THE CIRCULATION 



93 



arteries a layer of muscular tissue is placed outside the 

 endothelium. Outside of this muscular tissue, and forming 

 the outer surface of the artery, is a layer of connective 

 tissue containing both white fibers and yellow elastic fibers 

 (Fig. 90, also Fig. 32). In fact, some of the yellow elastic 

 fibers (Fig. 32) are found also in the other coats. The 

 muscular and connective tissue layers become still thicker 

 in the larger arteries. So the wall of an artery consists 

 of three layers: (i) the endothelium ; (2) the muscular coat ; 

 (3) the connective tissue coat on the outside (Figs. 90 and 

 92). The arteries are therefore very firm and elastic, and 

 do not collapse when they are cut, but stand open, and the 



-n 



A V 



FlG. 92. An Artery and Corresponding Vein cut across. 



A, artery ; V, vein ; e.c, endothelial cells ; nt, muscular coat ; c , connective tissue ; , nuclei 

 of endothelial cells. 



flow of blood through them is unobstructed. The walls of 

 the veins consist of the same three layers. Their walls are 

 not so thick as those of the arteries, for the muscular and 

 connective tissue layers are much thinner (Fig. 92). When 

 a vein is cut it collapses, that is, the thin walls fall together, 

 and the bleeding is stopped unless the vein is large. 



154. The Use of the Three Coats. The white fibers of 

 the connective tissue coat (see Fig. 90) give strength and 

 firmness to the vessel, and the yellow elastic fibers give elas- 

 ticity. The muscular, or middle, coat enables the arteries 

 and veins to change in size, and the inner, or endothelial, 



