ELEMENTS OF COMPARATIVE ZOOLOGY. 



FIG. 27. Brain of Bird. 



have forced the optic lobes apart and have covered over the 

 'twixt-brain. The eye is peculiar in 

 that it departs widely from the spheri- 

 cal form, being obtusely conical in 

 front, and in that a circle of bones are 

 developed in this conical portion. 

 There is a tube developed (external 

 meatus) leading from the side of the 

 head in to the ear, and this is sur- 

 rounded by a ring of regularly arranged 

 feathers. 



In the skeleton division into neck, 

 thoracic, sacral, and caudal vertebrae, 

 occur. The number of neck vertebrae 

 varies from eight to twenty-four. The 

 sacral are noticeable for their number, 

 and really embrace, besides the true sacrals, some of the 

 lumbars and caudals. The anterior caudal vertebrae are 

 free, but the last six or eight are coalesced into the pygo- 

 style or plowshare bone. The bodies of the vertebrae in 

 living birds are concave vertically, convex transversely 

 behind, the conditions being reversed on the anterior 

 faces. The cervical vertebrae bear short ribs, free in the 

 young but firmly united in the adult. Each of the true 

 ribs has a small plate (uncinate process) on the posterior 

 margin, which connects it with the rib behind. The breast- 

 bone (sternum) is large and broad, and in flying birds 

 possesses a strong ridge or keel below, to which the muscles 

 of flight are attached. In some flightless birds the keel is 

 lacking. 



The skull is noticeable from the great extent of the 

 fusion of the separate bones; for the single condyle for 

 articulation with the neck and for the suspension of the 



