THE ANATOMY OF BIRDS. — OSTEOLOGY. 



161 



front, are divided into several recognizable parts, ale, 

 als, aln, the latter being the external nostril; })}) is a 

 transverse partition between the orbital and nasal cham- 

 bers. The nasal cartilages ultimately become much 

 convoluted to form the nasal labyrinth, among the con- 

 volutions of which will be the superior and inferior tur- 

 binal cartilages, in addition to those already noted. 

 The ethmoidal wall ends behind a,t ps, the presphe- 

 noidal region, where the brain case begins ; below and 

 behind, it is deeply notched for the 02)tic foramen, 2. 

 The pituitary space forms a circular foramen, through 

 which the carotid arteries enter. The site of the orbit 

 of the eye is bounded behind and below by the post- 

 frontal process of the alisphenoid wing, pf of as. The 

 I)terygo-palatine rod is seen along the under border of 

 the skull, pg and pa. The quadrate, q, has acquired 

 nearly its shape, and the rest of the mandibular and 

 hyoidean parts are clearly displayed, mk, etc. The 

 proximal hyoidean element, st, is freed from the peri- 

 otic cartilage, leaving the fenestra ovalis (see last para- 

 graph). Below the general outline, pa to oc, is not 

 shown a mat of soft tissue, in which are to be devel- 

 oped the hasitemporal and parasphenoid bones which 

 underflocjr the whole skull, — the former making a plat 

 between the ears, fig. 69, bt, the latter forming the thick- 

 ened under edge of the rostrum of the skull rbs. 



At the third stage, about the middle of the second 

 week of incubation, the cartilaginous parts already 

 •described are neatly finished, and the skull is beginning 

 to ossify. The occipital parts are well formed; the 

 condyle is perfect ; the foramen magnum is circum- 

 scribed by the ex- and supra-occipitals, eo and so, fig. 

 09. Investing bones, formed in membrane without pre- 

 vious cartilage, are becoming apparent. The hasitem- 

 poral, bt, and parasphenoid, rbs, are engrafting upon 

 the base of the skull. The prenasal cartilage, pn, now 

 at its fullest growth, is beginning to decline ; on each 

 side of it is formed a three-forked bone, the premaxil- 

 lary, px, having superiorly nasal, and laterally palatal 

 and dentary processes. This bone is to grow to great 

 size, forming most of the upper beak, and starving out 

 the maxillary, which in mammals is the principal bone 

 of the ui)per jaw. The palatal, pa, and pterygoid, pg, 

 bones are ossified, and the quadrate, q, is ossifying. 

 Between the premaxillary and the quadrate are the 

 bones forming the zygoma, or jugal bar, developed in 

 the outer part of the maxillo-palatine bar of the earlier 

 embryo. They are the weak maxillary, mx, with its 

 ingrowing process, the maxillo-palatine bone, nixp; 

 next the jugal, j ; then the quadrato- jugal, qj; the 



11 



',:.f/fe' 





Fig. 69. — Skull of cliick, third stage, 

 viewed from belmv, x 6| diameters, pn, 

 prenasal cartilage, running behind into the 

 septum nasi ; on each side of it the premax- 

 illary, ;).r, of which the (inner) palatal and 

 (outer) dentary processes are seen (the upper 

 nasal process hidden) ; mx, the maxillary, 

 developing inner process, the maxillo-pala- 

 tine, mxp ; pa, the palatal, well-formed, ar- 

 ticulating behind with rbs, the sphenoidal 

 rostrum, its thickened under border, the 

 parasphenoid; this will bear the vomeral its 

 end when that bone is developed; J, jugal, 

 joining mx- and (/j, the quadrato-jugal, join- 

 ing ; and q, the quadrate ; mx to fj, the 

 jugal bar or zygoma ; pt). the pterygoid, 

 making with pa the pterygo-palatine bar, 

 joining 7 a,ndpx : bt, the hasitemporal, great 

 mat of bone from ear to ear, underflooriiig 

 the skull proper, as rbx, a similar formation, 

 does further forwani ; (V, outer end of carotid 

 canal, to run between the bt plate and true 

 floor of skull, and enter brain cavity at origi- 

 nal site of pituitary fossa (tigs. 64, 66, ic) ; tij, 

 tympanic cavity — external opening of ear; 

 a.t, alisphenoid, bounding much of brain- 

 box anteriorly, and orbital cavity posteri- 

 orly; psc, posterior semicircular canal of ear, 

 in opisthotic bone, which will unite with the 

 spreading eo, exoccipital, which will reach 

 the condyle shown in the middle line, above 

 the foramen magnum, fm, completed above 

 by so, supra-occipital; 8. foramen lacerum 

 posterius, exit of pneumogastric, glosso-pha- 

 ryngeal and spinal accessory nerve; 9, exit 

 of h\ poglossal nerve, in basi-occipital. (After 

 Parker.) 



