128 
MR. AV. K. PARKER ON THE STRUCTURE AND 
foramen magnum (/.to.) the notochord ( nc .) still projects. It is embedded in the 
middle of a roughly jDentagonal bony plate, the basioccipital ( b.o.). 
The structure of this fundamental and most instructive skull will be still better 
understood by referring to the upper aspect (fig. 2). 
Here the snout ( al.n .) is seen to be much shorter than below, and partly pinched off 
from the next or intermediate nasal region, the aliseptal ( al.sp .), which, in turn, passes 
a little suddenly into the enlarged olfactory region, proper, the aliethmoidal ( al.e .). 
Here all is finished, the crested intertrabecula being confluent with the nasal roofs 
throughout their whole extent; a peculiarly Mammalian structure. The hind margin 
of the double labyrinth, above, is elegantly bracket-shaped, the proper roof ending 
thick-edged, in front of the huge lozenge-shaped, perforated, secondary roof, or cribri¬ 
form plate ( cr.p .). The partition wall, septum nasi ( s.n .), in front, and perpendicular 
ethmoid (p.e.), behind, is, at first, scarcely apparent, above ; then in the aliseptal region 
it thickens out considerably, and the tract between the upper turbinals does this again, 
but to a lesser degree. This top of the wall thickens out in front of the cribriform 
plate, to fill in the space between the retiring roof; it then, in the rhinencephalic 
fossa, narrows considerably, to swell out again as the presphenoid ( p.s .). The perfora¬ 
tions for the olfactory nerves are simply countless, two crescentic rows lie back to 
back, close to the septum, and then about five more valleys, full of holes, run forwards 
and inwards from the postero-external margin of the great fossa. On a higher level 
than this valley full of holes are the roots of the orbitosphenoids ( o.s .) which are not 
so broad as the basal beam from which they arise, the presphenoidal region (p.s.); 
these frond-like growths of cartilage run up to, and beyond, the most bulging part of 
the skull in front, and form a good floor and -wall to the region of the fore-brain. 
Confluent with the edge of the cribriform plate in front, they are free behind, and 
have a thrice-notched margin there; their selvedge looks upwards, and, in front, 
melts into the general ethmoidal roof; whilst, behind, it is continuous with the large 
pterotic or supra-auditory crest ( s.a.c .), which in turn passes insensibly into the 
supraoccipital (s.o.). The roof of this skull, therefore, although open, or only covered 
by the investing bones, rests upon a complete rim of cartilage, from the top of the 
perpendicular ethmoid, in front, to the middle of the supraoccipital cartilage, behind. 
But the mid-brain rests upon the bulging, broken w r all of the hinder sphenoid, and 
the alisphenoidal plate, right and left (al.s.), is seen, in this view, away from the eye 
and partly hidden by the orbitosphenoid (o.s.). Down in the floor we see the optic 
passages (II.), the sphenoidal fissure for various nerves (V 1,3 .), the foramen ovale (V 3 .), 
and the fissure between the cochlea (chi.) and the alisplienoid, through the inner part 
of which the internal carotid artery enters. 
Then in the auditory capsule itself the sieve for the 7th and 8th nerves (VII., 
VIII.); between the capsule and the skull the posterior lacerated foramen for the 9th 
and 10th nerves (IX., X.) ; and through the contiguous exoccipital tract the proper 
foramen fur the hypoglossal (XII.). 
