CHEMICAL EFFECTS OF RESPIRATION. 209 



upon only by that portion of air wliicli is contained 

 in the water respired, and which is less powerful 

 in its action than the same element in its gaseous 

 state. We may, in like manner, continue to trace 

 the connexion between the extent of these func- 

 tions and the degrees of vital energy throughout 

 the successive classes of invertebrate animals. The 

 vigour and activity of the functions of Insects, 

 in particular, have an evident relation to the effec- 

 tive manner in which the complete aeration of the 

 blood is secured by an extensive distribution of 

 tracheae through every part of their system. 



§ 4. Chemical Changes effected by Respiration. 



We have next to direct our attention to the chemi- 

 cal offices which respiration performs in the animal 

 economy. It is only of late years that we may be 

 said to have obtained any accurate knowledge as 

 to the real nature of this important function ; and 

 there is perhaps no branch of physiology which 

 exhibits in its history a more humiliating picture of 

 the wide sea of error in which the human intellect 

 is prone to lose itself, when the path of philosophi- 

 cal induction is abandoned, than the multitude of 

 wild and visionary hypotheses, devoid of all solid 

 foundation, and perplexed by the most inconsistent 

 reasonings, which formerly prevailed with regard 

 to the objects and the processes of respiration. To 

 give an account, or even a brief enumeration of 

 these theories, now sufficiently exploded, would be 

 incompatible with the purpose to which I must 



