SOME ECONOMICS OF NATURE. 31 



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 labour 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 bodies. They follow the movements of the chest 

 walls, and thus expand and contract they suffer dilatation and com- 

 pression 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 labour. 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 position 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 capacity of 

 the chest, and to further expel the air from within its contained lungs. 

 Laboured or excessive breathing, as most readers know, calls into 

 play extra help from muscles not ordinarily used in natural respira- 

 tion. This fact takes us out of the normal way of life into the con- 

 sideration of abnormal or diseased states, and demonstrates that the 

 economy of Nature disappears when phases of morbid action fail to 

 be subserved. In natural breathing, however, we see conservation 

 once more in the easy recoil which follows the muscular labour of 

 inspiration. The physiology of a sigh and its relief can be readily 

 appreciated on the basis which shows how the easy act of expiration 

 is correlated with the more laboured 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 

 certain animal forms, belonging to the Molluscan races, we may 

 discover equally admirable examples of economy in natural work. 

 Among the Cephalopods or cuttlefishes we observe such features. 

 Any one who has seen an octopus resting in its tank in an aquarium, 



