DEVELOPMENT OF THE SKULL'IN THE MAMMALIA. 
89 
skull at the sphenoidal fissure with the 2nd branch of the 5th (V 1 , 2 .). The pre- 
sphenoidal region ( p.s.) is still unossified; it can be seen between the diverging 
palatines. The posterior sphenoid stretches across the whole inter-glenoidal region, 
outside and inside the pterygoids. The basisphenoid (b.s.) has ossified three-fourths 
of its proper territory; it is emarginate both before and behind. Nearer the eye 
than the orbital wing ( o.s.), each alisphenoid (V 1 , 2 . to V 3 .) reaches the squamosal in 
front, and is then deeply notched for the 3rd branch of the 5th nerve (V 3 .). This 
notch is protected by bone behind, which runs between the nerve and the auditory 
capsule (chi.) ; on the outside the foramen ovale is finished by other parts, it merely 
notches the outer edge of the alisphenoid bone. In this figure the left annulus and its 
related parts are shown; whilst on the right side of the head—left of the figure— 
the lower face of the capsule is exposed. 
The annulus ( a.ty .) is a wide U-shaped tract, thickened at the end of its inturned 
cornua ; the foremost of these is notched to let Meckel’s cartilage (mk.) pass. 
This is drawn as cut across. The narrow annulus leaves a wide and exposed drum 
membrane ( m.ty .), in which we see a long manubrium mallei proceeding from 
the head of the malleus (ml.), which articulates with the incus (i.) under the double 
tegmen formed by the squamosal (sq.) outside and the capsule within. The cochlea 
is exposed on the inner side of the annulus, and where the pterygoid (pg.) overlaps 
these parts, there the Eustachian aperture opens into the throat. The cochlea and 
vestibule (chi., op.) are large and normal; they are quite unossified; the fenestra 
ovalis (fs.o.) is large; outside it is the passage for the 7th nerve {VII.), which escapes 
through the stylomastoid foramen behind the small epihyal (e.hy.) ; the rounded 
part behind that opening (op.) contains the ampulla of the posterior canal. A band 
of cartilage separates the fenestra ovalis (fs.o.) from the fenestra rotunda (fr.), which 
is seen behind the first or proximal coil of the cochlea (chi.). A deep groove runs 
between the occipital arch and the capsule, and on the inner side of the vestibule 
both parts are notched to make a common passage for the 9th and 10th nerves (IX., 
X.). Behind and within this is another passage or hole in the occipital arch within 
the exoccipital bone (e.o.), which is creeping forwards ; this is for the large hypoglossal 
nerve (XII.). Thence to the edge this part is ossified; and the bony matter runs 
right and left, up to and over the convex arcuate condyles ( c.oc ). But a very wide 
band of cartilage is seen continuous with, and in front of the condyles, margining the 
foramen magnum (fm.), and ending in a concave margin, where the basioccipital 
bone (h.o.) has broadened out to twice its interauditory region. That bone is rather 
pinched in towards its fore end, where it is notched, and has a fine tubular process 
in the notch which runs much of the distance between this bone and the basi¬ 
sphenoid (b.s.) ; that process is the ossified sheath of the fore part of the cranial 
notochord (nc.) 
The upper view (Plate 12, fig. 1) shows how greatly the investing bones have 
developed since the early stage (Plate 11, fig. 2). The facial region is very small 
MDCCCLXXXV. N 
