.302 THE VITAL FUNCTIONS. 



air which is thus consumed is unquestionably due 

 to Dr. Priestley. The exact quantity of oxygen 

 which is lost in natural respiration varies in dif- 

 ferent animals, and even in different conditions of 

 the same animal. Birds, for instance, consume 

 larger quantities of oxygen by their respiration ; 

 and hence require, for the maintenance of life, a 

 purer air than other vertebrated animals. Vau- 

 quelin, however, found that many species of insects 

 and worms possess the power of abstracting oxygen 

 from the atmosphere in a much greater degree than 

 the larger animals. Even some of the terrestrial 

 mollusca, such as snails, are capable of living for a 

 long time in the vitiated air in which a bird had 

 perished. Some insects, which conceal themselves 

 in holes, or burrow under ground, have been known 

 to deprive the air of every appreciable portion of its 

 oxygen. It is observed by Spallanzani, that those 

 animals, whose modes of life oblige them to remain 

 for a great length of time in these confined situa- 

 tions, possess this power in a greater degree than 

 others, which enjoy more liberty of moving in the 

 open air: so admirably have the faculties of ani- 

 mals been, in every instance, accommodated to 

 their respective wants. 



Since carbonic acid consists of oxygen and 

 carbon, it is evident that the portion of that gas 

 which is exhaled from the lungs is the result of the 

 combination of either the whole, or a part, of the 

 oxygen gas, which disappears during the act of 

 respiration, Avith the carbon contained in the dark 

 venous blood, which is brought to the lungs. The 

 blood having thus parted with its superabundant 

 carbon, which escapes in the form of carbonic acid 



