THE PIGEON 



405 



parts are bony in the pigeon, whereas in the rabbit the 

 sternal ribs are cartilaginous. On the hinder side of each 

 of the free ribs, except those of the last pair, is an uncinate 

 process. The skull is remarkable for the fusion of most of 

 its bones. There is a short, wide cranium, lying mainly 

 behind the large orbits, which are separated, not by the 

 cranium, but by an interorbital septum (p. 386). A scaffold- 

 ing of slender jawbones supports the beak. 



The hinder part of the cranium is formed by two exoccipitals at the 

 sides of the foramen magnum, a median basioccipital below and a 



£.Oc. 



An~ 



Fig. 295. — A diagram of a bird's skull, disarticulated. — 

 After Gadow. Membrane bones shaded. 



B.Oc, basioccipital; E.Oc, exoccipital ; S.Oc, supraoccipital ; 

 Pa., parietal; Fr., frontal; Na., nasal; Jim., premaxilla ; 

 M., maxilla ; Ju., jugal; Qj., quadrato-jugal ; Qu., quadrate ; 

 P., p alatine ; Pi., pterygoid ; pe. , periotic ; Sq., squamosal ; 

 AS., alisphenoid ; B.S., basisphenoid ; O.S., orbito- 

 sphenoid ; Pr.Sph., presphenoid ; zio., vomer; tOS., inter- 

 orbital septum; £., ethmoid; .5V., nasal septum; De., 

 dentary ; SJ>., splenial ; An., angular; S.A., supra-angular; 

 Ar., articular; MK., Meckel's cartilage. 



median supraoccipital above. There is one median occipital condyle, 

 formed mainly by the basioccipital. The roof of the cranium in the 

 middle and foremost regions is formed by the parietals and frontals. 

 In the region of the parietals the floor is formed by the basisphenoid, 

 which lies in front of the basioccipital, but is covered in below by a 

 broad membrane bone, the basitemporal, which perhaps corresponds 

 to the crosspiece of the parasphenoid. The side of the skull in this 

 region is formed mainly by the squamosal, from which a zygomatic 

 process projects forwards, lying free. Below the squamosal the wall 

 is derived in front from the alisphenoid and behind from the bones of 

 the auditory capsule united with adjoining bones, but the limits of 

 none of these can be made out. In the frontal region, the cranial 

 cavity is greatly restricted by the presence of the interorbital septum, 

 over which, however, it extends forward somewhat. The septum is 



