150 NATURE OF RESPIRATION. 



respiratory elements of food fat, sugar, starch, &c., yield these two pro- 

 ducts alone. The nutritive elements give rise to nitrogenized compounds 

 in addition. The conditions of life are such that carbonic acid can not 

 be permitted to accumulate in the system, and means have therefore to 

 be resorted to for its removal. The introduction of oxygen and excre- 

 tion of carbonic acid are accomplished by the same mechanism, the lungs, 

 the action of which is 'dependent on a physical principle. 



Under its simplest condition, respiration consists in the passing of car- 

 Respiration is b om * c ac id with the vapor of water from the system, and the 

 connected tvith reception of oxygen in exchange. The construction of the 

 porous maker apparatus which accomplishes this double duty in atmos- 

 on b"- pheric animals is such that it can deal with substances in 



the aerial state alone. Nothing can be introduced through the lungs or 

 escape therefrom except it be in the gaseous or vaporous form. All 

 those products of disorganization which are not presented under this con- 

 dition must therefore be removed by other organs, and this is more par- 

 ticularly done by the kidneys. 



But in aquatic animals, as in fishes generally, there is not this restric- 

 Coaiescence of tion or concentration of function, for the gill, being in contact 

 and^rhmrv^r- w ^ n water, offers a channel for the passing away of many 

 gans in fishes, products of waste which, from their non-aerial state, could 

 never escape through a lung, and so I regard this organ, the gill, as in a 

 measure sharing the duty of a kidney in eliminating nitrogenized and 

 perhaps saline matters. Comparative anatomists have long recognized 

 that the so-called kidney in fishes approaches in character the Wolffian 

 bodies largely developed in the foetal condition of man. I am disposed 

 to believe that the physical interpretation of this depends on the fact now 

 before us, and that the gill in fishes, and the placenta, in part, in mam- 

 mals, discharge at once the double office of a respiratory and urinary or- 

 gan. It is consistent with the scheme of organic design that there should 

 be this separation and concentration of function as development takes 

 place. 



These considerations would therefore lead us to expect that we should 

 find in the respiration of air-breathing animals that function in its purest 

 and least complicated form, and this is accordingly the case. If it be 

 merely the skin that is relied on,' as in the low orders of aerial life, or if 

 the mechanism be constructed on the type of carrying the air to the 

 blood, as in insects, or that of carrying the blood to the air, as in man, 

 the operation consists essentially in the escape of carbonic acid and 

 steam, and the reception of oxygen in return. 



Respiration, like circulation, furnishes us with a signal instance of 

 the employment of purely physical principles for the accomplishment of 

 physiological purposes. It is with the pressure of the atmosphere, the 



