We could quote numerous facts showing the serious nature of the accidents resulting from the absorption of this product, the exact nature of which is unknown, but which is extremely poisonous-the human miasma.

Very serious symptoms, and even fatal results have often been observed in persons who have spent a long time in a confined space. These effects of "stuffy air are not due to asphyxia, but to a direct poisoning by the human miasma.

Brown-Sequard and D'Arsonval, in a communication made to the Academy of Sciences, on January 10th, 1888, showed that human breath contains a most active poison, an alkaloid capable of killing in two hours an animal into which it is injected.

CHAPTER I.

LOCAL FATIGUE.

Experimental Fatigue-Absolute and Relative Fatigue-Fatigue in Ordinary Conditions of Work; it is always Relative-Examples of Relative and of Absolute Fatigue-Causes of Sensation of Fatigue-Causes of Muscular PowerlessnessInfluence of Waste-Products of Combustion; Transmission of Fatigue to Muscles which have not been Working-Use of Fatigue-Part played by the Brain in Fatigue. Unconscious Movements cause less Fatigue than Voluntary Movements; Practical consequences.

I.

IF we isolate one of the muscles of a living animal and pass an electric current through it, we see that it contracts as long as the passage of the current lasts. But if the experiment is prolonged, the muscle after a time contracts more feebly; a little later there comes a stage when the muscle does not contract at all: it is fatigued.

Fatigue is at first only relative, and the muscle will contract afresh if stimulated by a current stronger than the first. But there comes a time when fatigue is absolute, that is to say the muscle has lost the property of contracting under the influence of the most powerful electric stimuli.

A human muscle never reaches, in consequence of work, the condition of absolute fatigue, of complete powerlessness, which we observe in animals under experiment. What prevents this is the painful sensation experienced by the man before the time when the muscle becomes absolutely incapable of action. Under the influence of the suffering which the contraction causes, the work is

stopped and the muscle rests. Here is the capital difference between true absolute fatigue, such as can be produced by experiment on animals, and the fatigue observed clinically in a man at work.

That which dominates in the fatigue of a man performing any exercise is the subjective element, the painful sensation which prevents him from continuing his work until the muscle becomes absolutely exhausted. We can represent the effort made by a powerful man to carry his exercise to the last possible limits, as a combat between the will which commands and the sensibility which rebels.

The most energetic will is unable to use up the contractile power of a muscle as completely as do mechanical or physical agents. When a fatigued man ceases the effort which he has long been making, we say that his muscles are exhausted: this is not yet so. The proof of this is as follows :—

We know that one of the most tiring attitudes to assume is that which consists in holding the arm horizontally outstretched. The deltoid muscle in this case does most of the work. There are few men vigorous enough to be able to hold out an arm in this manner for more than five or six minutes. At the end of this time the deltoid can act no longer and the arm drops. But the muscle is not exhausted: its fibres still possess a great contractile force, and this is proved by the fact that certain agents, such as electricity, can bring into play this motor force over which the will has no longer any action. If, in a man who has been holding his arm outstretched, we wait till the sensation of fatigue becomes intolerable, and if, at the moment when the man declares he has used up all his power, and is about to let his arm fall, we apply a strong electric stimulus to the deltoid muscle, the fatigue seems to vanish, and the arm remains outstretched; the muscle had not yet lost its contractile power.

What is the cause of this local fatigue? A double answer is needed to this question; we must say why the work renders muscular contraction painful in a tired