CHAPTER I. THE ORGANS OF MOVEMENT. Exercise and Work—Muscle-Nerve: Avalanche Theory. The By bodily exercise, we mean work done with the object of perfecting the human organism from the point of view of strength, skill, or health. Scientifically speaking, there is no difference between the professional labour which circumstances demand from the peasant or workman, and the more or less refined exercise to which a sportman devotes himself. The manual labourer who chops wood, and the gentleman who fences, both perform muscular work. But the gentleman has his exercise at his own hours, regulates to his own taste the time he allots to it, following the calls of hygiene, diet and rest, while the poor man works too much, feeds badly, and sleeps little. This is why work wears out the one, while exercise strengthens the other. But what the workman does of necessity, the man enamoured of violent exercises can do by excessive ardour. In the two cases the result is the same, and the abuse of athletic exercise causes exhaustion and overwork as surely as does excessive labour. Bodily exercise and labour are then synonymous from the physiological point of view, and we shall blend them in our study by referring them to their fundamental principle, muscular contraction. I. The immediate agents in movement are the muscles, bundles of reddish fibres which collectively form the fleshy masses surrounding the different parts of the skeleton. The muscles form, by weight, more than half of the human body. Hence the importance of muscular exercise in modifying nutrition. Work in fact, changes profoundly the physiological condition and the chemical composition of muscles, and many exercises cause all the muscular regions of the body to work at once. We can understand that the whole system associates in the modifications produced in so important a member of the living tissues. The muscular tissues of the body are divided into larger or smaller bundles of fibres, which are generally of an elongated shape, and have two extremities, each usually ending in a tendon attached to a bone. Each of these masses forms a muscle, and every muscle consists of secondary fasciculi. Finally, these secondary fasciculi may be split up into primitive fibres, the fundamental elements of the organ. The primitive muscle-fibres are essentially made up of a kind of membranous sheath called the sarcolemma, enclosing the muscular juice. The muscular juice or plasma is quite fluid at a low temperature. Kühn gives us a curious proof of this: he saw a parasitic worm swimming in a lively manner in the interior of a primitive fibre. But we can only prove the fluidity of the plasma by the considerable degree of cold which congeals the other constituents of muscle. To see it in a liquid state it must be observed above 3° C. When cooled, it already tends to coagulate at o° C., and when heated up to 45° C. it suddenly becomes solid. The plasma does not only coagulate under the in fluence of heat. It also tends to solidify when treated with certain acids, notably lactic acid, which is formed. in muscles in action. We shall see, in discussing the phenomena of fatigue, how important a part in acute overwork is played by the coagulation of the muscle-plasma under the influence of excessive heat and of the numerous acid products which develop in the over-driven muscle. Muscles possess the property of contraction, that is to say, of shortening, and bringing their extremities nearer to each other, after the manner in which a stretched caoutchouc cord returns to its former size. When a muscle contracts it exercises a tension on the bones to which it is attached. Thanks to the varied effects of levers, pulleys, pivots, etc., of which the joints are made up, the fundamental movement of traction is very variously transformed, and the limbs are flexed, extended, turned and returned in all directions. The muscles are charged with the performance of movements, but they cannot bring them about independently, without the assistance of an agent which throws them into contraction. The contractile force of muscles is a latent energy comparable to that of gunpowder, which cannot explode without a spark. A muscle left to itself remains inert, and cannot arise from its inaction, from its repose, unless made to do so by some stimulus. The stimulus most commonly employed is the will, but many other agents can bring into play the contractile properties of muscle. Any mechanical, physical, or chemical action on a muscle, a blow, a pinch, an electric discharge, the contact of a strong acid, etc., can play the part of a stimulus, and cause contractions and movements. In order to bring into play the irritability of a muscle, the property which causes the organ to contract when stimulated, it is sufficient to apply the stimulus directly to the muscular fibre. Thus, in an animal just killed, it is enough to expose a muscle and pinch its fibres strongly, and the muscle is seen to contract, and to move the bones to which it is attached. |