4 THE STRUCTURE OF THE FOWL 



themselves are relatively small, firmly adherent 

 to the wall of the chest, and, when compared 

 with the lungs of mammals, remarkable in the 

 possession of little elasticity. That important 

 mammalian muscle of respiration, the dia- 

 phragm, moreover, is strikingly rudimentary. 

 These peculiarities render the possession of air- 

 sacs a necessity; not, as has been supposed, 

 to render the body lighter, but to subserve 

 the respiratory function. 



At the same time it must be pointed out 

 that the position of the lungs and air-sacs has 

 a marked influence upon the position of the 

 centre of gravity of the body, and thus con- 

 tributes materially to the bird's ability to 

 maintain its balance in the air. The light 

 lungs and air-sacs are placed in the dorsal part 

 of the body, while the heavy digestive organs 

 and pectoral muscles are ventral in position. 



The fore-limbs being useless for the purpose 

 of walking, the bird, when on the ground, is 

 forced to assume the upright position. The 

 joint between the pelvis and the thigh is 

 therefore so placed as to maintain the balance 

 of the body on the two hinder limbs. There 

 is considerable elongation of the ilium, and an 

 extensive fusion of it to the vertebral column. 



