THE CIRCULATION OF THE BLOOD 



tunica media, composed of yellow, elastic tissue; and (3) the 

 inner coat or tunica intima, composed of epithelium on base- 

 ment membrane. 



Veins. The veins also have 

 three coats, the external, middle 

 and internal, as the arteries; but 

 the middle coat is composed chiefly 

 of inelastic, fibrous tissue. Thus 

 the veins lack the elasticity and 

 contractility given to the arteries by 

 the middle coat. 



The Capillaries. As the arteries 

 get smaller we find them still com- 

 posed of the three above named 

 coats. Finally, though, in the 

 minutest vessels we find only the 

 innermost layer remaining. These 

 one-coated vessels are the capillaries, 

 and they have only one layer of 

 epithelial cells on a basement mem- 

 brane. This is in order to render 

 possible the interchange of material 

 between the blood current and the 

 lymph stream, so the tissues may 



be nourished and the waste prod- 

 FIG. 24 -Scheme of the uctg removed> 



circulation. 



a, right, b, left, auricle; A, right, 

 B, left, ventricle; i, pulmonary 



IMPORTANCE OF ARTERIAL 

 ELASTICITY. 



If an amount of fluid corre- 

 sponding to that of the " pulse 

 volume" be suddenly injected into 

 the end of a rubber tube already distended with liquid, the 

 tube will be further distended by the liquid injected ; but if a 



artery; 2, aorta; i, area of pulrno 

 nary, K, area of systemic, circula- 

 tion; o, the superior vena cava; G, 

 area supplying the inferior vena 

 cava; u; d, d, intestine; m, mesen- 

 teric artery; q, portal vein; L, liver; 

 h, hepatic vein. (Landois.) 



