TEACHING OF PHYSIOLOGY AND PUBLIC HEALTH. 39 



succeeding one is not yet at hand. A number of inter- 

 esting theories have been advanced to account for this 

 immunity. Pasteur held that there was a special food in 

 the body which these bacteria needed and which was com- 

 pletely used up in the first attack, thus making the second 

 one impossible; but as the blood changes in composition 

 so rapidly it seems impossible that such elements would not 

 again soon be introduced if they were normally in the 

 blood. Then Chauveau ventured the opinion that these 

 germs produce excretions which finally render the blood 

 unfit for them to live in and thus prevent the recurrence of 

 the disease. When we remember how quickly the system 

 normally throws off foreign poisons it seems hardly possible 

 that the body should retain these excretions for such a long 

 time as the immunity seems to last. L,ater, Grawitz pro- 

 posed the idea that the outcome of a disease was the out- 

 come of the battle waged between the bacteria in our bodies 

 and the living tissues, that if the tissues managed to 

 obtain the upper hand and destroy the bacteria the training 

 so acquired would increase their vital energy and resisting 

 power and so cut short a second attack, something as a 

 regiment of veterans would be less easily overcome in battle 

 than a corps of raw recruits. Succeeding this Buchner 

 gave as his explanation that in a first attack the places at 

 which the bacteria were introduced became more resistant 

 and so prevented the introduction of succeeding ones. This 

 went on the supposition that all kinds of bacteria had their 

 specific places through which alone they could enter the 

 body. It is not necessary to state that this explanation 

 found little support. 



Then came the now celebrated theory of Metchnikoff , 

 which holds that the white corpuscles of the blood and 

 lymph are scavengers going up and down between the tis- 

 sues, and picking up foreign particles wherever found. 

 This picking up is nothing more than the process of amoe- 

 boid digestion. This theory is now very generally accepted, 

 and observations clearly show that these corpuscles do take 



