74 PHYSIOLOGY. 



red color, but a thin layer of the same jelly, as when one 

 takes a spoonful on a plate, has a pale color, more yellow- 

 ish. The colorless plasma with the colored bodies in it 

 may be compared to a glass dish filled with cranberries 

 and water. 



Hemoglobin. The coloring matter in the blood, then, 

 is wholly in the colored corpuscles. Examination of these 

 corpuscles shows that their color is due to a substance 

 called hemoglobin. There is a small amount of iron in 

 the hemoglobin, and the presence of this small quantity 

 of iron appears to be essential to give the blood its color. 

 When we come to the study of respiration we shall see 

 that the hemoglobin in the corpuscles is the chief agent in 

 picking up the oxygen from the air in the lungs and carry- 

 ing it to the tissues in the body. 



The Coagulation of Blood. When the blood escapes 

 from its natural channels it usually changes from a liquid 

 to a jelly-like condition. This is known as coagulation. 

 It is due to the formation of threads of fibrin from the 

 plasma. These threads of fibrin entangle and inclose the 

 corpuscles, and the two constitute the clot, or coagulum, as 

 it is more technically termed. The liquid that afterward 

 separates from the clot is the serum, and differs from the 

 plasma only in the removal of the fibrin, which is exceed- 

 ingly small in quantity, though of great importance in its 

 action. Many experiments have been made, and much 

 has been written about the coagulation of the blood, and 

 perhaps its real cause is not yet clear. But we know that 

 the coagulation often serves to stop the flow of blood from 

 wounds, and this is its main use. 



Fibrin. If freshly drawn blood be stirred rapidly with 

 a bundle of wires (perhaps the most convenient stirrer is 



