2l8 THE CIRCULATORY SYSTEM. 



outer is the looser in structure, possesses few cellular elements, and 

 shows a circular arrangement of its fibers. The adventitia is also 

 made up of fibro-elastic tissue, but in this case with a still looser 

 structure and a longitudinal arrangement of its elastic fibers. In the 

 outer portion of the adventitia the white fibrous tissue is more 

 abundant. The adventitia is rich in blood-vessels. 



The medium-sized arteries differ in structure from the larger in 



that the elastic elements of the 



intima and media are replaced to 

 a considerable extent by nonstri- 

 ated muscular fibers. To this type 



u 



belong the majority of the arterial 

 vessels, ranging in caliber from the 

 brachial, crural, and radial arteries 

 to the supraorbital artery. In 

 these the intima shows, besides its 

 endothelium, only a single connec- 

 tive-tissue layer with numerous 

 Fig. 173. Precapillary vessels from longitudinal fibers, the subendo- 



mesenteryofcat: a, Precapillary artery ; tne lial layer, which is thin and is 

 l>, precapillary vein possessing no muscu- . . . , , A . r 



lar tissue. limited externally by the fenes- 



trated membrane of Henle (lamina 



elastica interna). The media no longer gives the impression of 

 being laminated, but consists of circularly arranged muscle-fibers 

 separated from each other by elastic fibers and membranes and a 

 small amount; of fibrous connective tissue in such a way that the 

 muscle-cells form more or less clearly defined groups. Here also 

 the media is limited externally by the external elastic membrane. 

 The adventitia, which becomes looser externally, is not so well de- 

 veloped as in the larger vessels, but presents in general the same 

 structure. In certain arteries (renal, splenic, dorsalis penis) it shows 

 in its inner layers scattered longitudinal muscle-cells, which, how- 

 ever, may also occur in other arteries at their points of division. 



With regard to the elastic tissues, the arteries of the brain differ 

 to some extent from those of the remainder of the body. The 

 elastica interna is much more prominent, the elastic fibers in the 

 circular muscular layer are fewer, and the longitudinal strands are 

 almost entirely lacking (H. Triepel). 



The walls of the smaller arteries consist mainly of the circular 

 muscular layer of the media. The intima is reduced to the endo- 

 thelium, which rests directly on the elastic internal limiting mem- 

 brane. Outside of the external limiting membrane is the adventitia, 

 which now consists of a small quantity of connective tissue. The 

 vasa vasorum have disappeared. To this type belong the supra- 

 orbital, central artery of the retina, etc. 



In the so-called precapillary vessels the intima consists only 

 of the endothelial layer. The internal elastic membrane is very 

 delicate. The media no longer forms a continuous layer, but is 



