186 MICKO-ORGANISMS AND DISEASE. [CH. xx. 



Whether this chemical substance has been elaborated di- 

 rectly by the bacilli, or whether it is a result of the chemical 

 processes induced in the body by the bacilli during the first 

 illness, matters not at all ; it is only necessary to assume that 

 the blood and tissues of the living animal contain this chemical 

 substance. 



Some observers (Grawitz, &c.) are not satisfied with this 

 theory, but assume that owing to the first attack the cells of 

 the tissues so change their nature that they become capable of 

 resisting the immigration of a new generation of the same 

 organism. There is absolutely nothing that I know of in 

 favour of such a theory ; it is impossible to imagine that the 

 cells of the connective tissues, of the blood and of other 

 organs, owing to a past attack of scarlatina, become possessed 

 of new functions or of some new power, as, for instance, a 

 greater power of oxidising or the like. Connective tissue-cells, 

 blood-corpuscles, liver-cells, and other tissues are, so far as we 

 know, possessed of precisely the same characters and functions 

 after an attack of scarlatina as before. 



On the whole then, we may, it seems, take it as prob- 

 able, that owing to the presence in the normal blood and 

 tissues in a living animal of a chemical substance inimical to 

 the growth of a particular micro-organism, this animal is un- 

 susceptible to the disease dependent on the growth and multi- 

 plication of this micro-organism ; and further, that in those 

 infectious maladies in which one attack gives immunity against 

 a second attack of the same kind, one attack produces a 

 chemical substance in the blood and tissues which acts 

 inimically to a new immigration of the same organism ; hence 

 the animal becomes unsusceptible to a new attack, or is " pro- 

 tected." This is not the case with all infectious maladies, for, 

 as is well known, in a good many instances a single attack does 

 not protect against a second ; and, as is also well known, a first 

 attack may protect but only for a limited period, or for a period 

 greatly differing in different individuals. All this would be 

 explained by our theory in the same way as it is explained by 

 the other theories ; viz., when one attack does not protect, no 

 inhibitory chemical substance has been produced ; while in 

 those diseases in which one attack does protect only fcr a 

 limited period, the necessary inhibitory substance has only 

 lasted for a limited period, and so on. 



