THE ANxiTOMY OF BIIWH. — OSTEOLOGY. 



155 



front, are divided into several recognizable parts, ale, 

 ah, 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 labyrintli, among the con- 

 volutions of which will be the superior and interior tur- 

 binal cartilages, in addition to those already noted. 

 The ethmoidal wall ends behind atp.s, the presphe- 

 n(]idal region, where the brain case begins ; behjw and 

 behind, it is deeply notchcMl tor the optic foramen, 2. 

 The pituitary space f'oi'ins a circular foramen, through 

 which the carotid arteries enter. The site of the orbit 

 of the eye is bounded beliirul and lielow by the post- 

 frontal process of the alisphenoid wiug, p)f of as. The 

 pterygo-pahitine rod is seen along the under bonk'r of 

 the skull, pfj and pa. The quadrate, q, has acf[uired 

 nearly its shape, and th(! rest of the mandibular and 

 hyoidean jiarts are clearly displayed, mJc, 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- 

 (jped the hasitemporal and paruspjlieimid bones which 

 nnderdoor 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 tlie skull r})s. 



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-occi[)itals, 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 (jf the skull. The prenasal cartilai/e, pn, now 

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

 side of it is formed a three-ft)rked lione, the premaxil- 

 lary, px, having superiorly nasal, and laterally palatal 

 and dentary pirocesses. This bone is to grow to great 

 size, f(jrming 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 premaxillary and the quadrate are the 

 bon(!s forming the zygoma, fir 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, mxp; 

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



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

 viewed from hahtw, y Gj^ rliameters. pn, 

 prenasal cartilage, running beliind into tbe 

 septum nasi ; on eacL side of it tbe premax- 

 illary, ji.c, of which the (inner) paiatal and 

 toutei) dentary processes are seen (the upper 

 nasal process hidden) ; w.r, the maxillary, 

 developing inner process, the maxillo-pala- 

 tine, m-rp ; pa. the palatal, well-formed, ar- 

 ticulating behind with rbn, the sphenoidal 

 rostrum, its thickened under bonier, the 

 parasphenoid ; this will hear the vomer at its 

 end when that bone is developed: j, jugal, 

 .loining H),r and r//, the quadrato-jugal, join- 

 ing i and (j, the quadrate ; mx to q, the 

 jugal bar or zygoma ; ptj, the pterygoid, 

 making with pa the pterygo-palaline bar, 

 joining q and px ; bt, the hasitemporal, great 

 m.at of bone from ear to ear, underflooring 

 the skull proper, as rb.t, 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 pituit.ary fossa (figs. 64, G6, ic) ; fi/, 

 tympanic cavity — external opening of ear; 

 a.s, 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 co, exoccipital, which will reach 

 the condyle sliown in the middle line, above 

 the foramen magnum, frn, completed above 

 by .s'o, supra-occipital ; 8. foramen lacerum 

 posterius, exit of pneumogastric, glosso-pha- 

 ryngeal and spinal accessory nerve; 9, exit 

 of In poglossal nerve, in basi-occipital. (After 

 Parker.) 



