298 THE VITAL FUNCTIONS. 



large a quantity of air throughout tlie body of 

 animals of this class presents an analogy with a 

 similar purpose apparent in the conformation of 

 insects, where the same object is effected by means 

 of tracheae.* 



Thus has the mechanism of respiration been 

 varied in the different classes of animals, and 

 adapted to the particular 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 pulmo- 

 nary organs. The Mammalia rank next below 

 birds, in the scale of vital energy; but they still 

 possess a double circulation, and breathe atmos- 

 pheric air. The torpid and cold-blooded Reptiles 

 are separated from Mammalia by a very wide in- 

 terval ; because, although they respire air, that air 

 inriuences only a part of the blood ; the pulmonary, 

 being merely a branch of the general 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 



* The peculiarities of structure in the respiratory system of birds 

 have probably a relation to the capabihty we see them possess, of 

 bearing with impunity, very quick and violent changes of atmos- 

 pheric pressure. Tims the Condor of the Andes is often seen to 

 descend rapidly from a height of above 20,000 feet, to the edge of 

 the sea, where the air is more than twice the density of that which 

 the bird had been breathing. We are as yet unable to tiace the 

 connexion which probably exists between the structure of the liiiigs, 

 and this extraordinary power of accommodation to such great and 

 sudden variations of atmospheric pressure. 



