1550 SLEEP. [BOOK iv. 



preserved ; and hence the man who in the prime of his manhood 

 was a ' martyr to dyspepsia ' by reason of the sensitiveness of 

 gastric nerves and the reflex inhibitory and other results of their 

 irritation, in his later years, when his nerves are blunted, and 

 when therefore his peptic cells are able to pursue their chemical 

 work undisturbed by extrinsic nervous worries, eats and drinks 

 with the courage and success of a boy. 



979. Within the range of a lifetime are comprised many 

 periods of a more or less frequent recurrence. In spite of the aids 

 of a complex civilisation, all tending to render the conditions of 

 his life more and more equable, man still shews in his economy 

 the effects of the seasons. The birth-rate for instance shews an 

 increase in winter, and most people gain weight in winter and lose 

 weight in summer. Careful observations of school children shew 

 that these increase in length rapidly in the spring but hardly at all 

 in the autumn, and very slowly in the winter, while their increase 

 in weight is most marked in the autumn, being very slight or even 

 negative in the spring, and not great in winter. Some of these 

 apparent effects of the season are the direct results of varying 

 temperature, but some probably are habits acquired by descent, 

 and in some again the connection is a very indirect or possibly not 

 a real one. Within the year, an approximately monthly period is 

 manifested in the female by menstruation, though there is no exact 

 evidence of even a latent similar cycle in the male. The phenomena 

 of recurrent diseases, and the marked critical days of many other 

 maladies, may be regarded as pointing to cycles of smaller duration 

 than that of the moon's revolution, save in the cases in which the 

 recurrence is to be attributed rather to periodical phases in the 

 disease-producing germ itself, than to variations in the medium of 

 the disease. 



980. Prominent among all other cyclical events is the rhyth- 

 mic rise and fall in the activities of the central nervous system ; most 

 animals possessing a well-developed nervous system, must, night 

 after night, or day after day, or at least time after time, lay them 

 down to sleep. The salient feature of sleep is the cessation or 

 extreme lowering of the psychical activity of the brain and of 

 the nervous processes which serve as the basis of that activity. 

 When sleep is at its height, the afferent nervous impulses which 

 external agents set going in the afferent somatic nerves such as 

 those of the special senses, are no longer the starting points of 

 complex cerebral processes; not only do they fail to excite con- 

 sciousness and to leave their mark on memory, but they may 

 be unable to call forth even a simple reflex movement. And 

 yet they are not wholly without effect ; for though a set of feeble 

 afferent impulses may produce no visible reaction and leave no 

 impression on the mind of the sleeper, yet impulses of the same 

 kind, if made stronger in proportion to the depth of the sleep, may 

 be followed by their wonted cerebral consequences, and may thus 



