V 



322 TEXT-BOOK OF PHYSIOLOGY. 



smallest vessels are capillaries. \The blood in the artery is of a 

 brighter color than the blood in the vein; the blood in the capillaries 

 is almost colorless. \The arterial blood-stream not infrequently 

 shows remittancy, an alternate acceleration and retardation, corre- 

 sponding to each heart-beat; the__caplllary and venous streams_are 

 uniform and continuous. The relative velocities in the three sets 

 of vessels are indicated by the movement of the red corpuscles. In 

 the arteries they pass before the eye so rapidly that they can not be 

 distinguished; in the capillaries they pass so slowly that both form 



and structure may be deter- 

 mined; in the veins, though 

 again moving rapidly, they 

 can often be distinguished. 



The relative positions of 

 the red and white corpuscles 

 in the blood- stream are also 

 apparent; the former occupy 

 the central, the latter the per- 

 ipheral portion, at the same 

 time adhering to the sides of 

 the vessel. Between the axial 

 portion of the stream occu- 

 pied by the red corpuscles and 

 the wall of the vessel there is 

 a clear still layer of plasma, 

 the result of an adheskm of 

 the plasma to the wall. lit is 

 this feature which gives rise to 

 the friction between successive 

 layers of the blood- stream, the 

 resistance to the blood-flow, 

 and the development of blood- 

 pressure. The relative breadth 



of the still layer and amount of friction are greater in small than in 

 large vessels. 



The volume of blood passing into any given capillary area is 

 determined by the degree of contraction of the arterioles. Thus on 

 the application of warm saline solution, which relaxes the arterioles, 

 there is a large increase in the inflow of blood; vessels previously in- 

 visible suddenly come into view as the blood with its corpuscles passes 

 into them. On the application of cold water, which contracts the 

 arterioles and diminishes the inflow, many of the smaller vessels 

 entirely disappear from view. The alternate contraction and re- 

 laxation of the arterioles will therefore determine the quantity of 

 blood flowing into and through the capillary system. 



FIG. 154. THE VESSELS or THE FROG'S 

 WEB. a. Trunk of vein, and (b, b) its 

 tributaries passing across the capillary 

 network. The dark spots are pigment 

 cells. (Yeo's "Physiology.") 



