Licence: In copyright
Credit: Hygiene: a manual of personal and public health. Source: Wellcome Collection.
Provider: This material has been provided by London School of Hygiene & Tropical Medicine Library & Archives Service. The original may be consulted at London School of Hygiene & Tropical Medicine Library & Archives Service.
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![CHAPTER XXXVIII. PERSONAL HYGIENE {continued)—R^BT AND SLEEP. Physiological Considerations.—Life is made up of alterna- tions of rest and action. The exercise of any organ is followed by a necessary period of repose, during which the oxidised materials produced by functional activity are removed by the blood, and carried to the excretory organs ; while at the same time fresh nutritive material is supplied by the blood to make good the losses thus sustained. The only apparent exceptions to this rule of alternation of rest and exercise are the heart and lungs, and some less important organs acting out of the control of personal volition. But even these organs obey the universal law. The difference is that their rest is very frequent and momentary ; the heart having to contract sixty or seventy times per minute, rests of a second each second, or more than thirteen hours in the twenty-four. The lungs and respiratory muscles rest a shorter time than this, but probably about three hours per day. The necessity for rest is well shown by the sense of taste. If salt is kept in the mouth for a considerable time, the power of tasting it disappears, and only returns in its original strength after several hours. The gustatory nerve has been exhausted. The other sense-organs illustrate the same principle. Persons are not uncommonly made deaf by the sounds of machinery. After looking at a particular colour for some time, the nerves receiving impressions from this colour are exhausted, and only its complementary colour is visible. Rest may be either partial or general. The principle of partial rest has very useful practical bearings. Such rest is illustrated by the student who takes a walk, or uses methodical gymnastic exercis'^s ; a concert may provide agreeable exercise for the auditory nerves and the part of the brain connected with them, while allowing the over-tired intellectual part of the brain to rest in peace; similarly, light literature may prove a pleasing rest after severer studies. Walking is more especially the exercise of the brain-worker. Partial rest is the same thing as change of occupation, and by a careful regulation of the relative amount of cerebral and muscular work, energy can be largely economised. The horse, which exercises chiefly his muscles, requires only five or six hours to recuperate his energy ; and our muscles require less sleep than our brain. Newsholme's Hygiene.] 18](https://iiif.wellcomecollection.org/image/b21357675_0269.jp2/full/800%2C/0/default.jpg)
