THE BLOODVESSELS ARTERIES. 



503 



elastic lamellae of the middle coat of the large arteries. The elon- 

 gated fissures between its fibres produce its fenestrated appearance. 

 (Fig. 334.) In the largest arteries this is reinforced externally by 

 a coarsely fibrous layer of elastic tissue, shown by Fig. 335. 



1. In the large arteries the fenestrated layer is thickened inter- 



t/ / J 



nally by one or several clear layers the striped lamellce, (ffenk) 1 

 appearing, when nucleated, like coalesced epithelial cells ; and when 

 not so, homogeneous, like pale elastic membranes. In the axillary 

 and popliteal arteries, smooth muscular fibres have also been found 

 in this layer. (Fig. 244.) In the medium-sized and the small arteries, 

 the fenestrated layer, or elastic stratum, is T2 otf of an inch or more 

 thick, and smoothly stretched beneath the epithelium during life. 

 In collapsed arteries it presents a number of strong folds, and 

 frequently also fine transverse rugae. It may be traced down into 

 vessels only ? J of an inch in diameter; but cannot be regarded 

 as fully developed in those less than 2 J of an inch. 2. The epi- 

 thelial layer of the tunica intima consists of a single layer of flat- 

 tened nucleated cells. These in the large arte- 

 ries are usually shorter than in the smaller ves- 

 sels, though still fusiform, and s^Vu to ijo^ f 

 an inch long. (Fig. 336.) In the medium-sized 

 and small arteries, they are fusiform, pale, with 

 long oval nuclei, and readily separated in con- 

 nection, in fragments, or in perfect tubes. The 

 epithelium may be traced in vessels of only 

 TJTJV or even y^V^ of an inch in diameter. 

 Here, however, the cells cannot be isolated, and 

 its presence can be recognized by its closely 

 placed elliptical nuclei alone. 



All the arteries (and the veins), above fa of 

 an inch in diameter, have nutritive vessels (vasa 

 vasorum), derived from minute contiguous arte- 

 ries, forming a rich capillary network in the 

 external tunic, and which extend into the outer portion of the mid- 

 dle coat of the large vessels, but not to its inner portion, nor to 

 the inner coat at all. Nerves also, both sympathetic and spinal, are 

 distributed to many arteries, but often only accompany them. 



Fig. 336. 



Epithelial cells from the 

 aorta of an ox. (Magnified 

 400 diameters.) 



1 In the large vessels the striped lamellae are sometimes not covered by, but are 

 continuous with, the epithelium. 



