504 



OF RESPIRATION. 



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 ( 402.) 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 temperature, in- 

 creasing w?.th its diminution (as more heat must then be generated), and dimin- 

 ishing with its increase ; the effect of external heat being thus precisely opposite, 

 in the warm-blooded animal, to that which it exerts on the cold-blooded ( 586). 

 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 ( 416). 



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

 are threefold. 1. The continual decay of the tissues common to all organized 

 bodies, which is favored by all that promotes their vital activity, and retarded 

 by every influence that depresses it. 2, The metamorphosis peculiar to the 

 Nervous and Muscular tissues, which is the very condition of the production of 

 their power, and which therefore bears a direct relation to the degree in which 

 they are exerted. 3. The direct conversion of the carbon and hydrogen of the 

 food into carbonic acid and water, which is peculiar to 

 warm-blooded animals ; and which varies in quantity, in 

 accordance with the amount of heat to be generated. 



540. The wonderful nature of the structural arrange- 

 ments which are made for the aeration of the blood in Man 

 (as in Mammalia generally), and the completeness of the 

 provisions whereby these are put into active operation, 

 will be best understood, if, for the sake of contrast, we first 

 bestow a brief survey on the Pulmonary apparatus of Rep- 

 tiles ; a class in which the demand for respiration is re- 

 duced to a comparatively low grade, by the absence of any 

 necessity for the maintenance of an independent tempera- 

 ture, by the general torpor of their habits (whence arises 

 a very small amount of " waste" in the nervo-muscular 

 apparatus), and by the very slow rate at which their or- 

 ganic functions are performed, and the life of the whole 

 body is carried on. The lungs of Reptiles are, for the 

 most part, simple sacs ; into which the bronchial tubes 

 open freely, and on the walls of which the pulmonary ves- 

 sels are distributed. The extent of surface is considerably 

 increased, however, by the formation of a number of little 

 pits or sacculi on the inner wall of the cavity, especially 

 at its upper part ; and between these we observe a sort of 

 cartilaginous framework, which is continuous with the 

 cartilage of the bronchus on either side. Thus it happens 

 that the network of pulmonary capillaries is exposed only 

 on one side to the influence of the air. The general dis- 

 tribution of these vessels is shown in the accompanying 

 figures. It will be seen that the trunk of the pulmonary 

 artery runs along one side of the sac, and that of the pulmonary vein along the 



Fig. 141f. 



* i 



I--4 



1 



Lung of Triton criftatus, 

 magnified about 3 ilium.; 

 a, pulmonary artery ; b, 

 pulmonary vein. 



