SOURCES OF EXCRETION OF CARBONIC ACID. 359 



the carbonic acid, which is one of the products of the continual decom- 

 position of its tissues. ' The softness of many of the tissues of Animals, 

 and the large quantity of fluid contained in their bodies, render them 

 more prone than Plants to this kind of decomposition ; and, in warm- 

 blooded animals, the high temperature at which the fabric is usually 

 maintained, adds considerably to the degree of this tendency; so that 

 the waste of their tissues, from this cause alone, is as much greater 

 than that of cold-blooded animals, as the latter is than that of Plants. 

 But when the temperature 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 the 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 

 frozen ( 136), or when their tissues are completely dried up ( 159), 

 the decomposition is for the time entirely suspended, and consequently 

 there is no carbonic acid to be set free. 



644. 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, during the period of their activity. It has been already shown 

 ( 361), that there is strong reason to believe the waste or decomposi- 

 tion of the Muscular tissue to be in exact proportion to the degree in 

 which it is exerted ; every development of muscular force being accom- 

 panied by a change in the condition of a certain amount of tissue. In 

 order that this change may take place, the presence of Oxygen is 

 essential ; and one of the products of the union .of oxygen with the 

 elements of muscular fibre, is carbonic acid. The same may probably 

 be said of the Nervous tissue ( 384). Hence it may be stated as a 

 general principle, that the peculiar waste of the Muscular and Nervous 

 substances, which is a condition of their functional activity, and which 

 is altogether distinct from the general slow decay that is common to 

 these tissues with others, is another source of the carbonic acid which 

 is set free from the animal body : and that the amount thus generated 

 will consequently depend upon the degree, in which these tissues are 

 exercised. In animals which are chiefly made up of the organs of 

 vegetative life, in whose bodies the nervous and muscular tissues form 

 but a very small part, and in whose tranquil plant-like existence 

 there is but very little demand upon the exercise of these structures, 

 the quantity of carbonic acid thus liberated will be extremely small. 

 On the other hand, in animals, whose bodies are chiefly composed of 

 muscle, and whose life is an almost ceaseless round of exertion, the 

 quantity of carbonic acid thus liberated is very considerable. 



645. We are enabled to trace the connexion between the amount 

 of muscular exertion, and the quantity of carbonic acid set free in the 

 act of respiration, in the class of Insects, better than in any other. 

 They have no fixed temperature to maintain ; and they are consequently 

 not in the condition of warm-blooded animals, in which the quantity of 



