DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 177 
sections a bulbous form, below. The cartilaginous crista galli (c7.g.) is a mere retral 
point of the great septum ; below that point the outline is concave, and between the 
large cartilaginous cribriform plates (c7.p.), it rises up at the meeting of the presphe- 
noidal region with the perpendicular ethmoid. 
From the middle of the presphenoidal tract (p.s.), to the foramen magnum, the skull 
is drawn as cut along the mid-line; the front part is left entire. Right of the pre- 
sphenoidal, the orbitosphenoid (0.s,) is seen helping to wall in the orbital region ; its 
stem is ossified, and near the hind margin of the bony tract the small optic foramen 
(II.) can be seen. Above the bone the stem broadens suddenly into an axe-blade of 
cartilage, which reaches to the top of the skull in front, and also to the top of the 
side wall, further back. 
Then from the hind margin of the stem to the middle of the auditory capsule there 
is an elegant archway of cartilage nearly equal to the orbitosphenoidal stem in 
width. Over the junction of this arched band with the auditory capsule the cartilage 
—supra-auditory (s.a.c.)—more than equals the great blade of the orbitosphenoid in 
size. In reality it is twice as large, but the hinder two-fifths of this large crescentic 
crest is ossified as the supraoccipital (s.o.). The roughly oval space below the orbito- 
sphenoidal archway is filled up, in its antero-inferior third, by a small ruptured and 
out-turned part of the side wall—the alisphenoid (q/.s.); the angle between this 
auriform flap and the cochlea (ch/.) is filled by the huge Gasserian ganglion (V.). 
The upper two-fifths of the space under the archway is void of cartilage, and is 
finished by the investing bones—frontal, parietal, and squamosal. There is no pre- 
sphenoidal bone; the orbitosphenoids will meet to finish that region ; the basi- 
sphenoid (0.s.) is already present as a short tract in the middle of its own region. The 
small lobulate alisphenoid is not ossified, it has two large foramina near its upper part, 
the f. ovale (V*.) and the f. rotundum (V*.). The auditory capsule is relatively very large 
and extremely oblique in position; it stretches from the hind margin of the ali- 
sphenoid upwards and backwards to the lower edge of the supra-oecipital. The large 
archway for the 7th and 8th nerves (VIT., VIII.) —the meatus internus—is very near 
the great orbitosphenoidal archway, where it becomes supra-auditory. The pupitorm 
cochlea (ch/.) lies in a clearly-margined space, right and left of which the basioccipital 
and basisphenoidal regions meet. The anterior canal (a.s.c.) has its crown looking 
backwards as much as upwards; it arches over a considerable fossa for the flocculus 
cerebelli ; it is arched over by a very elegant crescentic channel for the lateral sinus, 
which makes the cartilaginous crest very thin at that part. The gap (foramen 
lacerum posterius) for the 9th and 10th nerves (IX., X.) is large, and the small 
hypoglossal foramen (XII.) is seen close behind it in the occipital arch. Above that 
hole the exoccipital bony centre (c.0.) is seen to occupy about a third of the side of 
the arch, between the supra- and basioccipital centres (s.0., b.o.) ; the latter is a large 
lozenge-shaped tract. 
Various investing bones are seen in situ, namely, the nasal, frontal, parietal, mter- 
MDCCCLKXXV. 2A 
