92 THE DISEASES OF DOMESTIC ANIMALS. 



As many of you may have read, in the Catacombs of Rome and 

 other such places carbonic- acid gas is generally very abundant in 

 the lower strata of air in the passages, and while it is impossible for 

 dogs or cats to follow their owners in such places, yet the latter, 

 on account of the greater elevation of their respiratory apparatus, 

 can walk along in perfect safety. We know also how quickly un- 

 pleasant odors are dispersed by the atmosphere. 



A poisonous gas must be very soon so dispersed as to lose its 

 specific characteristics. 



Were the generation of cholera dependent on infectious gases, 

 we should find it rapidly extending over every part of a city or dis- 

 trict, instead of being confined, as is frequently the case, to certain 

 streets or districts. So in the case of rinderpest. If the infectious 

 elements were of a gaseous nature, our endeavors at stamping out 

 would be utterly futile ; yet we know we can frequently confine it 

 to a single stable in a village where many cattle are kept. Were 

 the infectious elements gases, all individuals with any disposition 

 to infection would contract given diseases, as they would be far 

 more likely to take up a correspondingly equal amount of infectious 

 material than if they were of an organized nature. 



Again, if they were gases, the infectious elements would soon be 

 so widely dispersed as to lose their activity. 



While we know the minuteness of many forms of bacterial life, 

 so minute that our strongest powers give us but the most inadequate 

 idea of their nature, may we not safely assume that there are many 

 forms which still escape our observation ? 



Are the elements of infection formless, or are they organic in- 

 dividuals — that is, objects having form and life ? 



We have two possibilities to consider : either they enter an or- 

 ganism in such quantities as to cause immediate action, or they 

 enter in very small quantity, and have the power of multiplying 

 within the organism. 



By poisoning the first takes place. A given quantity of a known 

 poison causes direct and specific action. This never takes place by 

 infection. 



Specific poisons have no period of latency. 



The most poisonous of substances act only in this manner. Minus 

 a given quantity, no poisonous action, though we may have what is 

 known as a physiological or medicinal action which we make use of 

 in many of them. If we carry these medicinal doses beyond a cer- 

 tain limit, a poisonous action frequently results, which is known as 

 the cumulative action of drugs — as with strychnine. 



