88 TKAXSPORTxVTIOX SYSTEM IxN ANIMALS 



larger vessels, the veins, whose union finally results in 

 one, or two, main vessels entering the heart. 



It will be seen from this brief description of the circula- 

 tion that the blood never leaves these vessels, and accord- 

 ingly at no point does it come in direct contact with the 

 cells of the body excepting those of the blood itself and 

 those lining the circulatory system. It is a closed system 

 and the transfer of nutritive materials to the tissues and 

 the removal of wastes to the blood stream is provided 

 for by a special process that will be more clearly under- 

 stood after we have examined some of the peculiarities 

 of blood. 



The Blood. — The familiar red fluid known as blood, 

 when examined under the microscope, is seen to consist 

 of a fluid, the plasma, in which there are floating cells 

 of two types, the red and white corpuscles. The plasma 

 consists largely of water in which food and waste sub- 

 stances are dissolved, together with certain other ma- 

 terials some of which aid in the destruction of disease- 

 carrying germs, while others regulate various activities 

 of the body cells. That it is more than water is also 

 indicated by its ability to coagulate or form a clot in an 

 injured vessel that, thus plugged up, is incapable of 

 further bleeding. The white corpuscles are likewise ac- 

 tive agents in engulfing and digesting germs in the blood 

 stream. In some instances they may even carry on this 

 work outside of the capillaries through whose walls they 

 have bored a way ; or they may carry off" and digest frag- 

 ments of injured cells whose removal paves the way for 

 repair processes. The red corpuscles owe their name to 

 the presence of a pigment, the hemoglobin, carried in 

 their substance. As this material is of the utmost im- 

 portance in the process of respiration, its general charac- 

 teristics will be considered in that connection. 



Lymph. — The blood capillaries are not only widely 

 distributed, but throughout the greater part of their ex- 

 tent they are surrounded by slit-like spaces, so that prac- 



