572 OF RESPIRATION. 



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

 Animals, there is another, which appears to be peculiar to the two highest 

 classes, Birds and Mammals. These are capable of maintaining a constantly 

 elevated temperature, so long as they are supplied with a proper amount of 

 appropriate food; and their power of doing so appears to depend upon the 

 direct combination of certain elements of the food, with the oxygen of the air, 

 by a process analogous to combustion ; these elements having been introduced 

 into the blood for that purpose, but not having formed a part of any of the 

 solid tissues of the body, unless they have been deposited in the form of fat. 

 The nature of these substances has been already noticed ( 641). It is quite 

 clear that they cannot be applied in their original form, to the nutrition of the 

 tissues that originate in proteine compounds; and it is tolerably certain that, 

 in the ordinary condition of the body, they undergo no such conversion, as 

 would adapt them to that purpose. The Liver seems to afford a channel, by 

 which some of the fatty matters are drawn off from the blood; but even these 

 seem, in part at least, to be reabsorbed ( 671), and to be thrown off by the 

 respiratory process. 



753. 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 quan- 

 tity which is generated by the waste or metamorphosis of the tissues is suffi- 

 cient 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 combus- 

 tion of the materials of the food. But in Herbivorous animals of comparatively 

 inert habits, the amount of metamorphosis of the tissues is far from being suf- 

 ficient; and a large part of the food, consisting as it does of substances that 

 cannot be applied to the nutrition of the tissues, is made to enter into direct 

 combination with the oxygen of the air, and thus to compensate for the de- 

 ficiency. In Man and other animals, which can sustain considerable variations 

 of climate, and can adapt themselves to a great diversity of habits, the quantity 

 of carbonic acid formed by the direct combination*of the elements of the food 

 with the oxygen of the air, will differ extremely under different circumstances. 

 It will serve as the complement of that which is formed in other ways; so 

 that it will diminish with the increase, and will increase with the diminution 

 of muscular activity. On the other hand it will vary, in accordance with the 

 external temperature; increasing with its diminution, as more heat must then 

 be generated; and diminishing with its increase. In all cases, if a sufficient 

 supply of food be not furnished, the store of fat is drawn upon: and if this be 

 exhausted, the animal dies of cold ( 896). 



754. To recapitulate, then, the sources of Carbonic Acid in the animal body 

 are threefold. i. The continual decay of the tissues; which is common to 

 all organized bodies; which is diminished by cold and dryness, and increased 

 by warmth and moisture; which takes place with increased rapidity at the 

 approach of death, whether this affect the body at large, or only an individual 

 part; and which goes on unchecked when the actions of nutrition have ceased 

 altogether. n. The Metamorphosis, which is peculiar to the Nervous and 

 Muscular tissues ; which is the very condition of their activity, and which 

 therefore bears a direct relation to the degree in which they are exerted. in. 

 The direct conversion of the carbon of the food into carbonic acid; which is 

 peculiar to warm-blooded animals ; and which seems to vary in quantity, in 

 accordance with the amount of heat to be generated. We shall now examine 

 into the manner in which this compound is set free, in the principal groups of 

 the Animal kingdom. 



755. Notwithstanding their diversity in external form, the organs of Re- 



