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 be comes 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.

At first sight we are disposed to think that the will, like other muscle-stimuli, acts directly on the motor organ.

Willing and doing seem sɔ intimately connected with each other, that they appear to fuse. At the slightest command our hand seizes an object, places or displaces it, and obeys with such punctuality and quickness, that the will seems to stimulate the muscles directly. But it is not so, and the faculty of will needs, for the transmission of its orders, a very complicated mechanism of intermediate organs, without which its action is ineffectual.

These intermediate organs are the nerves, the spinal cord, and the brain.

If the nerves of the arm are cut, the most energetic will in vain exhausts itself in the endeavour to move the limb; the muscles contract no longer.

It is generally stated that the section of motor nerves paralyses muscles. The expression is inaccurate; these muscles have not lost the power of contraction, but they are removed from the influence of the will, and receive its orders no longer. Acted on by other stimuli, they would still contract and move the bones to which they are attached. If we galvanise these muscles which seem paralysed, if indeed we simply pinch them strongly, we produce contractions and movements.

Section of the spinal cord, lesion of the brain, have the similar result of putting the muscles out of reach of the will, without, for all that, destroying their contractility.

Contractility is a force inherent in muscle, and is not supplied to it by its motor nerve. If we carefully destroy all the nervous filaments going to a muscle, the latter, reduced to its own elements, nevertheless retains the power of contracting under the influence of a stimulus.

A muscle has an individuality and a power peculiar to itself, independent of any nervous action.

If a muscle be detached from the leg of a recently killed dog, this muscle, thus isolated, being no more than

a fragment of the body of the animal, may be made to do work. If we attach the muscle by a nail at one end, and fasten to the other end a hanging weight, it is enough, the muscle being in this manner stretched, strongly to pinch the fibres to cause the muscle to contract and to raise the weight.

Muscle has great vital energy, and long retains the power of action, provided that it receives a sufficient stimulus. Thus in many cases the loss of power of action shown by a fatigued man must not be attributed to the muscular system. Almost always, in the ordinary course of life, it is the will-the stimulus of muscular contraction-which first gives out, long before the muscle has lost its contractile powers, under the influence of prolonged work.

II.

We may compare motor nerves to the wires which conduct electricity from an electro-motor to a receiving apparatus. They convey to the muscles the stimuli emanating from the brain. They convey also all stimuli which can come from outside agents. A pinch, an electric shock, the contact of an acid, can throw the muscle into action by the mediation of the nerve. If a nerve be electrified the effect produced on the muscle to which this nerve is distributed will be identical with that which would be produced by electrifying the muscle directly.

The will needs the help of the nerves to transmit to the muscles the orders to act. In the most vigorous and energetic man, it suffices to cut one of these slender little filaments, to see the muscles supplied by it become inert. The will then exhausts itself in useless efforts, and gives in vain repeated orders. Its call is unheard. Similarly the breaking of the wires between two telegraphic stations, renders all communication impossible.

The nerves have not in themselves any power of producing movements Their function is merely to transmit to the muscles the stimuli which bring their vital properties into play.

We must mention, however, that according to certain physiologists, a nerve has, besides the power of conducting a stimulus received by it, the further power of reinforcing that stimulus.

According to Pflüger, when a nerve is stimulated, whether by a mechanical shock, an electrical discharge, or by the action of the will, a phenomenon occurs which this physiologist calls the nervous avalanche. Just as a lump of snow detached from a mountain top grows as it descends the snowy slope, and when it reaches the valley is of larger size than when it set out, so the stimulus received by the nerve is amplified in its passage through the conducting filament, and is much more intense when it reaches the muscle than it was when first produced.

The nerve would then be a reinforcing as well as a conducting apparatus: it would increase the intensity of the stimuli which it transmits, as the microphone increases the intensity of the sounds which pass through it.

If Pflüger's theory is correct, and if the nerve really has the power of amplifying the stimuli which it conveys to the muscle, we may believe that this power is developed by exercise, like all the physiological functions of working organs. The motor nerves of a man who devotes himself to bodily exercise should then become more capable of reinforcing the voluntary stimuli.

This property could be a partial cause of the sometimes surprising increase of power displayed by trained men, which cannot always be explained by an increase of the muscular tissues; it would render it possible to produce, with a moderate effort of will, a more intense stunulation of the motor fibre, and consequently a more energetic contraction.

Of all nervous tissues the nerves have the simplest structure, for they have only one fundamental tissue, the white matter. This is made up of elongated elements in the form of hollow fibres, or tubes in which is seen, with the microscope, a kind of filament called the axis-cylinder. At the point where the motor nerve is distributed to a muscle, the axis-cylinder ends in a disc.