of "internal fat" which disappears, and of" external fat" which remains. Fat is equally distributed throughout the system; and if any cause, such as work, brings about a diminution in its quantity, the parts which work most are earliest free from it.

When we see fat men take to fencing in order to get thin, we notice that in them the internal fat is far from being the first to disappear; the abdomen is the region which retains with most tenacity its supply of fat. There can be nothing more awkward than the shape of the body of a man in this ungrateful period when the fat has disappeared from his arms, his chest and his legs, while his belly remains as large as ever. The regions which have been made to work in fencing have lost their fat; the arms and legs appear slender, and the chest, which has been freed by the work of the pectoral muscles, appears shrunken by comparison with the abdomen which has remained of its original size. It is only by continuing his exercise for some weeks still, that the fat man is able to gain what he above all desires, the diminution in the size of his belly. Now the respiration has become easy and breathlessness has diminished long before the disappearance of the fatty masses of the abdomen, the persistence of which must singularly hinder the vertical enlargement of the thorax.

The diminution of breathlessness in the trained man or horse, is much less due to the greater freedom of the lungs owing to the removal of the fat in their neighbourhood, than to the lessened production of carbonic acid which results from the disappearance of the provisions of combustible tissue which give rise to too great a quantity of this gas.

Persons who practise running very soon gain the power of not losing breath, for running is the exercise which most quickly causes the disappearance of the fatty tissues.

The combustion of the fatty tissues during work gives a satisfactory explanation why the practice of muscular exercise lessens the tendency of a man to become

breathless. The man in training has no longer in his system the elements capable of producing this excessive quantity of carbonic acid, as was the case before he began regular exercise. The disappearance of the tissues rich in hydrogen, explains also the lessened perspiration of the man in training. Sweat mostly consists of water, and the burning of a substance rich in hydrogen must tend to cause the formation of an excess of aqueous secretion.

Breathlessness is not the only form of fatigue, just as fats are not the only reserve materials. There are, produced by work, nitrogenous products of combustion which cannot be derived from fatty substances. There are, among the reserve materials, proteid substances, and it is from them that the nitrogenous substances excreted by the urine are derived, which diminish when the organism has for some time assiduously practised

muscular exercise.

These substances play the same part in the production of stiffness as belongs to fats in the production of breathlessness. Fat, in being burned, gives rise to carbonic acid and other products rich in carbon and hydrogen; proteids, in being burned, produce a whole series of nitrogenous compounds of which uric acid and the various extractives are the type. We believe—and we hope that we have proved it by sufficiently cogent arguments that the nitrogenous waste-products of combustion which are formed during work, and which stay sometimes in the blood when work is done, are the cause of the general disturbances, febrile or non-febrile, which constitute consecutive fatigue or stiffness.

*

In the man accustomed to work there are no disorders following exercise. This is one of the most remarkable results of training, and would be quite inexplicable unless we were to admit that daily exercise causes the disappearance from the body of the organic substances to which the phenomena of consecutive fatigue are due.

* See pages 111 et seq.

We believe that these substances are the nitrogenous reserves contained in muscle. We think that these nitrogenous reserves are contained in the muscles themselves; observation shows that men accustomed to one definite kind of exercise have no immunity from fatigue, and can experience the general effects of stiffness, if they suddenly take to another exercise, bringing into play muscles which have not yet been modified by work and which have not yet lost their reserve materials.

Exercise modifies the muscle then not only by increasing the size of its elements, but also by changing its structure, by eliminating from the organ not only fat, but also the nitrogenous substances capable of giving rise to superabundant waste-products of combustion, which cause auto-intoxication and general consecutive fatigue.

Thus the more we analyse the phenomena of work, the more we see that the frequent repetition of muscular movement produces, in the nutrition of the living tissues, material changes which are able to shelter the system from the various disorders of fatigue.

CHAPTER III.

MODIFICATION OF FUNCTIONS BY WORK.

Increase in the Contractile Force of Muscle-Probable increase in the Power of Conduction of Nerves-Improvement in Faculties of Co-ordination of Movement-Education of the Muscular Sense-Domination of Reflex Actions by the Will; Regulation of the Respiratory Movements—Changes produced in the Nervous System by Muscular Exercise-Material Changes in the Nervous Tissue: Are they purely hypothetical?—An_observation of Luys - Functional Changes in the Nervous System— The Memory of the Spinal Cord; its Use in the Performance of often Repeated Actions Psychical Modifications due to Habituation to Work - Skill-Physical Courage - Incredible Energy of Prize-fighters.

WE have studied the material changes which take place in the system in consequence of work; we have just scen how exercise, powerfully influencing the process of nutrition, has the power of changing the organs and of profoundly altering the structure of the tissues of the body. A man used to work shows important peculiarities in his external conformation, and has further very characteristic ones in his internal structure. All the wheels of the human machine have been gradually adapted to the performance of the great quantity of work daily demanded from them, and have undergone a material improvement which renders them more fit for their function.

Under the influence of exercise man is physically transformed, and we may sum up in a few words the changes which have taken place in his system, in passing from an inactive mode of life to habits of work. We may say that all parts of the organism capable of favouring the performance of work have been developed, and all the materials which could be a cause of hindrance in the performance of movements have been diminished in quantity and tend to disappear.

From these two orders of organic modifications there result two different powers acquired by habituation to work power of producing more energetic movements, owing to the greater development of the motor organs; power of better enduring prolonged muscular efforts, owing to the disappearance of the reserve materials, the products of the dissimilation of which, when too abundant, lead to auto-intoxication of the body, the most important cause of the phenomena of fatigue.

The material changes which take place in the human body when work is regularly performed, can explain in great part the increase in strength and in power of resisting fatigue. But we should have an incomplete idea of the benefits due to habituation to work if we did not take into consideration another advantage gained by a man who exercises his muscles; it is the improvement in all the functions which take part, directly or indirectly, in the performance of work.

I.

Under the influence of well-planned daily exercise the muscles do not only become larger and firmer, their power of contraction also increases. "The muscles of a man in training contract with extraordinary force under the influence of the electric current * said RoyerCollard many years ago in his Paper on the training of English prize-fighters. Muscular fibre gains by exercise an increase of its power of contraction and can obey more vigorously the orders of the will, as well as the stimuli from an electric battery. We observe that two muscles being of equal size, the muscle which is in the . habit of contracting is stronger than the one which has long remained inactive.

The same improvement is observed in the respiratory function under the influence of work. The lungs have not only been increased in volume by exercise; they have further gained a greater power of performing their movements with calmness and regularity amidst the violent disturbances in the system which are caused by

* Royer-Collard. Paper read before the Academy, 1842.