﻿8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 59 



cite an example, from experimental medicine, to show how important 

 these external defenses are. Epidemic poliomyelitis, or infantile 

 paralysis, is, as far as we know, a disease of human beings exclusively. 

 The evidence available is to the effect that the microbic cause of the 

 disease enters the body by means of the nose and throat and locates 

 in the central nervous organs, where it multiplies and ultimately 

 brings about the tissue changes that account for the paralysis and 

 other symptoms. Of the lower animals, monkeys alone are really 

 susceptible to the disease, but only upon inoculation. They never 

 take the disease spontaneously; that is, they can be exposed to the 

 infection in any degree and yet they remain free from it. But when 

 the cause of the disease, in most minute quantity, is made to pass 

 the external defenses, by being inoculated into the nervous system 

 or into the interior of the nasal mucous membrane, they not only 

 acquire poliomyelitis, but they develop a far severer form than occurs 

 spontaneously in human beings. The deduction from this fact is 

 that monkeys, as compared with human beings, possess a far more 

 efficient system of external mechanisms to exclude the poliomyelitic 

 virus; but human beings, as compared with monkeys, possess a far 

 more efficient internal agency of destruction for the virus, after it has 

 gained access to the interior of the body. 



But for the existence of this second line of defenses, the internal, 

 as we may call it, the infected body would be wholly at the mercy of 

 invading parasitic causes of disease. But this second line is, indeed, 

 even more efficient than the first, and of more varied potentialities. 

 It consists of a group of substances contained within the blood, but 

 not produced there, and passing from the blood into the lymph, where 

 they exert influence on the cells composing the organs and upon 

 parasites in the interstices of the tissue. The chief site of the pro- 

 duction of these protective and healing principles — immunity prin- 

 ciples, so-called — is the very place where the blood corpuscles them- 

 selves are formed ; namely, in the bone-marrow, spleen, and lymphatic 

 glands. The principles are divisible into two classes of substances : 

 one soluble and dissolved in the fluid plasma, and the other corpus- 

 cular and suspended in it. The latter consists of the motile white 

 cells or phagocytes ; and together, in virtue of their soluble form 

 and ready motility, they are able quickly to reach most parts of the 

 body where their peculiar properties may be exerted. Not only 

 are these immunity principles preformed in all individuals in whom 

 they operate against intending infections, but they become rapidly 

 increased when infection has been established ; and the ultimate issue 



