314 THE VERTEBRATE SKELETON. 



the bones of the cranium proper and those forming the '-rest 

 of the skull, for the bones forming the cranium proper are 

 closely fused together, the sutures between them being nearly 

 all completely obliterated in the adult, while the bones form- 

 ing the face are loosely connected with the cranium proper ; 



(3) by the prolongation of the face into a long toothless beak ; 



(4) by the size of the orbits, and their position entirely in 

 front of the cranium, so that they are separated from one 

 another only by a thin interorbital septum. 



For purposes of description the skull may be divided into 



(1) The cranial portion. 



(2) The facial portion. 



(3) The mandible. 



(4) The hyoid. 



(1) THE CRANIAL PORTION. 



This is a rounded box expanded dorsally and posteriorly, 

 but tapering antero-ventrally. In the young skull the divi- 

 sional lines between the several bones can be easily seen, 

 but in the adult they are quite obliterated. 



(a) The dorsal surface is rounded, expanded in front and 

 behind, but encroached upon in the middle by the cavities of the 

 orbits. There is a prominent divisional line in front, separating 

 it from the facial part of the skull. It is formed mainly by 

 the frontal (fig. 59, A, 6) and parietal bones, but the frontals 

 diverge a little anteriorly and enclose between them the ends 

 of the nasal processes (fig. 59, A, 4) of the premaxillae. Just 

 in front of the orbit the outer margins of the frontals are 

 either notched or pierced by a pair of foramina. 



(b) At the posterior end of the cranium the most promi- 

 nent feature is the large, almost circular foramen magnum, 

 through which the spinal cord and brain communicate ; this in 

 young birds is seen to be bounded by four distinct bones, dor- 

 sally by the supra-occipital, ventrally by the basi-occipital, 

 and laterally by the exoccipitals. 



