30 STUDIES AV LIFE AND SENSE. 



but also conserves and economises 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 carti- 

 lages, 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. Essen- 

 tially, the chest is a bony cage, possessed of high elasticity. Even in 

 the dried skeleton, pressure from above, downwards or backwards, 

 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 keynote 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 labour ; 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 

 labour in the human domain is again subserved. 



Breathing, then, means that we enlarge the chest by the action of 

 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 

 accomplished forcibly, against gravity, and by aid of very consider- 

 able muscular power. We are said to perform no less than twenty- 



