receive a similar protection. A nerve is surrounded by a fibrous sheath, the neurilemma; the elements forming this protective envelope probably participate in the more energetic nutrition which goes on in all the fibrous tissues in consequence of work; thus, doubtless, may be explained the steady decrease of the pain felt in a region of work. The repeated muscular contractions become less painful because the nerve filaments which traverse the muscle are better protected against the pressures and frictions by a firmer neurilemma.

To sum up, muscular exercise has a considerable influence on the process of nutrition, and it is to this influence that are due the changes which occur in the conformation of a person whose muscles are habitually in action.

The body of a man or an animal, under the influence of a regular exercise progressively increased, is modified in a manner which makes the performance of the work Herein lies the secret of "habituation.”

more easy.

CHAPTER II.

MODIFICATION OF THE ORGANS BY WORK.

Function makes Structure-Disappearance of Organs when their Function has Ceased; Maintenance of Organs by Persistence of Function-Why Gymnasts remain Supple even in Old Age - Modification of Motor Organs by Exercise-Modification of Organic Apparatus associated with Movement; Amplification of the Lungs-Changes in the Living Tissues due to Work— More active Assimilation; Growth of Muscles-More rapid Dissimilation; Diminution of Reserve Materials-Increase of Strength through Growth of the Muscular Tissues-Lessening of Fatigue; It is due to the gradual Disappearance of the Reserve Materials-How this Result is Explained-Fat causes Breathlessness-By what Mechanism? Insufficiency of admitted Explanations-Theory of Trainers: the "Internal Fat.” Objections to this Theory-Practical Observation-Reserve Fat and Constitutional Fat-Fat Runners-Easy Dissimilation of Reserve Fats, causes Breathlessness by Excessive Production of Carbonic Acid-Reserve Proteids and Consecutive Fatigue -Disappearance of Stiffness of Fatigue in Connection with the Disappearance of Urinary Deposits after Exercise-Personal Observations of Fatigue.

I.

THE physiologists say that "function makes 'structure." This means that the human body adapts itself, by changes of conformation, to any frequently repeated action.

It is always difficult to get a clear idea of anything enunciated in a general and abstract form. We will try to give definite form to the idea we have mentioned by an example. Suppose that a man has suffered from a dislocation at the shoulder-joint, the head of the humerus has slipped out of the articular cavity, and is somewhere in the neighbourhood of it. If not reduced, the luxation

will not heal of itself, the head of the bone will not return into the hollow from which it has escaped, the limb will be motionless in the false position due to the accident, and after some months anchylosis occurs. If the surgeon is now called in, it is too late to replace the luxated bone; and he can only give this advice, to move the arm as much as possible in the hope of restoring its powers. In fact by regular daily movement of the arm, its functions may be re-established, and although in an abnormal position, it once more becomes capable of action.

We see patients in whom the head of the humerus has lodged between the clavicle and the upper ribs, an inch away from the normal joint cavity, and who yet recover by daily exercise of the arm, a great part of the movements of the limb. If we have the opportunity of making an autopsy in such a case, we find that the head of the humerus has hollowed out a new cavity for itself at the expense of the clavicle and ribs with which it is in contact. If the dislocation is old, and the patient has done much work with the limb since the accident accurred, the new joint-cavity has all the appearance of a normal one. A synovial membrane, articular cartilages, fibrous capsule, in fact all the elements which form a joint, have developed in an abnormal position.

This is known as a false joint. A joint, an organ indispensable to movement, can then be created by movement itself.

Thus "function makes structure." But there is a corollary to this law, which may be thus formulated, "the cessation of the function leads to the disappearance of the structure."

In the case just quoted for instance, the articular cavity from which the head of the humerus has escaped, being no longer the seat of movement, soon loses its shape and ordinary structure; the synovia which served to lubricate the surfaces of the joint, being no longer of use, is no longer secreted; the synovial membrane itself disappears; further, the articular cartilages are gradually

replaced by bone, and after a time the whole articular cavity fills up and disappears. The cessation of function has caused the disappearance of the structure. The law of the intimate connection of the existence of an organ with that of its function is nowhere more evident than in muscular work. It not only applies to the formation of a new organ by a new function, but also to the improvement of an already existing organ by the fact of more frequent use.

The phenomena observed in a man who performs regular muscular work fully bears out the law we have just enunciated. Muscular work tends to modify the nutrition of all the motor organs and to give to them a structure which is favourable to the performance of movements. If we review all the organs which concur in the performance of work, we see that all are subject to this physiological law of adaptation to function, or, in other words, of improvement by work. We also see, by observing the converse phenomena, that defective function of an organ leads to its decline and to arrest of its development.

Muscles increase in size through work, at the same time as their fibres become free from any clogging materials, and are freed from the fat which hinders contraction. Repose, on the other hand, causes atrophy of the muscular tissue, and the muscle which remains inactive too long becomes infiltrated with fat.

The joints are the parts of the body whose perfect action is of the utmost importance for the proper performance of movement. And there are none which feel more fully the influence of muscular exercise. To learn this we must compare a joint which has long been motionless with one which has undergone repeated movements. That which has been much at work has acquired a marvellous ease of movement; that which has remained inactive becomes in the end anchylosed, that is, the bones forming the joint become ossified together. Exercise of a limb preserves the mobility of its joints, and it is for this reason that gymnasts preserve in old age supple movements and youthful attitudes.

Age tends to cause the deposit of calcareous salts in all the tissues of the economy; the arteries of the old man are hard and deficient in elasticity; his fibrous tissues tend to become indurated, and his ligaments are more and more ossified. But continual movement of a joint opposes the tendency to calcareous degeneration; work renders anchylosis and calcareous degeneration of the fibrous tissues impossible; as long as a man keeps his muscles at work he remains able to use his limbs. The persistence of function preserves the integrity of the organ.

The internal organs, under the influence of muscular exercise, also undergo changes which favour the performance of the often-repeated action.

The lungs, the air-cells of which are brought more into play by a more active respiration, expand, and push outwards in all directions the osseous walls of their prison; the thorax expands, the ribs are raised, and the chest assumes a very characteristic convex shape. All professional gymnasts show a kind of vaulting of the part of the chest corresponding to the upper ribs and to the clavicle. Measurements of young soldiers at the Gymnasium at Joinville have been taken, and in a few months an increase has been noted of several centimetres in the circumference of the chest under the influence of muscular exercise.

It is easy to understand how respiration must be facilitated by this increase in the size of the thorax. A much greater volume of air is introduced into the lungs, and the elimination of the respiratory waste-products takes place over a much larger field, breathlessness during exercise diminishes.

The heart likewise undergoes a change in size and structure. Its muscular fibres become larger, and the whole tissue becomes firmer and denser; it frees itself from the fat which oppressed it, and diminished the elasticity of its fibres. This change favours the performance of exercise, for a vigorous heart drives the blood more energetically, and makes it traverse the capillaries without difficulty. The more energetic