236 THE VITAL FUNCTIONS 



Thus, has the mechanism of respiration been varied in 

 the different classes of animals, and adapted to the particu- 

 lar element, and mode of life designed for each. Combined 

 with the peculiar mode of circulation, it affords a tolerably 

 accurate criterion of the energy of the vital powers. In 

 birds, the muscular activity is raised to the highest degree, 

 in consequence of the double effect of the air upon the whole 

 circulating blood in the pulmonary organs. The Mamma- 

 lia rank next below birds, in the scale of vital energy; but 

 they still possess a double circulation, and breathe atmo- 

 spheric air. The torpid and cold-blooded reptiles are sepa- 

 rated from mammalia by a very wide interval, because, al- 

 though they respire air, that air only influences a part of 

 the blood; the pulmonary, being only a branch of the gene- 

 ral circulation. In fishes, again, we have a similar result, 

 because, although the whole blood is brought by a double 

 circulation to the respiratory organs, yet it is acted upon 

 only by that portion of air which 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 man- 

 ner, continue to trace the connexion between the extent of 

 these functions 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 effective manner in which the 

 complete aeration of the blood is secured by the 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 chemical of- 

 fices which respiration performs in the animal economy. It 



the density of that which the bird had been breathing 1 . We are, as yet, una- 

 ble to trace the connexion which probably exists between the structure of 

 the lungs, and this extraordinary power of accommodation to such great 

 and sudden variations of atmospheric pressure. 



