How Science Helps the Body 169 



fections seem to indicate that the extracts of 

 alien leucocytes used in this way may be of 

 great value. 



The incitants of small-pox and hydrophobia 

 are as yet unknown, though they are prob- 

 ably not bacteria. But we have methods of 

 preventive inoculation in both of these serious 

 diseases which are remarkably effective. 



These methods both depend upon the 

 gradual adaptation of the body cells to an 

 infectious agent. At first material of slight 

 virulence is used; then that which is more 

 potent. Thus presently the body cells, with- 

 out the acquirement of the disease by the 

 individual, have become so fully adapted to 

 the new conditions, that the body is protected 

 from the disease altogether. 



In small-pox the virus of diminished viru- 

 lence is secured by passing it through the 

 body of an insusceptible animal, the calf. 

 Then, while it does not induce small-pox 

 in man, it protects man from it. 



In hydrophobia the unknown living in- 

 fective agent, which is present in the nervous 

 system of rabic animals, through the method of 



