change in capacity when the pressure in the artery 

 corresponds, as we shall see, to the average arterial pressure in the 

 normal animal. In the vein, on the other hand, the capacity, which is 

 nothing at zero pressure, becomes considerable on raising the pressure 

 to 1 mm. Hg. A further rise of pressure to 10 mm. Hg causes a 



