SUSCEPTIBILITY AND IMMUNITY. 247 



troduced upon the surface of the brain, and immunity is established 

 during the interval so-called period of incubation which usually 

 occurs between the date of infection and the development of the 

 disease. That the immunity in this case also depends upon the 

 introduction of a chemical substance present in the desiccated spinal 

 cord of rabbits which have succumbed to rabies, which is used in 

 these inoculations, is extremely probable. But, as the germ of rabies 

 has not been isolated or cultivated artificially, this has not yet been 

 demonstrated. Wooldridge claims to have made susceptible animals 

 immune against anthrax by inoculating them with an aqueous ex- 

 tract of the testicle or of the thymus gland of healthy animals. 



We may mention also the interesting results obtained by Em- 

 merich, Freudenreich, and others, who have shown that an anthrax 

 infection in a susceptible animal inoculated with a virulent culture 

 may be made to take a modified and non-fatal course by the simul- 

 taneous or subsequent inoculation of certain other non-pathogenic 

 bacteria streptococcus of erysipelas, Bacillus pyocyaneus. 



In a series of experiments made by the writer some years ago 

 evidence was obtained that, under certain circumstances, immunity 

 from the effects of one pathogenic bacillus may be obtained by the 

 previous injection of a pure culture of a different species. In the 

 experiments referred to injections into the cavity of the abdomen of 

 a culture of Bacillus pyocyaneus protected rabbits from the lethal 

 effects of Bacillus cuniculicida Havaniensis, when subsequently in- 

 jected into the cavity of the abdomen in such amount (one cubic 

 centimetre of a bouillon culture) as invariably proved fatal in rabbits 

 not protected by such injections. 



Before considering the theories which have been offered in expla- 

 nation of acquired immunity it is desirable to call attention to certain 

 observations which have been made during the past few years relat- 

 ing to " chemiotaxis. " 



The term chemiotaxis was first used by Pfeffer to designate the 

 property, observed by himself and others, which certain living cells 

 exhibit with reference to non-living organic material, and by virtue 

 of which they approach or recede from certain substances. The 

 chemiotaxis is said to be positive when the living cell approaches, and 

 negative when it recedes from, a chemical substance. As examples 

 of this we may mention the approach of motile bacteria to nutrient 

 material or to the surface of a liquid medium where they find the 

 oxygen required for their vital activities ; and of leucocytes to cer- 

 tain substances when these are introduced beneath the skin of warm- 

 er cold-blooded animals. This subject has recently received much 



