RESPIRATION. 255 



dages placed round the mouth excited the current by their 

 constant motion, a very inadequate means, even were it 

 true, which it is not, that these organs are in continual mo- 

 tion. They are, however, like the gills, clothed with cilia ; 

 and therefore are not undeserving the appellation they have 

 sometimes got of " accessory gills." 



The purpose of the respiratory organs and of the currents 

 just described, is, to expose the blood freely to the purifi- 

 cative action of the atmospherical air, that it may be purged 

 of some noxious qualities which it has acquired during its 

 circulation through the venous system, and fitted again for 

 the continuance of the life of the individual. In the ver- 

 tebrate animals the blood is altered, even in its outward 

 appearance, by this process, from a dark it becomes a 

 bright red fluid, but no perceptible change is operated on 

 the white serous blood of the mollusca. Yet that it has 

 experienced a similar purification is not to be doubted ; 

 for the air breathed by these creatures is similarly dete- 

 riorated, as it would have been had it been breathed by 

 the quadruped or bird ; the oxygen has disappeared, and 

 its place become occupied by an equal bulk of carbonic 

 acid gas. This had been proved by the well-known expe- 

 riments of Spallanzani and other physiologists ; and though, 

 in general, the proposition holds good, yet it appears, from 

 the recent experiments of Treviranus, that the absorption 

 of oxygen is not always proportional to the excretion of 

 carbonic acid, the proportion of the one to the other de- 

 pending on the strength of the respiration, the time of its 

 continuance while the respirability of the air is diminish- 

 ing, and the volume of the air in which the respiration is 

 performed. " The more carbonic acid," says Treviranus, 

 " there is developed while breathing in the open air, and 

 the less the power of continuing in a medium deficient in 

 oxygen, the less is the proportion of the consumption of 

 oxygen to the production of carbonic acid gas, whence a 

 small quantity of atmospheric air is respired for a moderate 

 period. But when the respiration is continued for a longer 

 period in the same air, and the strength of the individual 

 begins to sink, the excretion of the latter diminishes more 

 rapidly than the absorption of the former. We know 

 that the higher classes of animals, when enclosed in a cer- 

 tain quantity of air, die long before all its oxygen has been 

 exhausted. The case is very different with many of the 

 mollusca under the same circumstances ; for they not only 

 consume all the oxygen, but actually continue afterwards 

 to exspire carbonic acid gas ; consequently, after the respi- 



