336 THE VITAL FUNCTIONS. 



For our knowledge of the fact of the dis- 

 appearance of oxygen we are indebted to the 

 labours of Dr. Priestley. It had, indeed, been 

 long before suspected by Mayow, that some 

 portion of the air inspired is absorbed by the 

 blood ; but the merit of the discovery that it is 

 the oxygenous part of the air which is thus con- 

 sumed is unquestionably due to Dr. Priestley. 

 The exact quantity of oxygen, which is lost in 

 natural respiration, varies in different 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. Vauquelin, 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 terres- 

 trial 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 con- 

 ceal themselves in holes, or burrow under ground, 

 have been known to deprive the air of every 

 appreciable portion of its oxygen. It is ob- 

 served by Spallanzani, that those animals, whose 

 modes of life oblige them to remain for a great 

 length of time in these confined situations, 

 possess this power in a greater degree than 

 others, which enjoy more liberty of moving in the 



