Physiology of Pathogenic Microbes. 65 



way of excitation (hyperkinesia developed by the 

 toxines of tetanus), or of depression (coma, somno- 

 lence, in fowl cholera and charbon ; paralysis consecu- 

 tive to diphtheria and to the pyocyanic disease). The 

 heart, the respiratory centers, and the vaso-motor cen- 

 ter may also feel the influence of these substances, 

 from which may result sometimes an increase, some- 

 times a diminution, of the functional activity of these 

 organs. 



RECEPTIVITY. 



Heceptivity, or aptitude to contract infectious dis- 

 eases, varies in accordance with a large number of 

 circumstances, and, especially, with the species and 

 the mode of inoculation, with the individual, age, 

 heredity, causes of depression, the quantity and the 

 quality of the virus, and the association of the virus 

 of more than one disease. 



1. Influence of species and of the mode of inocula- 

 tion. — The susceptibility of a given animal species 

 to a disease, when experimentally inoculated, does 

 not necessarily imply the liability of that species to 

 contract the disease spontaneously. 



Tubercu losis develops spontaneously, with great ease 

 and frequency, in man, cattle, and birds ; it is much 

 rarer in the horse, the pig, and the dog under the 

 same conditions; as for the rabbit and the guinea 

 pig, which it is very easy to inoculate experimentally, 

 they do not contract the disease except by inocula- 

 tion. 



Symptomatic charbon, appearing spontaneously 

 only in bo vines, is inoculable to the sheep, the goat, 

 and the guinea pig, but not to other animals. 

 6 



