of oxygen is renewed. There is also a deficiency of water in the system as the result of excessive elimination, and this must be replaced. Since the bodily functions require the presence of a considerable amount of water in the system, it seems essential that during exercise the loss of water should be compensated by the administration of small amounts of water at short intervals. The fatigue of the muscular system can only be relieved by a period of rest. The heart-muscle, under ordinary work, has a rest between the contractions, which is about twice as long as the time consumed by the contractions, and hence it requires no additional rest to recover itself.

Effect on the Nervous System.-The effect of exercise on the nervous system is more indirect than direct in its nature. Moderate exercise assists in maintaining all the bodily functions in their normal condition, and hence the nervous system is in a position to act most efficiently. This fact has had abundant demonstration in recent years since athletic sports have become such an important feature in college life. It has been found that, as a rule, the best athletes are rather above the average in their class records.

Overtraining is of course detrimental to the nervous system, because it undermines the general health. The effect of active exercise upon the mental activity is dependent to a certain extent upon individual conditions; but, as a rule, it is believed to be perfectly allowable in students that are able to keep up with the majority of their class. It is better to attain somewhat lower averages in class standing than to risk a breaking down of the nervous system as the result of overstudy or the ruination of the general health because of too close application to study.

Effect on the Elimination of Nitrogen.-A large number of experiments have been made to determine the relative amounts of nitrogen eliminated during rest and exercise. The results obtained indicate that during exercise the amount of nitrogen assimilated is increased per

ceptibly. The metabolism of nitrogen is influenced somewhat by the period of work and rest, and the severity of the work performed. During a period of active exercise the amount of nitrogen eliminated from the kidneys is slightly diminished, and after the exercise there is a slight excess of nitrogen excreted, continuing for some time. During severe exercise the amount of nitrogen eliminated appears to be increased. Voit and Krummacher are of the opinion that usually work does not directly produce a greater breaking down of proteid matter, but that an increase in the proteid cleavage is caused by the increased combustion of the nitrogen-free materials which protect the proteid matter. If it were possible, during the period of work, to supply the cells continuously with a sufficient amount of nitrogen-free material, then there would be no increase in the quantity of proteid material broken down. But this is a very difficult matter. Krummacher believes that the after-effect of muscular labor is not due to the continued excretion of nitrogenous cleavage-products, but to the fact that the nitrogen-free materials in the body were used up, and that it takes some time to provide the body with a new supply. In active exercise, therefore, an increased amount of nitrogen must be supplied in the food, as well as an increased amount of carbon. There must also be an increased supply of salts, especially sodium chlorid and potassium phosphate, to supply the loss in these salts during exercise.

Amount of Exercise that Should be Taken.-A good day's work for an average man is considered to be 150,000 kilogram-meters. Haughton has shown that walking on a level surface at the rate of 3 miles (4.8 kilometers) per hour is about equal to raising one-twentieth of the weight of the body through the distance walked.

In order to determine the work performed in walking 32,000 meters per day, we multiply the weight of the body in kilograms by the distance travelled, the result being the kilogram-meters of work performed. If a

pedestrian walks 32,000 meters a day, without a load, the energy expended, assuming him to weigh 70 kilograms, is 32,000 X 70= 2,240,000 kilogram-meters. Haughton divides the work performed into "fatigue work," the effort necessary to carry the weight of the body, and "useful work," the energy expended in performing labor. For instance, Coulomb observed that the work done by porters employed to carry goods 2000 meters, returning unloaded, amounted to 348 kilograms in six journeys, or 58 kilograms at a time. The useful work performed was, therefore, 2000 X 348 meters; the fatigue work was 2000 X 2 X 70 X 61,680,000 kilogram-meters. This allows 70 kilograms as the weight of the porter, and takes into consideration that the body is carried in both directions, or 4000 meters. The total energy expended was 2,376,000 kilogrammeters. He also found that pedlars, who always travelled loaded with their packs, were able, with a load of 44 kilograms, to travel 19,000 meters per day. Assuming their weight of 70 kilograms, we find

696,000 kilogram

useful work. fatigue work.

In athletic exercises it is essential that the amount of energy expended be carefully determined, in order to ascertain whether the exercise is likely to prove beneficial or otherwise. The amount of energy expended in athletic sports should not exceed that expended by laborers in hard manual labor-that is, about 150,000 kilogram-meters. In athletic contests, of course, the energy expended is often in excess of this amount; but a period of comparative rest must supervene the contest,

in order to allow the body to recuperate from the fatigue induced by the contest. The harmful effects of the large amount of energy expended in some athletic contests is frequently seen when these contests take place at too short intervals, allowing insufficient time for the contestants to recover from the fatigue of the previous contest. The same effects are also noted in soldiers who are compelled to make frequent forced marches over long distances.

From what has been learned of the effects of exercise, it will be seen that athletic training should aim to increase the breathing-power; to strengthen the power of the heart's action; to make the muscular action more. vigorous and enduring; and to decrease the amount of fat. These results are obtained by careful dieting; by regular and systematic exercise; and by increasing the action of the eliminating organs, especially the skin.

CHAPTER IX.

CLOTHING.

THE objects of clothing are to protect against the weather-heat, cold, and dampness-and to protect against injury. All other uses of clothing have no direct hygienic interest, only indirectly in so far as they may be injurious to health.

Protection against Cold.-The most important use of clothing in cold climates is to protect against cold. Clothing serves this purpose by diminishing the radiation of heat from the body. The radiation of heat from the body diminishes with the number of layers of clothing worn, and is also dependent upon the nature of the clothing worn. If we take the amount of radiation of heat from the naked body as 100, the radiation is reduced to 73 by means of a woollen shirt; to 60 by means of both a woollen and a linen shirt; to 46 by means of a woollen and a linen shirt and a vest; to 33 by the addition of a coat. Rubner found that if the radiation at 15° C. is taken as 100, at 23° C. it is only 69, at 29° C. it is 56, and at 32° C. it is 31.

The radiation of heat is directly dependent upon the thickness of the layer of clothing. If we take the loss of heat as 100, a thickness of 1 millimeter of cotton allows a radiation of 77 per cent.; 2 millimeters, of 68 per cent.; 3 millimeters, of 65 per cent.; 4 millimeters, of 57 per cent.; 5 millimeters, of 53 per cent.; 10 millimeters, of 41 per cent.; 15 millimeters, of 30 per cent.

The thickness of clothing, in our climate, must not be so great as to increase perceptibly the air-pressure by compression, nor so thin as to decrease perceptibly the air-pressure. The thickness of the clothing is, therefore,

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