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. 
Fic. 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 vertebra, have between 
them plates of bone, called Epiphyses, which are 
