there is a close analogy between the effects of mental fatigue and those of muscular fatigue. This first conclusion should be already enough to render us very circumspect in the application of physical exercise to persons suffering from mental overwork.

But if we descend to details, if we make a summary analysis of the chief exercises in general use in our own time, we shall find that the analogy between mental work and bodily exercise becomes more and more striking. In the difficult movements of gymnastics, in riding, and in fencing, we shall see that the office of the brain and the nerves is as important as that of the muscles.

CHAPTER III.

EXCITO-MOTOR WORK.

Necessary association of the Nerve-Cell and the Muscular Fibre in Movements-Origin of Motor Stimuli-Nerve Centres-The Spinal Cord a Centre of Unconscious Movements, the Brain a Centre of Voluntary Movements-Office of the Grey Matter of the Brain-The Dog of Professor Goltz-A counter-proof: the Observation of Dr. Luys-Muscular Work and Nervous Work in Voluntary Movements-Frequent Disproportion between the Effort of Will and the Muscular Exertion-Conditions which make the Relation vary between the Expenditure of Nervous Energy and the Mechanical Work of the Muscles-Diminution of Muscular Irritability—Muscular Fatigue.

I.

THE intimate relation which exists between the brain, the organ of thought, and muscle, the instrument of movement, is not as a rule sufficiently recognised. We wish to show here how the cerebral cell is intimately associated with the activity of the muscular fibre, and how the intellectual faculties are far from remaining inactive during the performance of various gymnastic movements which are nowadays in high favour.

To understand the importance of the part which work of the brain may play in a bodily exercise we must first get an exact idea of the organic apparatus by the aid of which movements are performed. This apparatus is essentially made up by: 1 Nerve-centres in which the motor stimuli are elaborated; 2 Conducting organs charged with the transmission of these stimuli: the motor nerves; 3 Organs whose office it is to respond to the stimuli emanating from the nervecentres, and to perform movements: the muscles.

To these organic agents of movement we must add

another, as unknown in its essence as it is indispensable for the performance of conscious muscular actions: the Will.

The Will orders and the muscles execute: but it is important to understand that the principal agent of movement has no direct relation with its subaltern. The will needs, for the transmission of its orders to the muscle, the whole complicated chain of nerve-centres and nerves. When we wish to move the foot, the order of the will sets out from the grey matter of the brain, passes down the spinal cord, and along the nerves of the leg and thigh. It is only after it has traversed this long succession of cells and nerve-fibres, that the vibration produced by the impulse of the will finally reaches the muscular fibres and determines their contraction. in this course the nervous impulse finds an interruption in the continuity of the conducting tissues; if the spinal cord or the motor nerve are cut, the stimulus stops at the point of lesion, and fails to reach its destination: the muscle does not act notwithstanding the exertion of the will, for it never hears the call. Thus are explained the paralyses of movement which follow lesions of the spinal cord or of the motor nerves.

If

The will has then no direct action on muscle-nor has it any such action on the motor nerves, nor on the spinal cord.

But, on the other hand, muscle has no power of its own and cannot act spontaneously. The force contained in its fibres is latent, and resembles that of gunpowder. The gunpowder will not detonate without a spark; the muscle will not contract without a nervous stimulus. A work of nervous stimulation must then precede the work of the muscle.

Experimentally we may replace the nervous agent, the natural stimulus of muscle, by artificial stimuli, of which the one in commonest use in physiology is electricity. The phenomena which occur on electrifying the organs of movement are quite similar to those of voluntary contraction, and we have here a valuable analogy which allows us to study precisely the work of

the muscles. We shall have to appeal to this remarkable analogy for the explanation of certain phenomena of bodily exercises.

In the living man the stimuli which cause the muscles to act come from the nerve-centres, that is from certain parts of the nervous substance which are endowed with a peculiar energy, and which have no need to borrow their power from any other part of the system. There exist two nerve-centres for the muscles of animal life: these are the Spinal cord and the Brain.

on.

The Spinal cord is the centre for reflex stimuli and unconscious actions: we shall speak of its office further The Brain is the organ in which the stimuli sent by the will to the muscles arise. It is solely from this organ that the orders transmitted to the muscles by the nerve-fibres set out.

The will acts only through the brain, and especially on the thin layer of grey matter which forms its outer surface and which is the essential organ of thought as well as the indispensable instrument of motor stimuli.

Curious experiments have shown that the removal of the grey matter of the brain leads to the abolition of all voluntary action, without however necessarily causing death. Professor Goltz in 1881, brought from Strasburg to the congress in London a dog which he had been able to keep alive after having removed all the brainsubstance. The animal was no longer capable of making any voluntary movement. Like an automaton, it walked straight forwards without ever turning aside, without seeking to avoid obstacles placed in its way, against which it struck, though its visual faculties were intact. Its muscles had not lost the faculty of action, but they were no longer directed by the will, and were no longer under the influence of external stimuli; they only performed reflex movements, or actions which habit had rendered automatic.

By the side of this experiment of Goltz showing the abolition of voluntary movements when the cerebral cortex has been removed, we may quote another observation which is no less curious, and which shows by a

sort of counterproof that the grey matter of the brain atrophies when its movements are abolished.

"I have been able to demonstrate," says Luys,* "that in persons who had undergone amputations at a distant date, subjects who had been long deprived of an upper limb, for instance, there existed certain long disused portions of the brain, coincident, very distinctly localized atrophies. I have, moreover, demonstrated that the atrophied regions of the brain are not the same in the case of the amputation of a leg, and in that of the amputation of the upper limbs."

To understand the value of this observation we must recall the fact that inactivity of an organ is always followed by its atrophy. If then the disappearance of certain muscular movements through the removal of a limb leads to the "silence," and in consequence to the atrophy, of certain regions of the brain, this is clearly because the functions of that organ are intimately associated with those of the muscles; the brain works when the body acts.

II.

Thus it is within the grey matter of the brain that the nervous work which precedes, provokes and accompanies all voluntary muscular actions is produced.

If we have succeeded in expressing our ideas clearly, the reader will now understand that every voluntary movement involves a double expenditure of force, or in other words, a double work: a work of the muscle which contracts, and of the brain which excites the contraction.

The work due to the muscular contraction is apparent, outwardly visible, and may be measured by the dyna

mometer.

The work due to the excitement of the motor cells is a hidden work, which cannot be determined de visu and can have no common measure with muscular work, because its nature is not mechanical, but physiological.

* Luys. Trench & Co.

The Brain and its Functions, p. 53. Kegan Paul,