Читать книгу The Body at Work: A Treatise on the Principles of Physiology онлайн

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Lymph is the reservoir of nutriment upon which every cell in the body draws. It is improbable that in health and under normal conditions the activity of any organ is ever restricted for want of sufficient food. As food is removed from lymph, it is instantly replaced by fresh food from the blood. There is some evidence—not very clear—that the removal of waste products offers greater difficulty than the renewal of supplies of food. When the activity of muscles has been excessively prolonged they ache. It has been supposed that their unwillingness to do more work is due, not to the exhaustion of the food which they use up when contracting, but to the inadequacy of the lymph and blood to carry off all refuse. This, at least, is the explanation of fatigue which is usually offered, although it is difficult to understand why the arrangements for removing waste products which have worked to perfection for eight hours should during the ninth hour become rapidly ineffective.

If a frog’s muscle, cut out of the body, has been made to contract until it refuses to work any longer, it again responds to stimulation after a solution of salt has been passed through its bloodvessels. The salt-solution brings no food; the only thing it can do is to wash away waste products. But this experiment upon a tired, isolated muscle does not necessarily throw light upon the nature of fatigue in muscles under normal conditions. The isolated muscle is using up, in contracting, food which it has stored. Cut off from the circulation, it has no means of getting rid of the lactic acid and other products into which food is changed. They may well have accumulated to a poisonous extent long before all the food has been used up. Hardly more cogent is the argument based upon the benefit which a tired man experiences from hot baths, massage, and the like. They take away the feeling of tiredness, but it does not follow that this result is due to the removal of waste products. Quickening the circulation of blood brings about renewal of the lymph. Renewal of lymph means fresh supplies of food as well as removal of waste products. Even human muscles are not perfect as machines. They will not work for an unlimited spell. There comes a time when they must have rest. Something goes wrong in the admirable adjustment which has hitherto provided exactly the right amount of food and exactly the necessary freedom from the products of action. A feeling of fatigue is the signal that the apparatus is not in a condition to work longer; but whether this feeling is due to a dislocation of the balance of supply and loss, or to some deterioration of the apparatus which calls for rest and renovation, it is at present impossible to say. It is not due to the exhaustion of muscle food. A more powerful stimulus, the urgency of fright or some other strong emotion, or an electric current applied directly to the muscle or its nerve, will still induce vigorous contraction. The muscles of a hare that has been coursed until it can run no farther still contain glycogen, muscle food.