THE BLOOD 31 



most abundant constituent of the blood; it is, in some 

 respects, the most important. It is the liquefying por- 

 tion of the blood, holding in solution the constituents of 

 the plasma and floating the corpuscles. Deprived of its 

 water, the blood becomes a solid substance. Through the 

 movements of the blood the water also serves the purpose 

 of a transporting agent in the body. The cells in all parts 

 of the body require water and this is supplied to them 

 from the blood. Water is present in the corpuscles as 

 well as in the plasma and forms about 80 per cent of the 

 entire volume of the blood. 



Coagulation of the Blood. If the blood is exposed to 

 some unnatural condition, such as occurs when it escapes 

 from the blood vessels, it undergoes a peculiar change 

 known as coagulation! In this change the corpuscles are 

 collected into a solid mass, known as the clot, thereby 

 separating from a liquid called the serum. The serum, 

 which is similar in appearance to the blood plasma, differs 

 from that liquid in one important respect as explained 

 below. 



Causes of Coagulation. Although coagulation affects all 

 parts of the blood, only one of its constituents is found in 

 reality to coagulate. This is the fibrinogen. The forma- 

 tion of the clot and the separation of the serum is due 

 almost entirely to the action of this substance. Fibrino- 

 gen is for this reason called the coagnlable constituent of 

 the blood. In the plasma the fibrinogen is in a liquid 

 form ; but during coagulation it changes into a white, 

 stringy solid, called ^rz>/. This appears in the clot and is 

 the cause of its formation. Forming as a network of exceed- 



1 Coagulation is not confined to the blood. The white of an egg coagulates 

 when heated and when acted upon by certain chemicals, and the clabbering o( 

 milk also is a coagulation. 



