PREFACE vii 



temerity for some time. Had he, however, begun 

 with a whifif or two at a cigarette, and persevered for 

 a week or so in giving himself up at intervals to this 

 form of doubtful enjoyment, it is probable that he 

 would at length have been able to withstand the 

 effects of the stronger weed. 



One fact stands out prominently in this example — 

 viz., that it is the influence of a first impression even 

 more than the actual strength of the tobacco which 

 disturbs the functioning of the nervous system, and 

 thus causes vomiting, headache, or whatever other 

 consequences may follow. Other cases might also be 

 cited to illustrate the same truth. Take, for instance, 

 that of the opium-eater who accustoms himself by 

 degrees to enormous doses, and thus, while weakening 

 his nervous system, nevertheless acquires for it pro- 

 gressively immunity from the effects which would 

 otherwise result therefrom. 



How does the nervous system govern the meta- 

 bolism of the tissues? There can, it would seem, be 

 only one conceivable answer to this question: the 

 nervous system must regulate all the various processes 

 connected with tissue change by virtue of the force 

 generated and developed in its own metabolism. 

 Being the most readily mobile of all the tissues, the 

 anabolic and katabolic changes in it give rise, we 

 must suppose, to a continuous series of rhythmic 

 movements the influence of which is felt throughout 

 the body. The conditions essential to the perfect 

 functioning of the whole system are, therefore, that 



