362 OF RESPIRATION. 



Paul Bert 1 it clearly appears that whilst the muscular tissue on account of 

 its large mass plays the most important part in those internal processes which 

 he has termed the " respiration of the tissues," yet that analogous changes 

 occur in every tissue and organ in the body. It must not, however, be too 

 hastily assumed that the phenomena here witnessed are those of simple oxida- 

 tion, since the absorption of Oxygen and the extrication of Carbonic acid are 

 probably only the first and last links in a long series of decompositions. 



282. 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 gen- 

 erated by the oxygeuation of the components of their disintegrating tissues, 

 and of their blood, 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 process. The quantity of carbonic acid 

 that is generated directly from the elements of the food, seems to vary con- 

 siderably 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 comparatively little 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 undergo com- 

 bustion with great facility, 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 oxy- 

 gen 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. It will also vary in an inverse ratio to the external tem- 

 perature, increasing with its diminution (as more heat must then be gen- 

 erated), and diminishing with its increase ; the effect of external heat beiug 

 thus precisely opposite, in the warm-blooded animal, to that which it exerts 

 on the cold-blooded ( 281). 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. 



283. To recapitulate, then, the sources of demand for Oxygen and pro- 



1 P. Bert, Le9ons sur la Physiologic comparee de la Respiration, Paris, 1870, p. 30 

 et seq. In these experiments M. Bert found that: 



Cub. cent, of Cub. cent, of Tar- 



Grammcs. Oxys'-u. bonic Acid. 



100 of muscle, ..." ] f 508 ] f 5U. 8 



" of brain, ... | 45.8 | | 4'J.8 



" of kidney. . . A , ! 37.0 w v i, ,i,.i I 15.1} 



.. " - Absorbed 4 .-> o - Jixnaled - ,- , 



" of spleen, . . . . | | 27.3 | Lo.4 



" of testis, ' . . . . | IS.:; | | 27.5 



" of bone and marrow crushed, ) I 17.2 J {_ 8.1 



It is worthy of notice that M. Bert finds in corroboration of Spallanzani'a experi- 

 ments that the larger the proportion of oxygen present in the air to which the tissues 

 a iv exposed, the more freely it is absorbed ; the proportion of carbonic acid gas ex- 

 haled, however, is not correspondingly increased. 



