BIRDS. 



353 



are thickly clothed with down and are able to run and to 

 feed themselves (prcecoces). 



The brain is large, and, in comparison with the lower 

 forms already studied, is noticeable for the great develop- 

 ment of the cerebrum and cerebel- 

 lum, which by their growth have 

 forced the optic lobes apart and 

 have covered over the 'twixt-brain. 

 The eye is peculiar in that it de- 

 parts widely from the spherical 

 form, being obtusely conical in front, 

 and in that a circle of bones is usu- 

 ally developed in this conical por- 

 tion. There is a tube (external 

 meatus) developed leading from the 

 side of the head in to the ear, and 

 this is surrounded by a ring of regu- 

 larly arranged feathers. FIG. 148. Brain of Bird. 



In the skeleton, division into neck, thoracic, sacral, and 

 caudal vertebrae occurs. The number of neck vertebrae 

 varies from eight to twenty-four. The sacrals are notice- 

 able for their number, and really embrace, besides the true 

 sacrals, some of the lumbars and caudals, which become 

 united with the pelvis. The anterior caudal vertebra 

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

 pygostyle or plowshare bone. The bodies of the vertebraa in 

 living birds are saddle-shaped, that is, 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 



