NATURE OP RESPIRATION. 259 



Plants than to Animals; for they also die when excluded 

 from it : and thus its presence may be stated to be a general 

 condition, necessary for the continuance of the life of all 

 organised beings. There is, however, a marked difference in 

 the rapidity with which the deprivation of air occasions 

 death in different animals ( 310). 



298. At first sight it might be thought that Animals which 

 always live beneath the surface of the water, such as Fishes, 

 Zoophytes, and many Mollusca, are removed from the influence 

 of the air ; and that they hence constitute an exception to this 

 general law. But such is not the case ; for the liquid which 

 they inhabit has the power of absorbing, and of holding dis- 

 solved in it, a certain quantity of air, which they can easily 

 separate from it, and which is sufficient for the maintenance 

 of their life. They cannot exist in water which has been 

 deprived of air (as by boiling, or by being placed under the 

 exhausted receiver of an air-pump) ; for they then become 

 insensible and die, just as do Mammalia and Birds when 

 prevented from inhaling air in the ordinary manner. 



299. The changes which result from the exposure of the 

 blood or nutritious fluid of Animals to the air, either in the 

 atmosphere, or through the medium of water, form a very 

 important part of their vital actions ; and the changes them- 

 selves, together with the various mechanical operations by 

 which they are effected, constitute the function of Respiration. 

 The nature of these changes will be first explained ; and the 

 structure and operations of the organs by which they are 

 performed will be afterwards described. 



Nature of the Changes essentially constituting Respiration. 



300. Atmospheric air, it has been stated, is necessary to 

 the continued life of all animals ; but this fluid is not com- 

 posed of one element alone. By the science of Chemistry, 

 it is shown to be a mixture of three gases possessing very 

 different properties. Besides the watery vapour with which 

 the atmosphere is always more or less charged, the air con- 

 tains 21 parts in 100 of oxygen, and 79 parts of nitrogen or 

 azote ; with about l-5000th part of carbonic acid gas. The 

 question immediately presents itself, therefore, whether these 

 gases have the same action on animals ; or, if their actions 



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