X PREFACE 



chance of any particle of the lymph being still left in 

 the system grows ever less. Yet the period of im- 

 munity is generally recognised to be from seven to 

 ten years. But if it is very difficult, or even impos- 

 sible, to rest the case for vaccination upon a material 

 basis, three facts in regard to it must strike one as 

 undeniable : firstly, that the disturbance caused by 

 it, lasting as it does for several days, must produce 

 both a marked and a specific impression on the 

 nervous system ; secondly, that such impressions are 

 likely to be of a very durable kind ; and thirdly, that 

 one such impression may modify considerably subse- 

 quent ones of like or allied nature. 



Turning from preventive to curative medicine, we 

 meet with an equally striking example of the employ- 

 ment of measures which are in reality of an essentially 

 dynamic nature. For ages medicated baths of various 

 kinds, containing salts, sulphur, iron, and other sub- 

 stances, have been adopted as a means of relief in 

 certain pathological conditions, with results which, 

 leaving success in the treatment of this or that disease 

 out of the question, demonstrate, nevertheless, beyond 

 all doubt that the physiological action of these reme- 

 dies is not imaginary. Yet physiologists of repute 

 tell us unhesitatingly that not one particle of iron or 

 of salt or of sulphur ever enters the body through the 

 skin. There is no absorption of these inorganic sub- 

 stances, so that if they act upon the system, as we 

 know they do, they cannot act quantitatively, and 

 therefore they must act dynamically — they must, that 



