DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
127 
the widening skull-beam ( b.s .), now becoming parachordal, and also osseous, there is a 
large leafy growth, a broad petaloid, ear-shaped tract; this is the free, outlying 
alisphenoid ( al.s.). This “ wing ” is broadly falcate, tbe handle being made by the 
cutting away of its proximal part to widen the sphenoidal fissure ; the blade has its 
antero-external, sharp edge equal to a quadrant; its point is somewhat rounded ; and 
its back, looking towards the auditory capsule (chi.), is gently concave. This hinder 
wing does not quite reach to the margin of the continuous skull-wall ( o.s., s.a.c.). 
Half-way from the pituitary opening (pi).) to the edge, and nearer the hind than the 
fore margin of the wing, we see the foramen ovale for the third branch of the 
trigeminal nerve (V 3 .), but there is no separate passage, as yet, for the second —the 
foramen rotundum. 
One-third of the hind margin of the alisphenoid is ossified; the bony matter forms 
a selvedge both to the foramen and the hind edge of the wing. Also behind the 
pituitary hole there is a pyriform ossification as large as the two alisphenoidal centres 
together; its broad fore end is perforated—part of the pituitary space; this bone is 
the basisphenoid (b.s.). 
Behind this bone the basis cranii shows three wide convex tracts, margined by a 
lesser concavo-convex tract, right and left. ' The least convex of the three main parts 
is cut away, so to speak, right and left, in a perfect semicircle, by the more convex 
masses, each margin being bevelled down to a sharpish edge. This region is the 
“ spheno-occipital synchondrosis,” which gradually becomes less and less until the basi¬ 
sphenoid bone (b.s.) meets the basioccipital (b.o.). 
The large swollen part and the lesser sinuous margins, right and left, both belong to 
the auditory capsule, the inferior surface of which is well displayed in this aspect. The 
cochlea (chi.) shows three coils, in front of which there is a fissure through wdiich vessels 
pass, and also the 7th nerve, which runs inside the eave or outer thickening of the ear- 
capsule under the tegmen tympani (t.ty,). That archway is ended by the epihyal 
(e.hy.) in its confluence with the opisthotic region of the ear-capsule, and under it the 
7th nerve (VII.) runs, and behind it this nerve escapes ; its exit is through the stylo¬ 
mastoid foramen, and before its exit it gives oft* its returning fork, the chorda 
tympani. In front of the epihyal the fenestra ovalis (fs.o.) is seen, behind it the 
fenestra rotunda ( fr,), and inside that the enlarged fissure for the 9th and 10th 
nerves (IX., X.) 
Behind this cranio-auditory chink, the occipital arch is perforated a little nearer the 
mid-line, for the hypoglossal nerve (XII.), and behind the epihyal that arch has a 
definite paroccipital thickening of an oval shape. 
Here the lateral ossification (e.o.) has taken up much of the tract between the par- 
occipital swelling and the condyle (oc.c.); it reaches the condyloid foramen (XII.). 
The enlarged cartilaginous tracts, right and left, that form the condyles give the hind 
margin of the basis cranii an emarginate outline ; into this emargination under the 
