216 OF THE BLOOD. 



result of decomposition either within or without the body ; such as that of the 

 " pustule maligne," or of the flesh of animals suffering under disease, on the 

 one hand, or the a cheese poison/' u sausage poison/' &c. on the other. It may 

 be admitted that our belief in a specific material cause for a great part of the 

 effects set down to the action of " morbid poisons/' is merely inferential; and 

 there are many persons, to whom their exhibition in a tangible form seems to 

 afford the only convincing evidence of their existence. But it must be remem- 

 bered that the evidence of chemistry itself is often purely inferential ; for we 

 recognize the presence of a chemical substance, not merely by obtaining it in a 

 separate form, but by witnessing the reactions which it displays with various 

 tests ; and there is one substance, fluorine, which has never yet been isolated, 

 and of whose existence, however, no chemist would hint a doubt. Now ifr is 

 the human body which forms the appropriate testing apparatus of " morbid 

 poisons;" and even if we could always obtain them in a separate state, and 

 could subject them to chemical analysis, we should know much less of their 

 most important properties, than that which we can ascertain by observation of 

 their actions in the system; this alone affording the means of judging of their 

 dynamical character, which is of far more importance than a knowledge of their 

 chemical composition. In the case of those poisons which are capable of being 

 introduced by inoculation, we have indeed, the required proof of their material 

 existence ; and this proof is capable of being extended by a safe analogy to 

 infectious diseases generally. For if smallpox can be communicated by the 

 inhalation of an atmosphere tainted with the exhalations of a person already 

 affected with it, as well as by the introduction of the fluid of the cutaneous pus- 

 tule into the blood of another, it can scarcely admit of a question, that the same 

 poisonous agent is transmitted in both cases, although through different media, 

 and that it has as real an existence in the transferred air, as in the transferred 

 pus. Diseases, then, which are capable of being transmitted in both these 

 methods, form the connecting .link between those resulting from ordinary toxic 

 agents, and those which must be assumed to depend upon a subtile poison, of 

 which the air alone is the vehicle such, for example, as malarious fevers ; this 

 assumption being required by all the rules of logic, as the only one which will 

 account for the phenomena to be explained, and therefore possessing a claim to 

 be accounted an almost certain truth. There is a strongly-marked difference, 

 however, between the modus operandi of the toxic agents whose action has been 

 previously examined, and that of the morbid poisons we are now considering ; 

 for whilst the former possess a certain definite action, the intensity of which 

 (cseteris paribus) is proportionate to the quantity that is in operation, and which 

 is usually determined, in virtue of the " elective affinity" already spoken of, to 

 some particular organ or tissue the latter act primarily upon the blood, in- 

 fluencing the system at large through the changes which they produce in its 

 constitution, and their potency depends rather upon the susceptibility of the 

 blood to their peculiar influence, than upon the quantity of the poison that may 

 be introduced into it. 



210. Of the existence of such susceptibility, as a " predisposing cause" of 

 Zymotic 1 disease, there cannot be the slightest doubt. In the case of the Ex- 

 anthemata and Hooping-cough, we see that it is congenital, and is usually re- 

 moved by the occurrence of one attack of the disease (although this is not a 

 uniform protection) ; but the liability even to these varies greatly in different 

 individuals, and at different times in the same individual. And with regard to 



1 The term zymotic is a very convenient designation, which has of late gained general 

 currency, for that class of diseases whose phenomena may be attributed to the operation 

 of a morbid poison of the nature described above ; this operation bearing a strong analogy 

 to that of "ferments." 



