7^ MEMOIRS OF 



in a manner incompatible with the modifications 

 of others, that being could not exist. Modern 

 experiments have shown, that one of the prin- 

 cipal uses of respiration is to reanimate muscular 

 force, by restoring to the muscular fibres their 

 exhausted irritabihty, and, in fact, among the 

 animals which breathe the air in a direct man- 

 ner, we see those with double circulation, and 

 not an atom of whose blood can return to the 

 parts till after it has been respired. Mammalia 

 and birds not only live always in the air, and 

 move in it with more force than other animals 

 with red blood, but each of these classes enjoys 

 the faculty of moving, precisely according to 

 tlie quantity of its respiration. Birds, for in- 

 stance, are as much impregnated with air within 

 as without ; not only the cellular parts of their 

 lungs are very considerable, but these organs 

 have bags, or appendices, which are prolonged 

 throughout the body. Thus, in a given time, 

 they consume a much greater quantity of air in 

 proportion to their size than quadrupeds; and 

 doubtless it is this wliich gives to their fibres a 

 prodigious and instantaneous force, and which 

 renders their flesh fit to act powerfully on those 

 violent movements which sustain them in the 

 air, by the simple vibrations of their wings." 

 In the concluding part of this first lecture, 



