ELEMENTS OF PHYSIOLOGY 



FIG. 90. Transverse Section of Part of the Wall 

 of an Artery, highly magnified. 



, epithelial (endothelial) layer, or inner coat; c, muscular 

 layer or middle coat; d, outer coat, consisting of connective 

 tissue. 



nary veins, to the left auricle, then through the bicuspid, or 



mitral, valve to the left ventricle, thence into the aorta by the 



semilunar valve, thence to the capillaries of the system, 



thence to the veins, and through them it returns to the heart, 



completing the circulation. 

 152. Structure of 



Blood Vessels. In 



order to understand 



how the arteries, 



veins, and capilla- 

 ries are adapted to 



their work, we will 



study their anatomy. 



We shall find three 



kinds of tissue (Fig. 



90) used in their 



construction, epithelial tissue to prevent friction, connect- 

 ive tissue to give both strength 

 and elasticity, and muscular tissue 

 to enable the vessels to change in 

 size. 



153. The epithelial tissue forms 

 the innermost layer of the vessels. 

 The endocardium or inner lining 

 of the heart is formed of this mem- 

 branous layer, and is continued 

 throughout the arteries, capilla- 

 ries, and veins. In these vessels 

 it is called the endothelium. The 

 epithelial cellsforming this smooth 



FIG. 9 i. - Epithelial Ceils forming layer are t h m and flat, and serve to 



the Walls of Capillaries. J . . . . 



Thenucleiandtheontlinesofthecells dimmish friction. The Walls of 



may be seen. The forming of the {he capillaries, the Smallest VCSSCls, 

 thin tubular walls of the capillaries . ... . 



is one of the most curious among the COnSlSt Simply Of thlS epithelial 



ayerful ^ accomplished membrane (Fig. 91 ). In the small 



