AETERIES 



85 



/ 



The internal coat, tunica intima, presents three layers, the 

 innermost being the layer of endothelial cells, the outermost a 

 layer of elastic tissue, ^ 



the fenestrated coat ^r ^*ll-l- --'----{ J 



of Henle, or internal '-^^. kWx^\Y ' 



elastic membrane ; be- 

 tween these is a deli- 

 cate fibrous mem- 

 brane, which consti- 

 tutes the middle layer. 



The endothelium 

 comprises only a single 



layer of flattened or 



-Mi-, 6 ' v ' ^->-%;j-vc- ^-^-;^;^- ., 

 squamous cells, placed \ ^ x xxc>i;?;c -: > --\--5'''t--^ ' 



edge to edge to form a - " '* 



FlG. 86. A SMALL ARTERY FROM THE CONNECTIVE TIS- 

 SUE OF THE ANTERIOR CERVICAL REGION OF MAN. 



a, tunica adventitia; i, tunica intima; w, tunica 

 media; n, a small non-medullated nerve trunk; , a 

 minute venule. Hematein and eosin. x 370. 



continuous membrane 

 of simple pavement epi- 

 thelium. These cells 

 are irregularly polygo- 

 nal in outline, and are 

 somewhat elongated in the direction of the axis of the vessel. They 

 are loosely attached to the elastic membrane by the middle layer 

 of fine fibrillar connective tissue, in whose ground substance small 

 branching connective tissue cells are found. The thickness of this 

 connective tissue layer varies proportionately to the size of the vessel. 

 The internal elastic membrane is a layer of elastic tissue, con- 

 sisting of an intimately united fibrous mass, which completely en- 

 circles the artery. In the smaller vessels the elastic fibres of this 

 layer form only a reticulated structure, but in the larger arteries 

 they are so abundant and so closely interwoven as to form a com- 

 plete membrane, which can be readily stripped from the subjacent 

 tissue. If the membrane thus prepared is examined microscopic- 

 ally, it will be found to present numerous small openings at points 

 where the elastic tissue is deficient. It is this appearance which 

 led to its description as a " fenestrated membrane." The internal 

 elastic membrane is intimately united to the tunica media, upon 

 which it rests ; in fact, it may perhaps be better considered as the 

 innermost layer of this tunic, for, in the larger arteries, e. g., the 

 aorta, it can only with difficulty be distinguished from the adjacent 

 layers of elastic tissue which form a large portion of the tunica 

 media of these vessels. 



