332 THE VITAL FUNCTIONS. 



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 cir- 

 culating blood in the pulmonary organs. The 

 Mammalia rank next below birds, in the scale 

 of vital energy ; but they still possess a double 

 circulation, and breathe atmospheric air. The 

 torpid and cold-blooded Reptiles are separated 

 from Mammalia by a very wide interval ; because, 

 although they respire air, that air only influences 

 a part of the blood ; the pulmonary, being only 

 a branch of the general circulation. In Fishes, 

 again, we have a similar result ; because, al- 

 though 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 manner, 

 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 an extensive distribution of tracheae 

 through every part of their system. 



