ORGANIC PROCESSES OF PLANTS AND ANIMALS. 1015 



character of the chemical action of animals is exceedingly well 

 defined, and the reverse of that of plants. The animal frame 

 may be looked upon as an apparatus of combustion, in which 

 the reduced hydrogen and carbon of plants are again oxidated 

 as in a furnace, and returned to the atmosphere in the form of 

 water and carbonic acid. Thus are sustained the animal heat, 

 and the powers of locomotion of animals. While carbonic 

 acid and nitrogen in the form of salts of ammonia are sup- 

 plied to the vegetable world. 



Animals require azotised food for their growth, for all the 

 great constituents of the animal frame, such as its fibrin 

 albumen, and casein are azotised matters ; nothing indeed is 

 found in the soft parts of the body which is not azotised, except 

 water and fat, neither of which is organised. For the renewal 

 of these parts, a constant supply of azotised food is also neces- 

 sary. Gum, starch, and sugar, which contain no nitrogen, are 

 incapable alone of supporting life for any considerable period, 

 and animals fed exclusively upon the latter substances event- 

 ually succumb with all the appearances of death from starva- 

 tion. 



Respiration. But elementary substances of the amylaceous 

 class although they afford no element to the body, supply 

 carbon to be burned in respiration, consisting as they do of 

 carbon with oxygen and hydrogen in the proportions of water. 

 In the lungs of the higher animals, the dark venous blood is 

 exposed to air through a thin and humid membrane, permea- 

 ble to oxygen from its solubility ; that gas is absorbed by the 

 blood, and imparts to it a fine florid red colour, and the* 

 characters of arterial blood. There is no reason to believe 

 that any considerable oxidation occurs in the lungs although 

 the gas is dissolved there by the blood. The latter containing 

 free oxygen is carried by the circulation to the extreme 

 capillaries, where the processes of secretion to which it 

 contributes are most active, and where it will enter into com- 

 bination in largest quantity. Indeed it has been found by 

 experiment that venous blood absorbs oxygen and becomes 

 red and arterial, without producing the smallest trace of heat. 



Carbonic acid being formed is carried by the venous blood 

 to the lungs where it escapes, at the same time that the blood 

 obtains oxygen from the air and is arterialised. From the 



