88 



NORMAL HISTOLOGY. 



coat, is of exceptional strength and toughness. It should be noted that the 

 endothelial tube is the fundamental and primary structure in all cases, the 

 other coats being secondary and variable according to the size and character 

 of the vessel. The customary division of the wall into the three coats is 

 more or less artificial and in the larger vessels often uncertain. The recog- 

 nition of an inner endothelial and an outer musculo-elastic coat frequently 

 more closely corresponds to the actual arrangement than the conventional 

 subdivision into the three tunics. 



The endothelial lining of the arteries consists of elongated spindle- 

 shaped plates united by sinuous lines of cement substance, which, after 

 silver-staining, map out the contours of the cells with diagrammatic clearness 

 (Fig. 124). Within the veins, the endothelial plates are shorter and broader 



Endothelium 



Intima 



Internal elastic 

 membrane 



Med 



Advenlitia 



FIG. 125. Transverse section of artery of medium size. X 150. 



than in the arteries. The demarcation of the endothelium into distinct cells 

 is less evident in the capillaries than in the larger vessels, in some cases a 

 continuous syncytial sheet replacing definitely outlined plates. The presence 

 of small oval nuclei is readily demonstrated by suitable stains. 



The involuntary muscle varies in amount from the imperfect single 

 layer of muscle-cells found in the arterioles to the robust muscular coat of many 

 lamellae in the larger arteries. It is relatively best developed in arteries of 

 medium size, where the muscle occurs in distinct broad or sheet-like bundles 

 between the strands of elastic tissue. The component fibre-cells are short 

 and often branched and, for the most part, circularly disposed. The distri- 

 bution of the muscular tissue is much less regular and constant in the veins 

 than in the arteries, since in many it is scanty, in some entirely wanting, and 

 in a few excessive, occurring as both circular and longitudinal layers. The 



