8 70 PHYSIOLOGY 



causes the flow of blood through the system must decline as we pass from 

 the arterial to the venous side. The chief function of the large arteries is 

 to serve as elastic conduits, whereas the small arteries or arterioles leading 

 from the arteries to the capillaries have in addition the function of regu- 

 lating the amount of blood flowing through the capillary area of the organs 

 which they supply. The veins have the function of conducting blood at 

 a low pressure from capillaries to heart and of storing up any excess of 

 blood which is not immediately taken up by the heart. Corresponding 

 to this difference in function we find variations in the structure of the 

 blood-vessels according to their situation in the circuit. 



The vessels which carry the blood from the heart to the tissues, the 

 arteries, are thick- walled, and contain an abundance of muscular and elastic 

 elements in their walls. The typical medium-sized artery is described as 



FIG. 372. Transverse section of part of the wall of the posterior 



tibial artery ( x 75). 



a, endothelial and sub- endothelial layers of intima ; b, lamina of elastic tissue ; 

 c, media consisting of muscle fibres ; d, adventitia. (SCHAFEB.) 



consisting of three coats (Fig. 372) : an intima lined by a continuous layer 

 of flattened endothelial cells, which rest on a well-marked lamina of yellow 

 elastic tissue ; a media composed of unstriated muscular fibres arranged 

 longitudinally and circularly ; and an external coat or adventitia of fibrous 

 tissue, with a number of longitudinal elastic fibres. Near the heart, in the 

 great vessels such as the aorta and its larger branches, there is a pre- 

 ponderance of elastic tissue as compared to the muscular ; and we find 

 in the media alternate layers of muscle fibres and fenestrated elastic mem- 

 branes. In the smallest arteries, on the other hand, the arterioles, the 

 elastic element entirely disappears, so that the wall consists of muscle 

 fibres, 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 distinguished 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 and arteries is of considerable import- 

 ance for the understanding of the distribution of pressures in the vascular 

 system, since the distensibility and reaction to pressure of these vessels 



