viii PREFACE 



these rhythmic movements shall possess a given 

 character, and that they shall be regular. These 

 movements, however, are easily disturbed and ren- 

 dered irregular. Thus, exposure of parts of the body 

 simultaneously to two degrees of temperature, when 

 sudden, is often, as we know, sufficient to upset the 

 rhythm of cutaneous metabolism, and produce a cold. 

 When bacteria in certain quantities enter the system 

 there is a similar disturbance of rhythm. The 

 nervous metabolic rhythm is one form of activity, 

 the vital processes going on in the bacteria constitute 

 another. Both the activity of the latter and their 

 waste products bring about a change in the en- 

 vironment of the nervous system, and hence -the 

 disturbance. The capacity of an individual to resist 

 the attack of an infective disease depends, one may 

 say, on the power of the nervous system to preserve 

 a certain regularity of rhythm in the presence of the 

 bacteria, and, above all, to preserve a rhythm in 

 which the excretory tendency is strongly accentuated. 

 One may indeed compare the cells of the nervous 

 system under these conditions to the members of a 

 small settlement on which a party of savages has 

 suddenly made a descent. If it be a matter of a first 

 and unexpected attack, the excitement and confusion 

 would naturally be greater than if it were a second 

 or third occurrence of the same kind. So it is with 

 the nerve-centres when bacteria of a virulent type in- 

 vade the system for the first time. If they become, 

 as they are likely to do, very excited, they lose in 



