ANATOMY OF HETERODONTUS: ENDOSKELETON 453 
From the fontanelle anteriorly the walls are extended by the 
rostral and basitrabecular cartilages. 
Along the ventral margin the floor or basis cranii also shows 
extremes in thickness. Directly under the anterior fontanelle 
it is relatively thick. It then becomes thinner and thinner 
posteriorly until it reaches the foramen for the internal carotid 
artery (f.i.c.). As we have said (p. 450) this foramen divides 
the basis cranii into two parts, the anterior of which is the 
embryonic trabecular, and the posterior the parachordal region. 
The parachordal or the part accompanying the notochord is 
greatly thickened. It extends to the posterior part of the 
cranium as a somewhat spool-shaped segment. Inside of this 
is the cranial notochord (ch.), and surrounding it posteriorly 
are calcified tissues. : 
Posterior to the end of the cranial notochord but not in the 
middle line appears the occipital condyle (o.cd.). Other struc- 
tures are seen below the basis cranii and in the background. 
These in front of the occipital condyles are the post- and pre- 
hyomandibular processes; under the socket is the down-curving 
basal plate; and under the anterior fontanelle, the margin of the 
palatal fossa. 
The foramina perforating the cranium are here seen to advan- 
tage. The most anterior of these is the large opening through 
which the olfactory tract passes (f.J). Midway between this 
and the occipital condyle is the optic foramen (f.J/); above and 
anteriorly is the anterior cerebral foramen. (f.a.c.). Almost di- 
rectly above the optic is the trochlear (f.JV). Slightly posterior 
to the optic are two foramina, the upper for the oculomotor 
nerve (f.JI1), the lower for the ramus anastomoticus artery (f.7-a.). 
Above the entrance to the internal carotid artery is the inter- 
orbital canal (7.0.). Above the anterior tip of the cranial 
notochord are two foramina, the upper of which is the large 
‘orbital fissure (0.f.); the lower of the two is a double foramen, 
the anterior division of which is for a part of the facial nerve; 
the posterior gives the acustic or eighth cranial nerve access 
to the ear (f.VIII). Posterior to this is the smaller foramen 
for the glossopharyngeal nerve, behind which is the larger 
