32 THE STRUCTURE AND LIFE OF BIRDS CHAP. 



this is filled by a projecting tongue from the second 

 or Axis vertebra, called the odontoid process, which 

 thus completes the cup. All mammals have two 

 condyles. The great freedom with which a bird 

 moves its head is due to the way in which, by its 

 single condyle, it articulates with the vertebra. 



\ 



r A ° 



J 



Fig. 9. — Skull of bird (Rhea) viewed from below. 

 c, condyle ; sp.C, entrance of spinal cord. 



(2) The lower jaw articulates with a bone called 

 the Quadrate, which may be easily recognised. It 

 roughly resembles a St. Andrew's cross. To the two 

 lower and shorter arms the lower jaw is hinged. To 

 the outside corner of the outer of these is attached a 

 long thin bone, which connects with the upper jaw. 

 The outer of the two upper arms fits into a hollow 

 in the bone called the Squamosal. In mammals the 

 quadrate is represented by an insignificant bone, the 

 Annulus of the ear (fig. 10, see p. 135). 



(3) In mammals the centra, the strong bases from 

 which spring the arches of the vertebrae, have between 

 them plates of bone, called Epiphyses, which are 



