Chap. IX] 



VASCULAR SYSTEM 



149 



tissues, and is found in both arterial and venous blood, but the 

 quantity is greater in venous blood. 



Enzymes. — An enzyme is a substance produced by living cells 

 and is capable of effecting chemical change without itself undergo- 

 ing alteration in the process. Each enzyme has a definite action at 

 a suitable temperature, and will only work in a medium of definite 

 reaction, either acid or alkaline. Further the products of the 

 action must be removed. Five enzymes have been found in the 

 blood. 



Internal secretions. — The blood serves as a medium to carry 

 internal secretions. (See page 136.) 



Immune bodies. — In addition to the phagocytes and opsonins, 

 the blood has been found to contain various other protective sub- 

 stances, which are described as antibodies. Just how they are 

 formed, and whether they are a natural constituent of the blood, 

 or whether they are developed only during an attack of disease, 

 are undecided questions. The antibodies, like the enzymes, are 

 specific in their action, that is, each variety will act against only 

 one form of bacterium or toxin ; for instance the antibody for 

 typhoid is of no service against pneumonia. 



THE CLOTTING OF BLOOD 



Blood when drawn from the blood-vessels of a living body is 

 perfectly fluid. In a short time it becomes viscid, and this vis- 

 cidity increases rapidly until the whole mass of blood becomes a 



Fig. 95. — Bowl of Recently 

 Clotted Blood, showing the Whole 

 Mass uniformly Solidified. (Dal- 

 ton.) 



Fig. 96. — Bowl of Clotted Blood 

 AFTER Twelve Hours, showing the 

 Clot Contracted and Floating in 

 the Fluid Serum. (Dalton.) 



complete jelly. If the blood in this jelly stage be left untouched 

 in a glass vessel, a few drops of an almost colorless fluid soon make 

 their appearance on the surface of the jelly.- Increasing in number 



