THE BLOOD AS A LIVING TISSUE 



195 



specific neutralizing or counteracting substances. This class of 

 poisons is now known by the name toxins (from a Greek word 

 meaning " poison ") and is illustrated by the venom of the 

 rattlesnake and of some other snakes, by a certain protein 

 found in the seeds of the castor-oil plant, and by one found 

 in the bark of the black locust. The substance produced by 

 the live cells under the influence of a toxin is called an anti- 

 toxin, and it is always specific ; that is, it will neutralize the 

 poison under whose stimulation it was produced, but no other. 



The best-known toxins 

 are those produced by cer- 

 tain bacteria, especially 

 those that cause lockjaw 

 and diphtheria. When 

 a quantity of toxin, not 

 enough to kill, is injected 

 into the blood of an ani- 

 mal (for example, a horse), 

 the cells begin to throw 

 off antitoxin. They will 

 produce more than enough 

 antitoxin to neutralize the 

 poison received. After 



the poison has all been destroyed, there will be a quantity of 

 antitoxin in the blood. If now a larger quantity of the poison 

 is introduced, some of it will be at once neutralized by the free 

 antitoxin ; and if the animal is in good health, an additional 

 quantity of antitoxin will be produced. In this way it is pos- 

 sible to increase the amount of antitoxin in the blood until 

 there is several hundred times as much as would be necessary 

 to neutralize very many fatal doses of the poison. 



In preparing antitoxin for diphtheria this is practically the method 

 followed. In two or three months the blood of the horse contains a 

 large amount of antitoxin. Blood is drawn from a vein in the neck 

 and is allowed to clot. The serum now contains all the antitoxin. 



FIG. 73. Danger in delay 



Each rectangle represents one thousand children 

 suffering from diphtheria. Antitoxin was admin- 

 istered to all the children on the days indicated. 

 The number of deaths in a group increases with 

 each day's delay in the use of antitoxin 



