360 OF RESPIRATION. 



carbonic acid set free is kept up to a more regular standard by the 

 provision to be presently noticed. On the other hand, they are pre- 

 eminent among all Animals, in regard to the energy of their muscular 

 power in relation to the bulk of their bodies ; and the waste of muscu- 

 lar tissue during their state of activity must therefore be very great. 

 Thus a Humble Bee has been found to produce one-third of a cubic 

 inch of carbonic acid in the course of a single hour, during which its 

 whole body was in a state of constant movement, from 'the excitement 

 consequent upon its capture : and yet during the whole twenty-four hours 

 of the succeeding day, which it passed in a state of comparative rest, 

 the quantity of carbonic acid generated by it was absolutely less. 



646. 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 maintain- 

 ing a constantly-elevated temperature, so long as they are supplied 

 Tfith 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 com- 

 bustion ; 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 ( 430). It is quite 

 clear that they cannot be applied in their original form, to the nutri- 

 tion of the tissues that originate in proteine-compounds ; and it is tole- 

 rably 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 ( 725), and to be thrown off by the respiratory process. 



647. 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 Carni- 

 vorous 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 

 materials of the food. But in Herbivorous animals of comparatively 

 inert habits, the amount of metamorphosis of the tissues is far from 

 being sufficient ; 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 deficiency. 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 



