358 OF RESPIRATION. 



atmosphere by a converse process the absorption of carbon and the 

 liberation of oxygen, their true respiration is commonly overlooked. 

 It may, therefore, be regarded as common to all Organized beings. 

 Every one is conscious, in his own person, of the imperative demand 

 for the due performance of this operation. If the breath be purposely 

 held for even a few seconds, a feeling of distress is experienced, which 

 increases every moment, and at last prompts irresistibly to the respi- 

 ratory movement. And if the admission of air to the lungs be in any 

 way prevented, the respiratory movements are at first increased in 

 energy, violent efforts are made to obtain the needed supply; these 

 are succeeded by irregular convulsive actions, and at the same time 

 insensibility comes on ; and within a short time all movement ceases, 

 the circulation of the blood is suspended, and a stop is put to all the 

 vital operations of the body. This state, which is termed Asphyxia, 

 usually comes on, in a warm-blooded animal, within ten minutes of 

 the time when the respiration is completely checked; thus affording the 

 most convincing proof of the importance of that function in the Animal 

 economy. In many cold-blooded tribes, however, a much longer sus- 

 pension may be borne with impunity; as also by warm-blooded animals, 

 when the general activity of their functions is lowered in the state of 

 hylernation ( 121). We shall now inquire into the sources of the 

 necessity for this interchange of oxygen and carbonic acid ; and the 

 mode in which the suspension of it acts upon the system at large. 



642. All Organized bodies, as already explained, are liable to con- 

 tinual decay, even whilst they are most actively engaged in performing 

 the actions of Life ; and one of the chief products of that decay is car- 

 bonic acid. A large quantity of this gas is set free, during the decom- 

 position of almost every kind of organized matter ; the carbon of the 

 substance being united with oxygen supplied by the air. Hence we 

 find, that the formation and liberation of carbonic acid goes on with 

 great rapidity after death, both in the Plant and in the Animal ; and 

 that it takes place also, to a very great extent, in the period that often 

 precedes the death of the body, during which a general decomposition 

 of the tissues is going on. Thus in Plants, as soon as they become 

 unhealthy, the extrication of carbon in the form of carbonic acid takes 

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

 atmosphere; and the same change normally takes place during the 

 period that immediately 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 car- 

 bonic acid set free. The same thing probably happens in the Animal 

 body, during the progress of many diseases which are attended with an 

 extraordinary tendency to decomposition in the solids and fluids ; for in 

 such cases the blood usually exhibits an unusually dark hue, indicating 

 that it has not been properly freed from the unusual amount of carbonic 

 acid which it has received from the tissues. It has not yet been accu- 

 rately determined, however, whether there is an increase in the amount 

 of carbonic acid actually thrown off in such cases. 



643. Hence, the first object of the Respiratory process, which is 

 common to all forms of Organized being, is to extricate from the body 



