70 INFECTIOUS AND PARASITIC DISEASES. 



least resistance to their attack. The tissues of the body 

 vary in susceptibihty to microbic invasion, the differ- 

 ence having a relation to the species of organism. Thus 

 there are certain bacteria which can exert a specific 

 action only when they gain entrance into the intestinal 

 tract ; to this class belong the typhoid bacillus and the 

 spirillum of Asiatic cholera. There are bacteria, on the 

 other hand, such as the tetanus bacillus, the gas bacillus, 

 and the common bacteria of suppurations, which may 

 be swallowed with impunity, in fact often are, their 

 characteristic effects being produced only by entrance 

 into a wound of the skin or a mucous membrane. 

 Similarly, the diphtheria bacillus has no effect upon the 

 unbroken skin, nor upon the mucous membranes of the 

 stomach or intestines, although, in the disease, countless 

 numbers are swallowed; it selects per force the mucous 

 membranes of the nose, the pharynx, and the posterior 

 nares chiefly, less often those of the conjunctiva and 

 the vulva. 



The channel of entry, therefore, occupies an impor- 

 tant position when the subject of a micro-organism's 

 pathogenicity is under consideration, since upon it will 

 depend the result of the presence of a pathogenic agent 

 whether the latter does or does not satisfy every other 

 condition necessary for its specific action. 



The real power of every species of micro- 

 ViRULENCE. organism which produces disease lies in 



its ability to secrete one or more toxins. 



