Exercise (the movement of any part of the body at the command of the will) is produced by contraction of the muscles of voluntary motion, and their alternate relaxation. For, although muscular contraction may be considered to be exercise, in the strict sense of the word; and although, when not alternated with relaxation, such contraction constitutes the most fatiguing way in which the muscles can be made use of; yet, in the more ordinary sense, muscular exercise implies a series of consecutive movements, involving and produced by alternating contraction and relaxation of the muscular fibres. So true is it, that muscular contraction, without such intervals of relaxation, is especially fatiguing (a fact, however, of which any one may be convinced, by attempting to hold the arms extended for more than a very few minutes at a time), that the extensor muscles are less powerful than the flexor muscles of the limbs, in order to secure the more complete repose of the body when not engaged in active

exertion.

The use of the muscles of voluntary motion must be preceded by so much of nervous influence being directed to them, as may serve the purpose of a stimulus to the fibres called into action; and this must involve a corresponding expenditure of nervous power, and a corresponding and simultaneous expenditure of the organised atoms of the nervous system itself: a preliminary condition, and source of additional expenditure, beyond that involved in the muscular contraction, which has been already noticed.

MECHANICAL EFFECT OF MUSCULAR ACTION. 5

Every action of the animal economy occasions an expenditure of some portion of its living organism; the atoms so expended undergoing important chemical changes; and either becoming directly excrementitious, or ministering, in the first instance, to the formation of secretions, which have ulterior uses to subserve,-or being resolved into nitrogenised matter, which becomes effete, and carbonaceous matter, which ministers to the demands of respiration.

Muscular contraction affects and alters the relative position and form of the fibres, in regard to one another, and in regard to the surrounding parts. The length of the muscle is diminished, and its width is increased; and a large amount of pressure is thereby exerted on the interstitial spaces between the fibres, and on the structures immediately around them. The mechanical effect of this pressure on all the capillary vessels, so liberally, minutely, and universally interspersed and distributed through all the tissues and structures,-constituting in fact a very large proportion of their substance, and filled with a fluid which is impelled a tergo by the action of the heart, and the contractile or elastic action of the larger arteries, to move in one direction, viz. from the heart to the minutest ramifications of the arterial system, where these infinitely small branches terminate in the veins,-must be to aid in driving the current of blood onward more rapidly in its destined course, to quicken the action of the vessels of the part by the extra stimulus of the more rapid current of blood, and to increase the amount of whatever

vital processes are accomplished in the respective structures by the passage of the blood through them, and by the chemical alterations in the blood which occur in its transmission through and from the arterial into the venous capillaries; these changes embracing the disengagement of certain of the organisable atoms from the current of the circulation, and their deposition and conversion into such portions of the organism as require additions or repair, from the expenditure that is consequent upon every action and process of the economy,-the removal of the expended atoms,—the formation of any required new combinations of their elements, or of the elements of the blood itself, for ulterior uses in the system (secretion), the taking away of such effete matters as have, as soon as possible, to be removed and conveyed out of the body (excretion), and the conversion of the arterial into venous blood, involving the combination of oxygen (obtained by respiration, and carried to the capillaries in the red particles of the blood) with the carbon (obtained from the destruction of tissue, or directly from the food), and the proportional evolution of heat. It follows from this, that, among the necessary results of muscular exercise, the powers of the nervous system are by so much expended, the expenditure of nitrogen is by so much increased,―the circulation of the blood is by so much quickened, or at least rendered more energetic, the secretions and excretions are promoted, the waste and expenditure of the system in these ways largely added to,-and the animal heat

EFFECT OF EXERCISE ON NUTRITION.

7

augmented; and all this, on intelligible and admitted principles.

The muscular fibres made use of are increased in size and power, in some degree of proportion to the amount of use made of them; all the vital processes going on in their tissue more rapidly, as well those of production as those of expenditure. And not only are the organic processes performed more quickly and vigorously in the tissue of the moving fibres themselves, or in the structures that are in their immediate vicinity; but all the processes of the economy, remote as well as contiguous,-the power and quickness of the general circulation of the blood; the equal distribution of the blood throughout the system; the amount of secretion and excretion from it,—are, as has been said, all added to. And thus, the vigorous action of the secreting organs of the stomach, and the consequent healthiness of the gastric and peptic secretions,-thus, the due solution and chymification of the food,-thus, the character and amount of the hepatic secretion,-thus, the healthiness and amount of mucus secreted from the whole lining membrane of the food and air passages, -thus, the separation of the chyle from the chyme, —thus, the conveyance of the chyle, by means of the lacteals and thoracic duct, to the subclavian vein, and the compensation, by the addition of newlyorganised atoms, for the expenditure of the system, -all come to be indirectly promoted by muscular exercise of the trunk and extremities. In the same way, the action of the skin, the kidneys, and other

emunctories of the body, is promoted by the quickened and more efficient passage of blood through their several textures; and muscular exercise is made to subserve, in a large degree, not only the expenditure and reproduction of the moving apparatus itself, but the wear and tear, and renewal, of every part of the system. But even this is not all the good effect of muscular exercise; for, as a consequence of this, the circulation of the blood being equalised, partial distributions of it and local congestions are prevented. This effect is very much promoted by the effect of exercise in determining the blood to the whole of the surface of the body,-independently of the ulterior and important influence of the augmented secretion from the skin on the expenditure of the system. It is needful to add to all that has been stated, that, by the increased rapidity with which the blood is made to circulate through the vessels of the brain, the tone and capabilities of the mind itself, by virtue of the marvellous and inexplicable connection and mutual dependance of mind and body, may be said to be increased; and, if this be aided by the will and effort of the individual, muscular exercise may be considered to play some part in quickening and strengthening even the faculties of the mind, or at least their capabilities of sustained exertion; and, even without any effort on the part of the individual, the effect of muscular exercise on the physical condition of the brain, leads very notoriously to the removal of such mental gloom and despondency, as, in so large a proportion of the cases in which they