THEIR MODE OF ACTION. 371 



uncleanliness, or even the use of decayed substances 

 as food. 



All these conditions for contagion must be con- 

 sidered as accidental. Their formation and accu- 

 mulation in the body may be prevented, and they 

 may even be removed from it without disturbing 

 its most important functions or health. Their 

 presence is not necessary to life. 



The action, as well as the generation of the 

 matter of contagion is, according to this view, a 

 chemical process participated in by all substances 

 in the living body, and by all the constituents of 

 those organs in which the vital principle does not 

 overcome the chemical action. The contagion, ac- 

 cordingly, either spreads itself over every part of 

 the body, or is confined particularly to certain or- 

 gans, that is, the disease attacks all the organs or 

 only a few of them, according to the feebleness or 

 intensity of their resistance. 



In the abstract chemical sense, reproduction of 

 a contagion depends upon the presence of two 

 substances, one of which becomes completely de- 

 composed, but communicates its own state of trans- 

 formation to the second. The second substance 

 thus thrown into a state of decomposition is the 

 newly formed contagion. 



The second substance must have been originally 

 a constituent of the blood : the first may be a 

 body accidentally present ; but it may also may be 

 a matter necessary to life. If both be constituents 



B B 2 



