THE ANATOMY OF BIBBS. — OSTEOLOGY. 



155 



front, are divided into several recognizable parts, ale, 

 als, aln, the latter being the external nostril; pp 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 wiU be the superior and inferior tur- 

 binal cartilages, in addition to those already noted. 

 The ethmoidal wall ends behind sXps, the presphe- 

 noidal region, where the brain case begins ; below and 

 behind, it is deeply notched for the optic 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 

 pterygo-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 

 hyoiddan parts are clearly displayed, mh, etc. The 

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

 otie 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 basitemporal and pa/rasphenoid bones which 

 underfloor the whole skull, — the former making a plat 

 between the ears, fig. 69, ht, 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 aye well formed; the 

 condyle is perfect ; the foramen magnum is circum- 

 scribed by the ex- and supra- oocipitals, eo and so, flg. 

 69. Investing bones, formed in membrane vidthout pre- 

 vious cartilage, are becoming apparent. The basitem- 

 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 upper jaw. The palatal, pa, and pterygoid, pg, 

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

 Between the premaxiUary and the quadrate are the 

 bones forming the sygoma, or jugal bar, developed in 

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

 embryo. They are the weak maxilla/ry, mx, with its 

 ingrowing process, the manMo-palatine bone, mxp; 

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



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

 viewed from below, x 6S diameters, pn, 

 prenasal cartilage, running behind into the 

 septum nasi ; on each side of it the premax- 

 iUary, px, 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 vomer at its 

 end when that bone is developed; j, jugal, 

 joining mx and qj, the quadrato-jugal, join- 

 ing 3 and q, the quadrate ; mx to q, the 

 jugal bar or zygoma ; pg, the pterygoid, 

 making with pa the pterygo-palatine bar, 

 joining q and pa; ,■ bt, the basitemporal, great 

 mat of bone from ear to ear, underflooring 

 the skull proper, as rbs, a similar formation, 

 does further forward ; ic, 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 (flgs. 64, 66, ic); ty, 

 tympanic cavity — external opening of ear; 

 as, 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 jn the middle line, above 

 the foramen magnum, /m, completed above 

 by so, supra-occipital; 8, foramen lacerum 

 posterius, exit of pneumogastric, glosso-pha- 

 ryngeal and spinal accessory nerve; 9, exit 

 of hypoglossal nerve, in basi-occipital. (After 

 Parker.) 



