4 8 



THE CIRCULATION OF THE BLOOD 



The Capillaries. As the arteries get smaller we find them 

 still composed 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 

 endothelial cells on a basement 

 membrane. This is in order to 

 render possible the interchange 

 of material between the blood cur- 

 rent and the lymph stream, so the 

 tissues may be nourished and the 

 waste products removed. 



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 like 

 amount of fluid be allowed to es- 

 cape at the other end the tube will 

 resume its original caliber. Thus 

 the pulse -volume enters -with 

 much force (the aorta or pulmon- 

 ary artery; the artery is very 

 elastic and expands under this in- 

 fluence, but immediately recoils 

 with a great pressure on the con- 

 tents. The pressure tends to 

 vessel in both directions, but its 

 is effectually prevented by the 



FIG. 24. Scheme of the 

 circulation. 



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

 right, B, left, ventricle; i, pul- 

 monary artery; 2, aorta; i, area 

 of pulmonary, K, area of syste- 

 mic, circulation; o, the superior 

 vena cava; G, area supplying the 

 inferior vena cava; u; d, d, in- 

 testine; m, mesenteric artery; q, 

 portal vein; L, liver; h, hepatic 

 vein. (Landois.) 



force the blood along the 

 return into the ventricle 



