HISTORIC 143 



phobia were even greater than in previous experiments. Here he was 

 dealing with a disease of unknown etiology, the causative agent of which 

 he could not cultivate artificially, but which he sought to attenuate by 

 a new process that of drying. 



Pasteur first established that the virus of rabies is contained within 

 the tissues of the brain and spinal cords of infected animals. 



He then invented a method of inoculating animals by making sub- 

 dural injections of an emulsion of these tissues. By repeated passage 

 of a virus through a number of rabbits a virus of fixed pathogenic power 

 (virus fixe) was obtained. By inoculating rabbits with this virus and 

 removing their spinal cords immediately after death and drying these 

 over a desiccating agent at room temperature, he found that he could 

 modify the virulence of the virus at will, depending on the length of the 

 period of drying. By emulsifying small portions of attenuated spinal 

 cord in salt solutions and injecting these he was able gradually to im- 

 munize animals against rabies, and finally he applied the treatment 

 successfully to the prevention of rabies in the human being. 



Antirabic vaccination is largely responsible for extending our knowl- 

 edge of the possibility of securing immunization. Pasteur has taught 

 us at least three different methods for modifying a virus in the prep- 

 aration of a vaccine, and that each disease, being itself a special 

 entity, having its own characteristics, must be dealt with along special 

 lines. 



These discoveries were largely empirical, and the explanations of 

 their mechanism are now only of historic interest. It was not until 1883, 

 when Metchnikoff shed light upon the problems of immunity by 

 making a series of remarkable studies on the role played by certain of 

 the body-cells in overcoming infection, and the part they played in the 

 processes of immunity in general, that the world was given a glimpse 

 into the dark problems of immunity. These observations were soon 

 followed by investigations showing the importance of the body fluids, and 

 since that time a great deal of work has been done upon these subjects. 

 As a consequence, a large amount of data of a wholly new order has 

 accumulated, accompanied by the introduction of a host of new terms 

 expressing diverse views and theories advanced by individual workers. 

 Of the many theories advanced from time to time to explain the phenom- 

 enon of immunity, two have claimed the most attention: one ascribes 

 protection and cure to the activity of certain body-cells; this is known 

 as the cellular theory; and the other attributes these qualities to the body- 

 fluids the humoral theory. The chief exponent of the former is Metch- 



