178 THERAPEUTICS AND PHARMACOLOGY 



treatment. This differs in its plan from that used in diphtheria. 

 In diphtheria the bacilli probably form a ferment which produces 

 a deadly poison by exercising its digestive powers on the material 

 it finds in the body. This poison is neutralized by the antidotal 

 serum which is formed in a horse and is injected into the patient. 

 In hydrophobia we have not been able to isolate the virus, but 

 from its mode of action we suppose it to be a minute organism. This 

 virus takes a long time to act in man, sometimes three weeks but 

 usually six weeks, but when cultivated successively in rabbits it 

 becomes very virulent indeed and acts much more quickly. It 

 apparently finds its chief nidus in the spinal cord. When the cord 

 is exposed to air the virus gradually becomes weakened and by 

 injecting with an extract of very weak cord on the first day and 

 with a stronger extract on each succeeding day the human body 

 becomes accustomed to the virus and forms its own antitoxins. 

 Thus by the time that the poison inoculated by the original bite 

 of the rabid animal has time to develop its action the person has 

 become immune. 



One of the most important problems of therapeutics, therefore, 

 is to render the human body immune against pathogenic microbes, 

 against the ferments they form, and the toxins they produce. The 

 two examples I have already given show how the toxins and pos- 

 sibly the ferments may be rendered innocuous by injecting anti- 

 dotal sera and thus producing what is called "passive immunity," 

 or by exciting the body to form antidotal substances itself and 

 thus produce what is called "active immunity." Both these meth- 

 ods have been used, and are being used, in regard to other diseases, 

 especially in those produced by micrococci of various sorts which 

 give rise to suppuration and inflammations. One great difficulty 

 in the way, however, is that the antidotal serum produced by one 

 coccus is not always efficient against the disease produced by 

 another, and so much is this the case that it would almost seem 

 as if an antidotal serum would require to be made for each par- 

 ticular patient. Nor are the sera altogether innocuous themselves 

 because their injection may be followed not only by annoying 

 rashes on the skin but by general swelling of the body like that 

 from advanced kidney disease, or by painful swelling of the joints 

 almost like rheumatic fever. Another of the problems of therapeu- 

 tics therefore is to obtain anticoccic sera which will not produce 

 any unpleasant or dangerous symptoms. 



Yet another is to confer on the tissues of the body the power of 

 resisting or destroying microbes, their ferments, and their toxins, 

 and thus protecting themselves or in other words acquiring immunity 

 against the diseases which the microbes would produce. In consider- 

 ing this question it may help us if we remember that the products 



