GENERAL FEATURES OF THE CIRCULATION 



915 



artery is described as consisting of three coats (Fig. 383) : 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 longi- 

 tudinal elastic fibres. Near the heart, in the great vessels such as the aorta and its 



FIG. 383. Transverse section of part of the wall of the posterior 

 tibial artery ( x 75). 



a, endothelial and sub-endothelial layers of ultima ; &, lamina of elastic tissue ; 

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



larger branches, there is a preponderance 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 mus- 

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

 unless it is distended by some internal pressure. 



Capacity in c.c. 



90 



mm. Hg. 



FIG. 384. 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 of 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.) 



The histological difference between veins and arteries is of considerable 

 importance for the understanding of the distribution of pressures in the 

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

 vessels are conditioned by their structure. In Fig. 384 is represented the 



