312 THE HUMAN BODY 



protein which is not repeated, there may appear a marked increase 

 of sensitiveness toward the immunizing substance, so that although 

 it may not have had any noteworthy effect on the Body formerly, 

 after this sensitization has occurred injection of the protein may 

 cause violent or even fatal disturbances. This reversal of the 

 ordinary course of immunization is called anaphylaxis. The tis- 

 sues which are most markedly affected are the involuntary muscles, 

 and death, when it occurs, is the result of cardiac or bronchial 

 spasms, or other smooth muscle involvements. 



Anaphylaxis is of practical importance because the administra- 

 tion of antitoxin involves the introduction of foreign proteins into 

 the system, and if sensitization should take place, a second dose 

 would have serious, or even fatal consequences. The serum (see 

 next paragraph) of horses forms the basis of diphtheria antitoxin. 

 Sometimes persons who are much about horses develop the condi- 

 tion, apparently from inhaling the effluvium from the animals. 

 Such persons cannot endure injections of antitoxin. They often 

 suffer disagreeable bronchial disturbances from the mere pres- 

 ence of horses. Hay fever is a similar sensitization toward the 

 proteins contained in the pollen grains of plants. 



The Coagulation of Blood. When blood is first drawn from the 

 living Body it is perfectly liquid, flowing in any direction as readily 

 as water. This condition is, however, only temporary; in a few 

 minutes the blood becomes viscid and sticky, and the viscidity 

 becomes more and more marked until, after the lapse of five or 

 six minutes, the whole mass sets into a jelly which adheres to the 

 vessel containing it, so that this may be inverted without any 

 blood whatever being spilled. This stage is known as that of 

 gelatinization and is also not permanent. In a few minutes the 

 top of the jelly-like mass will be seen to be hollowed or "cupped" 

 and in the concavity will be seen a small quantity of nearly color- 

 less liquid, the blood-serum. The jelly next shrinks so as to pull 

 itself loose from the sides and bottom of the vessel containing 

 it, and as it shrinks squeezes out more and more serum. Ulti- 

 mately we get a solid dot, colored red and smaller in size than 

 the vessel in which the blood coagulated though retaining its 

 form, floating in a quantity of pale yellow serum. If, however, 

 the blood be not allowed to coagulate in perfect rest, a certain 

 number of red corpuscles will be rubbed out of the clot into the 



