by a whole associated group. At times, however, associated movements are less intimately connected, and it is possible, by a great effort of the will and by continual practice, to learn to dissociate two movements which are ordinarily in combination.

V.

We have now considered the organs of movement. The nerves, the spinal cord, and the brain are so many instruments of transmission interposed between the will and the muscles.

The brain, with its motor centres, may be compared to a kind of keyboard, each key of which corresponds to a certain group of muscles, and on which the will strikes with more or less force, according as it wishes to produce a more or less energetic muscular effort.

How is the communication established between the will, a force of the psychical order, and a material substance like the grey matter of the cerebral convolutions?

This is a problem which is concerned in that of the relation between physical and moral phenomena, and which has not yet been solved. But, whatever may be its mode of action, the will is one of the most important factors in the performance of movements, one of the most active of the forces concerned in muscular work.

Muscle possesses a motor force, but an outside agent must intervene that this force may come into action.

Similarly, a bow contains a specific energy, capable of discharging an arrow, but an archer is needed to bring into play the elastic force of the wood. The will is just as necessary to bring the muscles into action as the arm of the archer to bend the bow and dispatch the shaft.

The will is the exciting cause of the movement, and the movement is always produced with a vigour proportional to that of the exciting cause. A muscle will remain inert if we endeavour to make it contract by means of too feeble an electric current; it will similarly be incapable of action if stimulated by a will without energy. Do we not see vigorous men lose all at once

their muscular power when their will is paralysed by a depressing emotion, such as fear? An exciting passion, like anger, on the other hand, increases muscular power, because it stimulates the will.

Thus is explained the great difference between the capabilities for work of two equally muscular persons. One, better gifted in the matter of will, can produce from his muscles a force which the other leaves in them, as it were, latent.

The will has no direct action on muscles any more than it has on the spinal cord or on the motor nerves. It can only act directly on the grey matter of the cerebral convolutions. It is powerless to bring the muscular fibres into action without the help of the brain, the only organ with which it is in immediate relation.

The brain is then as indispensable an organ for the performance of voluntary movements as for the accomplishment of mental work, and we must not attribute exclusively to intellectual occupations the privilege of making this organ work. Bodily exercises bring it into action every time they call for the intervention of the will.

By what mechanism is an order of the will transmitted to the muscles through the conducting fibres of the brain, the spinal cord, and the motor nerves?

It is now admitted that volition, or the act of will, produces a molecular disturbance in the cells of the grey matter, and that this disturbance, passing along the nerve fibres, is communicated by their means to the muscular fibres. This molecular movement has been compared to the wave produced on the surface of smooth water, which gradually invades the liquid sheet, as soon as one single point of the whole mass has been disturbed by a blow.

The production of an undulatory movement has not been actually demonstrated in the cerebral substance, the spinal cord, or the nerves: it is simply a very probable hypothesis.

On the other hand it has been very plainly seen in

muscle. "It has been observed in living muscle, that at stimulated points swellings or nodes are formed, which then run along the whole length of the muscle like a wave on the surface of water." *

By means of ingeniously constructed registering apparatus, Marey has been able to take a graphic tracing of the muscular wave. Aéby had already shown in 1862 that in living animals the swelling produced at the point stimulated, travelled to the ends of the muscle with a velocity of about one metre per second.

Every stimulus received by a muscle gives a shock to this organ which is transmitted by a muscular wave. If the stimuli follow each other quickly, it may happen that the first wave is still going on when the second begins. Then we see the two swellings run one after the other over the surface of the stimulated muscle. If the stimuli are repeated very quickly, the muscular waves are no longer seen, and one swelling occupies the whole muscle. This is then thickened and shortened uniformly it is in a state of contraction.

Between the moment when the will orders a contraction, and the moment when the muscle contracts, there always elapses an appreciable interval of time. This time is occupied by various physiological actions, and in the first place by the transmission of the nervous vibration. The disturbance of the cerebral cells does not instantaneously reach the muscle. It has first to traverse the fibres of the white matter of the brain, then the spinal cord, and then the whole length of the nervous filament which passes to the muscular fibre. The length of this course may be estimated in centimetres, and we know from the experiments of Helmholtz that the nervous vibration is propagated with a velocity of about 35 metres per second. It is then easy to calculate from these data, how many hundredths of a second must, for example, elapse, from the moment when a man wills to flex his foot, and the moment when the foot begins to move.

* Marey, La Machine animale.

But if we make this calculation exactly and then compare the results with those given by direct observation, we discover that the contraction of the muscle is retarded. An appreciable interval separates the instant at which the stimulus of the will reaches the muscle, and the instant at which the latter responds by a contraction.

This period, during which the already stimulated muscle has not yet begun to contract, is known as the period of latent contraction.

The period of latent contraction is not always of the same duration. Many circumstances may cause a variation, but increased intensity of the stimulus received by the muscle is the condition which most efficiently shortens the latent period. To a feeble stimulus, the muscle responds slowly, lazily; an energetic shock on the other hand is followed by a prompt contraction.

It was established by Helmholtz as a physiological law, that, the length of the latent period is in inverse proportion to the intensity of the stimulus received by the muscle.

When the will orders a muscle to contract, the latter obeys the more promptly, the more violent the nervous disturbance which transmits the order.

We shall see later what we shall deduce from these physiological data for the explanation of the great waste of nervous energy brought about by certain exercises which only represent a moderate expenditure of muscular energy, but which demand an instantaneous obedience of the muscles to the orders of the will: fencing for example.

CHAPTER II.

MOVEMENTS.

Associated Action of Different Regions during Work-How a Blow is given with the Fist-Coordination of Movements; Antagonistic Muscles; the Muscular Sense-Ataxic PatientsStatic Contraction-Stiffness in Exercises-Muscular Education. Association of the great Organic Functions with Muscular Movement-Effort-The Porter and his LoadEffort during Slight Expenditure of Force; too Hard a NutFrequency of Effort in Exercise—Long-Distance and Sprint Running. Influence of Movements on the Circulation-The Quickening of the Pulse; its Mechanism-The Pulmonary Circulation and the Active Congestion of the Lungs-Congestion of the Brain during Movement-Dancing DervishesA Runaway Horse.

I.

THE slightest movement performed by the human machine needs the employment of a great number of wheels. When a muscle contracts, the neighbouring muscles always, and distant ones sometimes, act with it, and are associated in its work.

Let us analyse the phenomena of a very simple

movement.

In order to be able to move the forearm, the arm must be fixed to give a point of application. The arm itself must be supported by the shoulder, and the shoulder by the vertebral column and the thorax. But the thorax and vertebral column being supported by the pelvis, and this by the lower extremities, the whole body is obliged to associate in the movement of the forearm. From head to foot, all the muscles participate in the most insignificant, and most localised work.