860 THE PHASES OF LIFE. 



force, and the movements of the intestine, bladder, and other organs, dimin- 

 ish in vigor. In the nervous system, the lines of resistance, which, as we 

 have seen, help to map out the central organs into mechanisms, and so to 

 produce its multifarious actions, become at last hindrances to the passage of 

 nervous impulses in any direction, while at the same time the molecular 

 energy of the impulses themselves becomes less. The eye becomes feeble, 

 not only from cloudiness of the medja and presbyopic muscular inability, 

 but also from the very bluntness of the retina ; the sensory and motor im- 

 pulses pass with increasing slowness to and from the central nervous system, 

 and the brain becomes a more and more rigid mass of protoplasm, the molec- 

 ular 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. 



815. 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 civili- 

 zation, all tending to render the conditions of his life more and more equable, 

 man still shows 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 recurrent diseases, and 

 the marked critical days of many other maladies, may be regarded as point- 

 ing 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. 



816. Prominent among all other cyclical events is the fact that most 

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

 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 hemispheres ; 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 of hibernation, 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 vibrations and lessens the explo- 

 sions of the protoplasm, not only of nervous but also of muscular and glandu- 

 lar structures ; indeed the activity of the whole body is lowered, in some 

 respects almost to actual arrest. At the same time that the labor of the cere- 

 bral molecules becomes insufficient to develop consciousness, the respiratory 

 centre is either wholly quiescent or discharges feeble impulses at rare inter- 

 vals, and the heart beats with a slow, infrequent stroke, not by reason of any 

 inhibitory restraint, but because its very substance in its slow molecular 

 travail can gather head for explosions only after long pauses of rest. And 

 such few and distant beats as do occur are amply sufficient to meet the needs 

 of the feeble metabolism of the several tissues. The sleep of every day 

 differs from the sleep of winter cold chiefly because the slackening of molec- 

 ular activities is clue in the former not to extrinsic but to intrinsic causes, 



