BLOOD- VESSELS. ,123 



defined fibrillated and of elastic fibres, among which 

 lie large, flattened branching cells. This layer is 

 called the intermediary layer of the intima, and is 

 sharply separated from the media by a fenestrated, 

 elastic membrane, called the membrana elastica in- 

 timce, which in contracted vessels is usually folded. 

 The media presents here quite a thick layer of 

 smooth muscle-cells, passing transversely around the 

 vessel, and among these we find a few elastic fibres, 

 which are connected with the elastic elements in the 

 intima and adventitia. The adventitia is thicker, 

 and consists chiefly of fibrillar connective tissue with 

 elastic fibres. 



In the larger arteries, such as the carotids, aorta, 

 etc., we find that the individual layers are consid- 

 erably less sharply defined. The three layers of the 

 intima are much less distinct ; in the media the 

 elastic tissue is very abundant, taking the place, to 

 a considerable extent, of the muscular elements ; it 

 is arranged in irregular lamellae and fibres, between 

 which lie fibrillated fibres, and connective tissue and 

 smooth muscle-cells, the latter no longer all lying 

 uniformly transversely to the axis of the vessel. In 

 the adventitia also of the large vessels the elastic 

 elements are very numerous, being most abundant 

 in the vicinity of the media. In the adventitia of 

 some of the large vessels, smooth muscle-cells occur, 

 arranged usually with their long axes parallel with 

 the axis of the vessel. 



