CHAPTER XV. 



PHYSIOLOGICAL RHYTHMS. 



Dependence of the central system Distribution of blood Physio- 

 logical rhythms The greater cycles Rhythms of the species 

 and the individual Long rhythms Daily rhythms Artificial 

 rhythms Variations in muscular power Minor rhythms 

 Pathological variations in reaction Automaticity. 



UP to this point the nervous system, and the changes 

 which occur in it, have been treated as though isolated 

 and independent, and no effort has been made to 

 emphasise the position of this system as one of several 

 mutually dependent upon each other for their common 

 existence. Nevertheless, the brain and cord are at the 

 mercy of the circulating blood, and through the distribu- 

 tion and quality of the latter the vigour and speed of 

 their reactions are entirely controlled. The central 

 system thus stands in the relation of an engine to its fuel 

 and furnace. The engine may be never so fine, but if 

 the motive power is not sufficient it will work poorly or 

 not at all. So in the animal body, let there be an im- 

 perfection in the working of the heart, rigidity or 

 flabbiness in the vessels, a delinquent gland disturbing 

 the nutritive conditions under which the nerves act, 

 and at once the entire system is affected. These facts 

 have a very direct bearing. When, for example, it is a 

 question of inferring the mental abilities of an individual 

 from the gross anatomy of his brain, it must always be 



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