674 THE POPULAR SCIENCE MONTHLY. 



" 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 circula- 

 tion. It therefore becomes of interest to note how, in the higher walks 

 of existence, the mechanical contrivances and actions of the body un- 

 dergo 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 Na- 

 ture's economics in higher life are frequently expressed in terms of 

 admirable mechanical contrivance. Primarily, in the case of respira- 

 tion, 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 breath- 

 ing, 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 mus- 

 cular trouble, so to speak. In the case of a deep inspiration, we exag- 

 gerate the effort seen in normal breathing, and we may therefore ap- 

 preciate 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 par- 

 takes, 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 



