IGO 



THE ORGANS 



occurs not only as fibrils but also as circularly disposed plates or 

 fenestrated membranes. 



3. The ADVENTiTiA (Fig. 95, C) is composed of loose fibrous 

 connective tissue with some elastic fibres. Occasionally there are 

 scattered smooth muscle cells. Both smooth muscle cells and 

 elastic fibres are arranged longitudinally. The adventitia does not 

 form a definitely outlined coat like the media or intima, but blends 



f 



i. 





I "I (J 



/-^ 







*-- ., 



t. ^ 



'A 



■J 



S^is >" 



v 





3 







Fig. 96. — From Transverse Section of Dog's Aorta. X60. (Technic 4, p. 164.) 

 Intima; Z;, media; c, adventitia; d, vasa vasorum; e, elastic tissue;/, endothelium. 



externally with the tissues surrounding the artery and serves to 

 attach the artery to these tissues. In some of the larger arteries 

 the elastic tissue of the adventitia forms an especially well-defined 

 layer at the outer margin of the media. This is known as the 

 membrana elastica externa. In general, it may be said that the 

 thickness of the adventitia and the amount of elastic tissue present 

 are directly proportionate to the size of the artery. 



Large arteries like the aorta (Fig. 96) have the same three coats 

 as small and medium-sized arteries. The layers arc not, however, so 

 distinct. Tin's is due mainh' to the excessive amount of elastic 



