THE BLOOD-VESSELS. 



155 



They are scattered irregularly throughout the middle coat. 

 Sometimes the intima and the adventitia also contain sparsely 

 distributed muscle-cells. The arterial muscular coat is dis- 

 tinctly separated from the intima by the interposition of the 

 internal elastic coat. Externally a sharp boundary is formed 

 either by the adventitia or by the external elastic coat. The 

 latter appears as a separate membrane in arteries of small and 

 medium size. There are, however, exceptions to this rule. The 

 external elastic coat consists of a close network of delicate 



FIG. 69. Longitudinal section of pulmonary artery. Mounted in glycerine and acetic acid after de- 

 siccation of the artery, a. Internal portion of intima ; 6, external portion of intima ; c, internal elastic 

 coat ; d, media, showing cross sections of muscle fibres and elastic tissue ; , adventitia. 



elastic fibrils, anastomosing with similar adventitial reticula. 

 The adventitia is composed of interlacing bundles of connec- 

 tive tissue, commingled with elastic lamellae of varying thick- 

 ness. 



The veins. From their origin in the capillaries to the point 

 where they enter the trunk proper, the veins preserve through- 

 out a uniform type of structure. But no sooner have they 

 penetrated into the visceral cavities of the body than we find 

 them undergoing considerable alterations, which may either 

 increase or diminish the complexity of their structure (Ran- 

 vier). The veins are far more numerous than the arteries. 

 They are also, as a rule, wider and more dilatable, and have 

 thinner coats. It is owing to the latter peculiarity that the 



