Page:Popular Science Monthly Volume 36.djvu/538

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OW is it that the lungs can increase in size through athletic exercise? By a mechanism well known in physiology, by the filling out of certain air-cells ordinarily inactive, which only come into play during forced inspiration. The expansion of the pulmonary vesicles is complete in proportion to the quantity of air introduced. The atmospheric air drawn into the lungs by a very powerful inspiration seeks out the most obscure corners, and inflates the air-cells of certain regions which ordinarily have no part in the respiratory function.

A definite increase in the volume of the lungs is the consequence of frequent repetition of this supplemental respiration. The air-cells which are as a rule inactive, and which are reserved for cases of excessive respiratory strain, arise from their inaction; their walls, which are usually collapsed, and even stuck together, separate and give entrance to the air which can not find room in the confined space sufficient for ordinary breathing.

If the forced inspirations are often repeated, the air-cells, the action of which has thus been accidentally solicited, come in the end to associate regularly in the ordinary respiratory movements. They are then very quickly modified in the sense most favorable for efficient working, according to the law we have so often pointed out, of the adaptation of organs to the functions they perform.

Thus, forced respirations result in a modification of the structure of certain regions of the lung, and in making them work better. Under the influence of unusual exercise the vesicles increase in size and contain more air. More blood is also supplied to them. Their capillary network becomes richer, and their nutrition more active. Thus in the end they take up more room.

It is in this manner that the regular working of a great number of air-cells, ordinarily inactive, can rapidly increase the size of the lungs.

If we follow out the modifications produced by forced respirations, we see that the lungs thrust outward the thoracic walls to make more room for themselves. During inspiration the ribs, by rising, favor the inflation of the lungs; but in this case it is the lungs which, having increased in size, thrust the ribs upward and keep them raised even in the condition of repose. Hence an increase in the circumference, and a vaulted conformation of the thorax.