Theoretically we must distinguish between mass and weight. The mass is the measure of the resistance offered by a body to the force which is required to alter its state by movement; the mass of a body is therefore the same wherever the body is placed in the universe. The weight, again, is the pressure which a body exerts on the underlying surface on account of the attraction of the earth, and varies with the distance of the body from the centre of the earth, which is somewhat different at different latitudes and at different heights above sea-level.

In the ordinary cultured, moderately active life we need rather over 30 calories in twenty-four hours per kilo of body weight, which we take in as protein fat, and carbohydrate in the proportions of a "good mixed diet." As representative of such a diet one may take in somewhat round figures the well-known scheme of Voit, i.e., 120 grm. protein, 60 to 90 grm. fat, and 300 to 500 grm. carbohydrate. This daily amount will suffice only at its maximum for a tolerably well-grown person doing hard work. Neither can it be taken as an absolute example of proportion. Teutonic races especially, often from choice, take more fat and protein than Voit's scheme allows.

Rubner gives the desirable proportions of the three important organic food substances as 1/6 protein, 1/6 fat, and 2/3 carbohydrate.

Forster's investigations give on an average 131.2 grm, protein, 88.4 grm. fat, 329.3 grm. carbohydrate, and 2945.9 grm. water. The amount of water necessary varies enormously with physical exercise and perspiration, and according to the food taken and the amount of salt it contains, but for a well-grown person under ordinary conditions amounts to 3 1/2 litres in twenty-four hours, and is taken in, as we all know, partly in the form of various drinks, partly in the solid food substances, which usually contain a large quantity of water.

A person, probably a very small person judging from the quantity of urine, lost, according to Pettenhofcr and Voit during rest

We receive from the food in gross value per gram of Protein. 4.1 calories Fat. 9.3 calories Carbohydrate. 4.1 calories and after subtracting the amount which leaves the body undigested in the excretions, on an average a net value of Protein. 3.2 calories Fat. 8.4 calories Carbohydrate. 3.8 calories.

While we increase our physical exercise we increase continually the amount of heat produced, and in order to maintain our equilibrium we must take in a much greater supply of calories of nourishment than that which would only correspond to the extra work. The need of food thus increases rapidly with physical exercise, and when the latter varies much the former is also very variable. A person lying in bed certainly requires barely 30 calories in twenty-four hours for every kilo of body weight. A fairly active person, not a manual labourer, disposes easily of 35 to 36 calories per kilo in twenty-four hours. The Swedish workman, who on an average weighs about 70 kilos and is generally very disinclined to overwork, generally uses up about 3,000 calories, i.e., nearly 43 calories per kilo body weight, in twenty-four hours. American students (in California), who, with an average weight of 70 kilos, played football a large part of the day, consumed daily per person 217 grm. protein, 416 grm. fat, and 710 grm. carbohydrate, which makes nearly 100 calories net per kilo in twenty-four hours. One calorie corresponds to 425 kilogrammetres, and about one-third of the calories must have been used up by the young men in physical work ! In general, 300,000 kgm. is considered a "fair," 400,000 kgm. a "stiff," day's work.

While physical exercise increases the need of food, it also increases the tendency to take in more food and the power of complete digestion. The effect of physical exercise in increasing the appetite belongs to those phenomena which may be noticed by any one; the same may be said of its effect upon defalcation.

Many people have tried to show the beneficial effects of physical exercise upon secretion in the glands of the alimentary canal, the pancreas, and liver. On the whole such an influence is highly probable. But while digestion is actually going on severe physical exercise is always to be avoided, because all the digestive glands are then secreting more than at other times, and are consequently the seat of a widespread hyperaemia, as is the case in all organs during increased activity. This causes blood to be taken from the muscles, and consequently counteracts (the desire for) movement.

Speaking of the effects of active movements upon the activity of glands, I must here put in a word as to their effects upon the kidneys, skin, and genital organs.

The functions of the kidneys are increased by physical exercise, in that the quantity of urine is first increased; at the same time the specific gravity of the urine usually sinks. After prolonged marches with profuse perspiration we often find a scanty, concentrated urine with high specific gravity. Zuntz and Schumburg found that a clinically. unimportant albuminuria, not uncommon in young persons, vanished during a march. But many researches prove that excessive marches, on the other hand, may often cause albuminuria, and that they easily produce relapse of recently cured nephritis, probably through irritation of the kidneys by the over-concentrated blood and urine after profuse perspiration.

On the genitalia physical exercise has a regulating effect. Fatigue is accompanied by lowered functional power, and the practitioner finds in energetic physical exercise a good means of lessening excessive sexual feeling in young men, and of promoting moderation in the exercise of the sexual functions, or even total abstinence, which is often difficult, though socially beneficial and hygienically harmless. On the other hand, the doctor often sees that systematic physical exercise in persons with sedentary habits and with weak potency produces a considerable increase of the latter.