SOME ECONOMICS OF NATURE. 675
certain muscles, that the pressure of air in the lungs becomes reduced as compared with that outside, and that in consequence air rushes into the lungs through the windpipe until an equality of air-pressure inside and outside the lungs is produced. This is the act which is accom- plished forcibly, against gravity, and by aid of very considerable mus- cular power. We are said to perform no less than twenty-one foot- tons of work by means of our respiratory muscles in twenty-four hours that is to say, the work of these muscles, extending over twenty- four hours' period, if gathered into one huge lift, would raise twenty- one tons weight one foot high.
By a little additional muscular labor we take in a deep breath, still further enlarge the chest, and inhale an additional quantity of air. The great muscle named the diaphragm or " midriff," which forms the floor of the chest, is the chief agent involved in the act of inspiration. It descends, while the ribs are elevated, and, as the chest enlarges, the inflow of air takes place. The lungs themselves are highly elastic bod- ies. They follow the movements of the chest- walls, and thus expand and contract they suffer dilatation and compression as the chest-walls move in the acts of respiration. But, when ordinary " breathing out" is studied, we see that it is as clearly a matter of recoil, as has been stated, as " breathing in " is a matter of exertion. Here elastic reaction steps in to complete the full act of breathing. Nature saves her energies and husbands her strength in this truly physiological division of labor. When we inspire, the lung-substance, elastic in itself, is put on the stretch ; the cartilages of ribs and breast-bone are similarly elevated and expanded, and the whole chest is, so to speak, forced into its posi- tion of unrest. Then comes the reaction. The muscles of inspiration cease their action ; they relax, and the elastic lungs recover themselves and aid in forcing out the air they contain. So, also, when the rib- muscles have come to the end of their tether in elevating these bones, the elastic recoil of the ribs and breast-bone serves to diminish the ca- pacity of the chest, and to further expel the air from within its con- tained lungs. Labored or excessive breathing, as most readers know, calls into play extra help from muscles not ordinarily used in natural respiration. This fact takes us out of the normal way of life into the consideration of abnormal or diseased states, and demonstrates that the economy of Nature disappears when phases of morbid action fail to be observed. In natural breathing, however, we see conservation once more in the easy recoil which follows the muscular labor of in- spiration. The physiology of a sigh and its relief can be readily ap- preciated on the basis which shows how the easy act of expiration is correlated with the more labored action and duty of enlarging the chest.
A phase of Nature which is by no means foreign to the foregoing illustration of the conservation of power in the human body is pre- sented to us in several aspects of lower life. In the breathing of cer-