CHAP, v.] THE PHASES OF LIFE. 691 



the retina ; the sensory and motor impulses pass with increasing 

 slowness to and from the central nervous system, and the brain 

 becomes a more and more rigid mass of protoplasm, the molecular 

 lines of which rather mark the history of past actions than serve as 

 indications of present potency. The epithelial glandular elements 

 seem to be those whose powers are the longest 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. 



Within the range of a lifetime are comprised many periods of 

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

 plex 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. Some of these are the direct results of varying 

 temperature, but some probably, such as the gain of weight in 

 winter and the loss in summer, are habits acquired by descent. 

 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 recur- 

 rent 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, unless we admit the view urged by some 

 authors that in these cases 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. 



Prominent among all other cyclical events is the fact that 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 of the 

 automatic activity of the brain ; it is the diastole of the cerebral 

 beat. But the condition is not confined to the cerebral hemi- 

 spheres; all parts of the body either directly or indirectly take 

 share in it. The phenomena of sleep are perhaps seen in their 

 simplest form in the winter-sleep or hybernation, to which especially 

 cold-blooded animals, but also to some extent warm-blooded animals, 

 are subject. In these cases the cold of winter slackens the vibra- 

 tions and lessens the explosions of the protoplasm, not only of 

 nervous but also of muscular and glandular structures ; indeed the 

 activity of the whole body is lowered, in some respects almost to 

 actual arrest. At the same time that the labour of the cerebral 

 molecules becomes insufficient to develop consciousness, the respi- 

 ratory centre is either wholly quiescent or discharges feeble im- 

 pulses at rare intervals, and the heart beats with a slow infrequent 

 stroke, not by reason of any inhibitory restraint, but because its 



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