THE ANATOMY OF BIRDS.— OSTEOLOGY. 
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 will be the superior and inferior tur- 
binal cartilages, in addition to those already noted. 
The ethmoidal wall ends behind at ps, 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 
hyoidean parts are clearly displayed, mk, ete. 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 basitemporal and parasphenoid bones which 
underfloor the whole skull, — the former making a plat 
between the ears, fig. 69, bt, the latter forming the thick- 
ened under edge of the rostrwm 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 cireum- 
scribed by the ex- and supra-occipitals, eo and so, fig. 
69. Investing bones, formed in membrane without 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 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 mavillo-palatine bone, map; 
next the jugal, 7; then the quadrato-jugal, qj; the 
155 
Fig. 69.—Skull of chick, third stage, 
viewed from below, x 63 diameters. pm, 
prenasal cartilage, running behind into tbe 
septum nasi ; on each side of it the premax- 
illary, 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, map; 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 j/ and g, the quadrate ; mx to q, the 
jugal bar or zygoma ; pg, the pterygoid, 
making with pa the pterygo-palatine bar, 
joining g and pz ; dt, the basitemporal, great 
mat of bone from ear to ear, underflooring 
the skull proper, as rds, a similar formation, 
does further forward; ic, outer end of carotid 
canal, to run between the b¢ plate and true 
floor of skull, and enter brain cavity at origi- 
nal site of pituitary fossa (figs. 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 in the middle line, above 
the foramen magnum, 7, 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.) 
