Skiill of Fowl. 97 



The skull of a Fowl contrasts greatly with that of the frog, 

 but it is formed out of precisely the same elements, save that 

 the labial cartilages are no longer represented. The most 

 obvious difference in the skull of a full-grown bird is its com- 

 plete ossification; it is only here and there that extremely 

 small portions of the original cartilage are left. From this it 

 follows that it is no longer possible to strip off the adherent 

 membrane bones ; they are firmly welded to the ot'her bones, 

 and, indeed, in the old skull the boundaries of the bones which 

 form the brain-case are no longer to be detected. 



In the bird's skull, not only is the primordial cartilaginous 

 cranium much more completely converted into bone than in 

 the frog, but the number of elements is greater. 



The hinder region of the skull (see Fig. 42) is a ring of 

 bone surrounding the foramen magnum, and is made up of 

 four originally separate bones — the basi-occipital (Fig. 42, b.o), 

 the suproroccipital {s.o), and two ex-occipitals (ed). The condyle, 

 by means of which the skull articulates with the backbone, is 

 single, and not double, as in the frog, and it is formed entirely 

 out of the basi-occipital. The cartilaginous base of the skull 

 in front of the basi-occipital is ossified to form three bones, one 

 in front of the other, which are at first distinct ; these are the 

 bast-sphenoid, pre-sphenoid, and the mesethmoid (etK). The walls 

 of the skull above these bones are formed by the ali-sphenoids 

 and the orbito-sphenoids {os). 



The auditory cartilage is ossified to form three bones, the 

 pro-otic, the epi-otic, and the opisthotic. 



The membrane bones, however, are hardly more numerous 

 than in the frog. Covering the skull above, and lying in front 

 of the supra-occipital, are the large parietals {p), in front of 

 which, but separate from them, are the frontals (/). At the 

 side of the frontals are the lacrymals (I), which are, as a rule, not 

 co-ossified with the skull, but are easily detachable. The wide 

 apertures of the nostrils are bordered behind by the bifid nasal 

 (11) bones, below by the maxillce (m.x), and in front by the 

 long nasal processes of the premaxillce (px). 



On the under surface of the skull, another s€t of membrane 

 bones are to be seen. Forming the base of the skull just in 



H 



