796 SPECIAL PHYSIOLOGY. 



carbon of the carbonic acid is derived from the blood and tissues, 

 themselves supplied bj the food. The chemical union of the oxygen 

 with carbon, and also with hydrogen, in the system, maintains the 

 movements and the temperature of the body, and is the source of its 

 nervous power and electricity. 



The function of respiration, therefore, has for its immediate effect, 

 the purification of the blood, and for its ultimate uses, the production 

 of Animal Heat, Motion, and Nervous Energy. 



In plants, as elsewhere mentioned, the respiratory process is reversed. Un- 

 der the action of light, the carbonic acid and water taken up, partly by the 

 leaves, but chiefly by the roots, are decomposed in the leaves ; the oxygen is 



and support their respiratory and other vital processes. 



We have just seen that the skin is the seat of a feeble respiratory 

 process, consisting of an interchange of oxygen and carbonic acid. A 

 small amount of oxygen may also be absorbed, and of carbonic gas 

 exhaled, at the mucous surfaces of the stomach and intestines ; for 

 atmospheric air is swallowed, mixed with the saliva and food, and dis- 

 solved in the drink. But in animals generally, excepting in the very 

 lowest, special respiratory organs; often consisting of a very compli- 

 cated apparatus, are present. 



The respiration of animals is performed, sometimes in air and some- 

 times in water, the former being termed aerial and the latter aquatic, 

 respiration. The Mammalia, including Man, all Birds, Reptiles, and 

 Amphibia, amongst the Vertebrata, the Pulmo-gasteropods belonging 

 to the Mollusca, and the Insecta, Arachnida, arid Myriapoda amongst 

 the Annulosa, are aerial breathers, and are provided either with com- 

 plex hollow organs named lungs, with simpler air sacs, or else with 

 minute air-tubes, or trachece, all these organs communicating directly 

 with the atmosphere. A certain number of the Amphibia, all the 

 Fishes, the Mollusca generally, except the pulmonated Gasteropoda, 

 all the Molluscoida, the Crustacea amongst the Annulosa, and all the 

 Annuloida, Coelenterata, and Protozoa, are aquatic breathers, and are 

 provided either with projecting organs named branchiae or gills, some- 

 times external, but more commonly concealed, or with internal ciliated 

 sacs or canals, or with external ciliated processes, discs, or surfaces, 

 always in contact with water. 



In aerial respiration, the source of the oxygen taken into the body, 

 is the atmosphere, into which the carbonic acid is given off. In aqua- 

 tic respiration, although the breathing is subaqueous, so that the oxy- 

 gen is taken up from, and the carbonic acid given off into, the water, 

 still the ultimate source of the oxygen is the atmospheric air dissolved 

 in that medium. The solvent power of water for air is- very great, and 

 owing to the greater solubility of oxygen than of nitrogen in water, 

 the air held in solution in this fluid contains- an unusual proportion of 

 oxygen. 



The great importance of the function of respiration to animal life 



