Exercise is a recognized necessity for the maintenance of all the functions of the body, including the mental functions, which are not of first importance with regard to the lower animals. As in the case of food, exercise may be beneficial or injurious according to its amount. Excessive exertion is perfectly well known to be followed by exhaustion, which is necessarily associated with loss of tone in the system and liability to disease. Failure of circulation in various parts will induce congestion and the accumulation of deleterious matters in the blood, and in this condition the body becomes remarkably subject to disease. On the other hand, want of exercise leads to a sluggish condition of all the functions, loss of muscular power, weakness of the heart in common with other muscular structures, feeble circulation, local congestions, inactivity of the respiratory functions, accumulations of fat, and what is perhaps of even more importance, an inactive state of the excretive organs which permits the accumulation in the blood of various deleterious products resulting from imperfect oxidation, which under a healthy condition would be rapidly eliminated from the system.

Temperature has a marked influence on the various functions of the body; heat causes debility by its stimulating influence upon the circulatory, respiratory, and nervous functions, the excessive activity being naturally followed by relaxation and exhaustion.

Excessive cold produces debility by its immediate sedative influence on the system, and the repellent effect of cold upon the surface diminishes the circulation in the superficial vessels of the skin and leads to an accumulation of blood in some of the internal structures, acting particularly upon some parts of the digestive and circulatory system. Its influence upon the nerve centres, whether it is direct or indirect, is shown in the rapid diminution of the vital powers of resistance, a condition which is eminently favourable to the action of exciting causes of disease.