164 OSTEOLOGY. 
three fossx, of which the cerebrum occupies the anterior and middle, whilst in the 
posterior is lodged the cerebellum. 
The anterior fossa is defined posteriorly by the sharp thin edge of the lesser 
wings of the sphenoid, which curve outwards and slightly upwards as well as back- 
wards to reach the region of the pterion externally. The floor is formed from before 
backwards, in the middle line, by the upper surface of the ethmoid and ‘the fore 
part of the body of the sphenoid ; laterally it is constituted by the orbital plates 
of the frontal and the lesser wings of the sphenoid. On these the under 
surface of the frontal lobes of the cerebrum rests. In front the fossa is divided 
mesially by the frontal crest, to which the falx cerebri is attached. This is con- 
fluent below with the fore part of the crista galli, from which, however, it is 
separated by the foramen cecum, which usually transmits a small vein from the 
nose. On either side of the crista galli there are grooves which vary considerably 
in depth and width: therein are lodged the olfactory lobes. The floor and sides of 
the groove are pierced by numerous foramina; of these the largest number transmit 
the olfactory nerves to the nasal fosse. In front an elongated slit, placed on 
either side of the crista, affords a passage to the nose for the internal nasal nerve 
and a small branch of the anterior ethmoidal artery which accompanies it. To 
the outer side of the olfactory groove and the cribriform plate, the anterior 
fossee communicate on either side by means of the two ethmoidal foramina with 
the cavities of the orbits. The anterior foramen transmits the internal nasal 
nerve and the anterior ethmoidal artery; the posterior affords passage to the 
posterior ethmoidal artery and the small spheno-ethmoidal nerve of" Luschka. 
External to the olfactory groove, the floor of the fossa, which here corresponds 
to the roof of the orbit, is very thin, as may be seen by holding the skull up to 
the light; it is convex from side to side, and bears the impress of the con- 
volutions of the under surface of the frontal lobes of the cerebrum which rest 
upon it. In front and at the side there are a number of vascular grooves for 
the branches of the anterior and middle meningeal arteries respectively. 
The middle fossa, which in form may be compared to the wings of a bird 
united by the body, is bounded in front by the curved thin posterior edge of the 
lesser wings of the sphenoid ; posteriorly, by the line of attachment of the tentorium 
cerebelli, extending from the posterior clinoid process along the superior border of 
the petrous portion of the temporal bone. The median part of the fossa, which is 
narrow, corresponds to the sella turcica and the olivary eminence of the sphenoid. 
It is limited anteriorly by a line connecting the anterior margins of the two optic 
foramina, and is overhung behind by the dorsum selle. In this area are lodged 
the structures which lie within the interpeduncular space on the base of the brain. 
The floor of the lateral parts of the fossa on each side is formed by the great wing 
of the sphenoid in front, the squamous part of the temporal bone to the outer side, 
and the superior surface of the petrous temporal behind. In the hollows so formed 
the temporal lobes of the cerebrum are lodged. On either side of the olivary 
eminence are seen the optic foramina; these pass into the orbital cavities and 
transmit the optic nerves and ophthalmic arteries. Immediately behind these 
openings the anterior and middle clinoid processes are sometimes united, so as to 
enclose a foramen. Through this the internal carotid artery passes upwards. 
Leading backwards from this, along the side of the body of the sphenoid, is the 
carotid groove, which turns downwards near the apex of the petrous temporal, to 
become continuous with the carotid canal, which here opens on the posterior wall 
of an irregular aperture, placed between the side of the body of the sphenoid and 
the summit of the petrous temporal, called the foramen lacerum medium. ‘Through 
the inner angle of this opening the carotid artery accompanied by its plexus of 
veins and sympathetic nerves passes upwards. Running through the fibrous tissue, 
which in life blocks up this opening, the large superficial petrosal nerve coming 
from the hiatus Fallopii passes downwards and forwards to reach the posterior 
orifice of the Vidian canal, which is placed on the anterior and inferior border of 
the foramen Jacerum medium. A small meningeal branch of the ascending 
pharyngeal artery also passes upwards through this foramen. In front and to the 
outer side of the foramen lacerum, and separated from it by a narrow bar of bone, 
