THE CIRCULATORY SYSTEM. 



FIG. 118. 



arteries as small, medium, and large. The first group includes the 

 terminal branches near transformation into capillaries, the second, 

 all the named arteries of the body, except those which, as the aorta 

 or the pulmonary artery, are recognized as belonging to the third 

 group of large arterial vessels. 



The inner coat, or intima, as seen in a typical artery of medium 

 size, comprises three layers : (a) an endothelial lining, made up 

 of long, lanceolate, nucleated plates, united by a sinuous line of 

 cement-substance and placed parallel to the axis of the vessel ; (b) a 

 sub-endothelial layer of delicate fibrous connective tissue, with 

 branched corpuscles ; (c) a band of elastic tissue the internal 

 elastic membrane which forms the most prom- 

 inent part of tKe~mtima, appearing in sections of 

 medium-sized ar- 



teries as a clear, FIG. 117. 



glistening, and 

 usually corrugated 

 band separating 

 the tissue of the 

 inner coat from 

 that of the media. 

 The sub-endothe- 

 lial tissue, which 

 separates the en- 

 dothelium from 

 the internal elastic 

 membrane, is wanting in the smaller arterioles, but appears in vessels 

 of medium size as a longitudinally disposed layer, becoming more 

 conspicuous with the increased calibre of the artery. In tubes of 

 large diameter, as in the aorta, the sub-endothelial tissue appears as 

 a stratum composed of layers made up of fibrous tissue, elastic net- 

 works, and flattened connective-tissue cells. Likewise, the elastic 

 tissue of the intima increases in amount and in complexity, in the 

 large arteries the broad elastic fibres becoming fused together to 

 form an almost continuous sheet the fenestrated membrane of 

 Henle. 



The middle coat, or media, is the muscular tunic, and consists 

 principally of circularly disposed bundles of non-striated muscle-cells ; 

 these elements, when isolated, appear as broad, nucleated, irregular 

 spindle-cells, presenting ragged outlines. In many arteries, con- 

 spicuously the subclavian, the inner portion of the media con- 

 tains additional muscle-cells longitudinally disposed. In the smaller 

 arteries the muscular tissue constitutes almost the entire media, but 

 an insignificant amount of intermuscular fibrous connective tissue 



Portion of the intima of the human aorta, 

 silver stained : the larger stellate figures are 

 the cell-spaces in the ground-substance be- 

 tween the elastic bundles and contain the 

 connective-tissue corpuscles. 



Portion of the elastic 

 tissue of the intima of 

 the human aorta ; the 

 fibres are so broad and 

 so closely grouped that 

 they constitute an elastic 

 sheet the fenestrated 

 membrane of Henle. 



