Chap, x.] THE HEART AND BLOOD-VESSELS. 



79 



especially in the part next to the media; they 

 form networks, and have chiefly a longitudinal 

 direction. 



The larger the artery the more insignificant is the 

 adventitia as compared with the thickness of media. 



In microscopic arteries (Fig. 47), the 

 adventitia is represented by thin bundles 

 of fibrous connective tissue and branched 

 connective tissue cells. 



Large and middle-sized arteries pos- 

 sess their own system of blood-vessels 

 (vasa vasorum), situated chiefly in 

 the adventitia and media ; lymphatic 

 vessels and lymphatic clefts are also 

 present in these coats. 



102. (c) The veins differ from the 

 arteries in the greater thinness of their 

 wall. The intima and media are similar 

 to those of arteries, only thinner, both 

 absolutely and relatively. The media 

 contains in most veins circularly ar- 

 ranged muscular fibres ; they form a con- 

 tinuous layer, as in the arteries, and 

 there is between them generally more 

 fibrous connective tissue than elastic. 

 The adventitia is usually the thickest coat, 

 and it consists chiefly of fibrous connec- 

 tive tissue (Fig. 46). The smallest veins i.e., before 

 passing into the capillaries are composed of a lining 

 endothelium, and outside this are delicate bundles of 

 connective tissue forming an adventitia. The valves 

 of the veins are folds, consisting of the endothelium 

 lining the surface, of the whole intima, and of part 

 of the muscular media. 



103. There are many veins that have no muscular 

 fibres at all, e.g., vena jugularis interna and externa 

 the vena subclavia, the veins of the bones and 



ig. 47. 



Microi 



Artery. 



Endothelium; t, 

 intima ; m, muscu- 

 lar media, com- 

 ingle 

 ircular- 

 nged non- 

 striped muscular 

 cells ; a, adventitia. 

 (Atlas.) 



IHT iiieaia, (. 

 posed of a si 

 layer of circt 

 ly-arranged i 



