Chap, hi.] THE PHASES OF LIFE. 1153 



the lines of resistance, which, as we have seen, help to map out 

 the central organs into mechanisms, and so to produce its mul- 

 tifarious 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 media and 

 presbyopic muscular inability, but also from the very bluntness 

 of 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 nervous 

 substance, 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. 



§ 719. 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 civilization, all tending to render the condi- 

 tions 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. 



§ 720. Prominent among all other cyclical events is the 

 rhythmic rise and fall in the activities of the central nervous 

 system ; most animals possessing a well-developed nervous sys- 



