212 ANIMAL MECHANISM. 



All the anterior surface of the thorax of birds is occupied 

 by powerful muscular masses, and especially by a large 

 iiiuscle, which by its attachments to the sternum, to the ribs 

 and the humerus, is analogous with the large pectoral musele 

 in man and the mammals ; its office is evidently to lower the 

 wing witli force and rapidity, and thus to gain from the air 

 the fulcrum necessary to sustain, as well as to move the mass 

 of the body. Underneath the large pectoral is found the- 

 medium pectoral, whose action is to raise the wing. On the 

 exterior, the small pectoral, acting as accessory to the large 

 one, extends from the sternum to the humerus. 



Since the force of a muscle is in proportion to the volume 

 of this organ, when we consider that these pectoral irmscles 

 represent about one- sixth part of the whole weight of the 

 bird, we shall immediately understand that the principal 

 function of flight devolves on these powerful organs. 



BoVelli endeavoured to deduce from the volume of the pec- 

 toral muscles the energy of which they are capable; he con- 

 cluded that the force employed \>y the bird in flight was equal 

 to 10,000 times its weight. We will not here refute the 

 error of Borelli ; many others have undertaken to combat his 

 notions, and have substituted for the calculations of the Italian 

 physiologist others whose correctness it would be difficult to 

 prove. Such great contradictions as are to be found in the 

 different estimates formed of the muscular force of birds have 

 arisen from the fact that these attempts at measurement were 

 premature. 



Navier, depending on calculations which were not based on 

 experiment, considered himself authorized in admitting that 

 birds develop enormous mechanical work : seventeen swallows 

 would exert work equal to a horse-power. "As easy would it 

 be," said M. Bertrand, facetiously, " to prove by calculation 

 that birds could not fly a conclusion which would rather com- 

 promise mathematics." 



l;< >iiles, we find that Cugniard Latour admitted, basing his 

 assertion on theory, that the wing is lowered eight times more 

 quickly than it rises. Experiment, however, pi'oves that the 

 wing of the bird is raised more quickly than it descends. 



f tin iiiuxciiliii- h'K-f <>f .'In I'ifil. \Ve must at the 



