THEIR MODE OF ACTION. 361 



All these conditions for contagion must be considered as acci- 

 dental. Their formation and accumulation in the body may be 

 prevented, and they may even be removed from it without dis- 

 turbing its most important functions or health. Their presence 

 is not necessary to life. 



The action, as well as the generation of the matter of conta- 

 gion is, according to this view, a chemical process participated 

 in by all substances in the living body, and by all the constitu- 

 ents of those organs in which the vital principle does not over- 

 come the chemical action. The contagion, accordingly, either 

 spreads itself over every part of the body, or is confined particu- 

 larly to certain organs, 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 decomposed, but communicates its own 

 state of transformation 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 con- 

 stituent of the blood : the first may be a body accidentally 

 present : but it may also be a matter necessary to life. If both 

 be constituents indispensable for the support of the vital func- 

 tions of certain principal organs, death is the consequence of 

 their transformation. But if the absence of the one substance 

 which was a constituent of the blood do not cause an immediate 

 cessation of the functions of the most important organs, if they 

 continue in their action, although in an abnormal condition, 

 convalescence ensues. In this case the products of the trans- 

 formations still existing in the blood are used for assimila- 

 tion, and at this period secretions of a peculiar nature are 

 produced. 



When the constituent removed from the blood is a product of 

 an unnatural manner of living, or when its formation takes place 

 only at a certain age, the susceptibility of contagion ceases upon 

 its disappearance. 



The effects of vaccine matter indicate that an accidental con> 



