THEIR MODE OF ACTION. 367 



when contagion results, that the blood contains a 

 second constituent capable of being decomposed by 

 the exciting body. It is only in consequence of the 

 conversion of the second constituent, that the 

 original exciting body can be reproduced. 



A susceptibility of contagion indicates the pre- 

 sence of a certain quantity of this second body in 

 the blood of a healthy individual. The susceptibility 

 for the disease and its intensity, must augment ac- 

 cording to the quantity of that body present in the 

 blood; and in proportion to its diminution or disap- 

 pearance, the course of the disease will change. 



When a quantity, however small, of contagious 

 matter, that is of the exciting body, is introduced 

 into the blood of a healthy individual, it will be 

 again generated in the blood, just as yeast is re- 

 produced from wort. Its condition of transformation 

 will be communicated to a constituent of the blood ; 

 and in consequence of the transformation suffered 

 by this substance, a body identical with or similar 

 to the exciting or contagious matter will be pro- 

 duced from another constituent substance of the 

 blood. The quantity of the exciting body newly 

 produced must constantly augment, if its further 

 transformation or decomposition proceeds more 

 slowly than that of the compound in the blood, the 

 decomposition of which it effects. 



If the transformation of the yeast generated in 

 the fermentation of wort proceeded with the same 

 rapidity as that of the particles of the sugar con- 



