GENERAL STRUCTURE OF THE RESPIRATORY ORGANS. 503 



inquire into the sources from which Carbonic acid is produced in the living body, 

 and the causes of the demand for Oxygen. 



536. It has been shown (CHAP, in.) that the vital activity of the organism at 

 large involves a continual change in its constituent parts ; and that those which (so 

 to speak) live the fastest, usually die the soonest, and pass most readily into 

 decay. Hence in the very performance of the Organic functions which concur 

 to effect the Nutrition of the body, there is a constant source of disintegration ; 

 and one of the chief products of the decay of the tissues, which is consequent 

 upon their loss of vitality, is Carbonic acid. Thus the most general object of 

 the Respiratory process, which is common to all forms of organized being, is 

 the extrication of this product from the system ; and the demand for aeration 

 hence arising will vary with the activity of the nutritive operations. Now the 

 rate of life, and consequently the amount of disintegration, in an$ organized 

 structure, depend in great measure upon the temperature at which it is main- 

 tained ( 123, 127); and thus it happens that the production of Carbonic acid 

 from this source, at the ordinary rate of vital activity, is much more rapid in 

 "warm-blooded" than in " cold-blooded" animals, and that the former suffer far 

 more speedily than the latter from the privation of air. But when the tem- 

 perature of the Reptile is raised by external heat to the level of that of the 

 Mammal, its need for respiration increases, owing to the augmented waste of its 

 tissues. When, on the other hand, the warm-blooded Mammal is reduced, in 

 the state of hybernation, to the level of the cold-blooded Reptile, the waste of 

 its tissues diminishes to such an extent, as to require but a very small exertion 

 of the respiratory process to get rid of the carbonic acid, which is one of its 

 chief products. And in those animals which are capable of retaining their 

 vitality when they are frozen, or when their tissues are completely dried up, 

 vital activity and disintegration are alike entirely suspended, and consequently 

 there is no carbonic acid to be set free. 



537. But another source of Carbonic acid to be set free by the Respiratory 

 process, and one which is peculiar to animals, consists in the rapid changes 

 which take place in the Muscular and Nervous tissues, in the very act of per- 

 forming their peculiar functions ] the development of the Muscular and of the 

 Nervous forces involving, as the very condition of their production, a change in 

 the substance of these tissues respectively ; in which change a large quantity of 

 Oxygen is consumed, and a large amount of Carbonic acid is generated. Hence 

 in Man, as in all Animals in which the Nervo-Muscular apparatus constitutes 

 the essential part of the organism, a powerful demand for Respiration is created 

 by its activity ; the amount of oxygen taken in, and of carbonic acid exhaled, 

 being determined, cxteris paribus, by the degree in which this apparatus is 

 exercised. 



538. Besides these sources of Carbonic acid which are common to all Animals, 

 there is another which is restricted (or nearly so) to the two highest classes, 

 Birds and Mammals ; these being distinguished by their power of maintaining 

 a constantly elevated temperature. A part of this Heat is generated by the 

 oxygenation of the components of their disintegrating tissues, the metamorphosis 

 of which takes place at a very rapid rate ; but where this is not sufficient, their 

 power of maintaining their temperature depends upon the direct combination of 

 certain elements of the food with the oxygen of the air, by the combustive pro- 

 cess. The quantity of carbonic acid that is generated directly from the elements 

 of the food seems to vary considerably in different animals, and in different 

 states of the same individual. In the Carnivorous tribes, which spend the 

 greater part of their time in a state of activity, it is probable that the quantity 

 which is generated by the waste or metamorphosis of the tissues is sufficient for 

 the maintenance of the required temperature ; and that little or none of the 

 carbonic acid set free in respiration is derived from the direct combustion of the 



