220 



THE CIRCULATORY SYSTEM. 



layer, although they sometimes occur as isolated fibers. The adven- 

 titia shows an inner longitudinal muscular layer, which may be quite 

 prominent and even form the bulk of the muscular tissue in the wall 

 of the vein. Otherwise the adventitia of the veins belonging to this 

 class corresponds in general to that of the arteries of the same, 

 size ; but here also we have, as in the intima, a preponderance of 

 white fibrous connective-tissue elements. 



In the crural, brachial, and subcutaneous veins, the muscula- 

 ture of the media is prominent ; while in the jugular, subclavian, 

 and innominate veins, and in those of the dura and pia mater, the 

 muscular tissue of the media is entirely wanting, and, as a conse- 

 quence, the adventitia with its musculature, if present, is joined 

 directly to the intima. 



In the smaller veins the vascular wall is reduced to an endothe- 

 lial lining, an internal elastic membrane, a media consisting of 

 interrupted circular bands of smooth muscle-fibers (which may be 

 absent), and an adventitia containing a few muscle-fibers. The 

 precapillary veins, which possess in general thinner walls than the 

 corresponding arteries, present a greatly reduced intima and ad- 

 ventitia, while the media has completely disappeared. 



Intima. 

 - Media. 



Adventitia xvith 

 nonstriated 

 muscle-cells 

 in cross-sec- 

 tions. 



Fig. 175. Section of small vein (human); X 6 4O- 



The valves of the veins are reduplications of the intima and 

 vary slightly in structure at their two surfaces. The inner surface 

 next to the blood current is covered by elongated endothelial cells, 

 while the outer surface possesses an endothelial lining composed of 

 much shorter cellular elements. The greater part of the valvular 

 structure consists of white fibrous connective-tissue and elastic fibers. 

 Flattened and circularly arranged muscle-cells are met with at the 

 inner surface of many of the larger valves. The elastic fibers are 

 more numerous beneath the endothelium on the inner surface of the 

 valves (Ranvier, 89). 



(c) The Capillaries. The capillaries consist solely of a layer of 

 endothelial cells, accompanied here and there by a very delicate struc- 

 tureless membrane, and rarely by stellate connective-tissue cells. The 

 connective tissue in the immediate neighborhood of the capillaries 

 is modified to such an extent that its elements, especially those of a 

 cellular nature, seem to be arranged in a direction parallel with the 

 long axis of the capillaries. When examined in suitable prepara- 



