"breathe out," the pressure in the large veins increases it is true, but a valve guards the entrance, which in inspiration is free, and untoward consequences are thus prevented. It is a notable fact that in many animals organs known as lymph-hearts are developed. As in the frog, these contractile organs assist the lymph in its return to the circulation. It therefore becomes of interest to note how, in the higher walks of existence, the mechanical contrivances and actions of the body undergo an evolution which not only avoids multiplication of parts and. organs, but also conserves and economizes the energy which has to be expended in the maintenance of life.
The function of breathing has been incidentally alluded to in the course of the foregoing remarks, and, in considering the details of this paramount duty of life, we find additional proof of the fact that Nature's economics in higher life are frequently expressed in terms of admirable mechanical contrivance. Primarily, in the case of respiration, we find the bony elements of the chest fitly developed in view of certain physical qualities, of which elasticity forms perhaps the chief. The front wall of the chest is practically composed of cartilage or "gristle." The "costal cartilages," or those of the ribs, intervene between the upper seven ribs and the "sternum" or breast-bone. The eighth, ninth, and tenth pairs of ribs also possess cartilages, but these run into and join the gristly extremity of the seventh pair; while the last two pairs of ribs (eleventh and twelfth) spring from the spine behind, but are not attached in front at all. Essentially, the chest is a bony cage, possessed of high elasticity. Even in the dried skeleton, pressure from above, downward or backward, applied to the front of the chest shows this quality of its structures in a marked fashion.
If we study, even superficially, the mechanism involved in breathing, we may gain an idea of the key-note of the process in so far as economy of force is concerned."Breathing in," if we reflect upon the nature of the act in our individual persons, is a matter of some trouble. It involves a large amount of labor; it gives us much muscular trouble, so to speak. In the case of a deep inspiration, we exaggerate the effort seen in normal breathing, and we may therefore appreciate still more exactly the expenditure of energy required to carry on this necessary function of vitality. But "breathing out" is a widely different matter. We let the chest "go," as it were, at the close of inspiration, and, without an effort, it returns to its position of rest. We expend force in "breathing in"; we appear to exert none in "breathing out." The former is a muscular act performed by a complex series of muscles, and participated in by the lungs and other structures connected with the chest. The latter is an act which partakes, even to the common understanding, of the nature of a recoil; and in this latter supposition we perceive how economy of labor in the human domain is again subserved.
Breathing, then, means that we enlarge the chest by the action of
