The effect of fatigue on nerve cells. a, healthy brain cell; b, fatigued brain cell.
Necessity of Food, Fresh Air, and Rest.—The nerve cells, like all other cells in the body, are continually wasting away and being rebuilt. Oxidation of food material is more rapid when we do mental work. The cells of the brain, like muscle cells, are not only capable of fatigue, but show this in changes of form and of contents. Food brought to them in the blood, plenty of fresh air, especially when engaged in active brain work, and rest at proper times, are essential in keeping the nervous system in condition. One of the best methods of resting the brain cells is a change of occupation. Tennis, golf, baseball, and other outdoor sports combine muscular exercise with brain activity of a different sort from that of business or school work. But change of occupation will not rest exhausted neurones. For this, sleep is necessary. Especially is sleep an important factor in the health of the nervous system of growing children.
Necessity of Sleep.—Most brain cells attain their growth early in life. Changes occur, however, until some time after the school age. Ten hours of sleep should be allowed for a child, and at least eight hours for an adult. At this time, only, do the brain cells have opportunity to rest and store food and energy for their working period.
Sleep is one way in which all cells in the body, and particularly those of the nervous system, get their rest. The nervous system, by far the most delicate and hardest-worked set of tissues in the body, needs rest more than do other tissues, for its work directing the body only ends with sleep or unconsciousness. The afternoon nap, snatched by the brain worker, gives him renewed energy for his evening's work. It is not hard application to a task that wearies the brain; it is continuous work without rest.
the senses
Touch.—In animals having a hard outside covering, such as certain worms, insects, and crustaceans, minute hairs, which are sensitive to touch, are found growing out from the body covering. At the base of these hairs are found neurones which send axons inward to the central nervous system.
Nerves in the skin: a, nerve fiber; b, tactile papillæ, containing a tactile corpuscle; c, papillæ containing blood vessels. (After Benda.)
Organs of Touch.—In man, the nervous mechanism which governs touch is located in the folds of the dermis or in the skin. Special nerve endings, called the tactile corpuscles, are found there, each inclosed in a sheath or capsule of connective tissue. Inside is a complicated nerve ending, and axons pass inward to the central nervous system. The number of tactile corpuscles present in a given area of the skin determines the accuracy and ease with which objects may be known by touch.
If you test the different parts of the body, as the back of the hand, the neck, the skin of the arm, of the back, or the tip of the tongue, with a pair of open dividers, a vast difference in the accuracy with which the two points may be distinguished is noticed. On the tip of the tongue, the two points need only be separated by 1/24 of an inch to be so distinguished. In the small of the back, a distance of 2 inches may be reached before the dividers feel like two points.