SOURCES OF CARBONIC ACID. 571 



place in greater amount than its fixation from the carbonic acid of the atmo- 

 sphere ; and the same change normally occurs during the period that immedi- 

 ately precedes the annual fall of the leaves, their tissue being no longer able 

 to perform its proper functions, and giving rise, by its incipient decay, to a 

 large increase in the quantity of carbonic acid set free. The same thing 

 happens in the Animal body, during the progress of many diseases which are 

 attended with an unusual tendency to decomposition in the solids and fluids, 

 such as eruptive fevers ; the quantity of carbonic acid set free in Respira- 

 tion is greatly increased, although the body remains completely at rest ; and 

 notwithstanding this, the blood frequently exhibits a very dark hue, indicating 

 that it has not been freed from the unusual amount of that substance which 

 it has received from the tissues. Hence the first object of the Respiratory 

 process, which is common to all forms of organized being, is to extricate from 

 the body the carbonic acid, which is one of the products of the continual de- 

 composition 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 con- 

 siderably 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 its tissues. When, on 

 the other hand, the warm-blooded Mammal is reduced, in the state of hyber- 

 nation, to the level of the cold-blooded Reptile, the waste of its tissues dimin- 

 ishes to such an extent, as to require but a very small exertion of the respira- 

 tory 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 they 

 are frozen, or when their tissues are completely dried up, the decomposition 

 is for the time entirely suspended, and consequently there is no carbonic acid 

 to be set free. 



751. But another sourceof 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 ( 586), that there is strong reason 

 to believe the waste or decomposition of the muscular tissue to be in exact 

 proportion to the degree in which it is exerted; every development of muscu- 

 lar force being accompanied 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 ( 292). Hence it may be stated as a general principle, that 

 the peculiar waste of the Muscular and Nervous substances, which is a con- 

 dition 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 b^jhctremely 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. 



