562 THE MECHANICAL FUNCTIONS. 



that every feather should be constantly anointed 

 with an oily secretion, which preserves it from 

 being wetted, and which is copiously provided for 

 that purpose by glands situated near the tail. 

 The flexibility of the neck alone would have 

 been insufficient for enabling the bird to bring 

 its bill in contact with every feather, in order to 

 distribute this fluid equally over them ; and 

 there is, accordingly, a farther provision made for 

 the accomplishment of this object in the mode 

 of articulation of the head with the neck. We 

 have seen that, in fishes, and in most reptiles, 

 this articulation consists of a ball and socket 

 joint ; a rounded tubercle of the occipital bone 

 being received into a hemispherical depression 

 in the first vertebra of the neck. In the mam- 

 malia the plan is changed, and there are two 

 articular surfaces, one on each side of the spinal 

 canal, formed on processes corresponding to the 

 leaves of the first cranial vertebra, and assimi- 

 lating it more to a hinge joint. In birds, how- 

 ever, where, as we have just seen, the most ex- 

 tensive lateral motions are required, the plan 

 of the ball and socket joint is again resorted to ; 

 and the occipital bone is made to turn upon the 

 atlas by a single pivot. So great is the freedom 

 of motion in this joint, that the bird can readily 

 turn its head completely back upon its neck, on 

 either side. 



As spinous or transverse processes of any 

 length wotdd have interfered with the flexions 



