THE UPPER SURFACE OF THE BASE OF THE SKULL. 165 
is the foramen ovale; through this pass both roots of the inferior maxillary nerve, 
the small meningeal artery, and some emissary veins. A little external and 
posterior to this is the foramen spinosum for the transmission of the middle 
meningeal vessels, together with a recurrent branch from the inferior maxillary 
nerve. Leading from the external extremity of the foramen lacerum there is a 
groove which passes outwards, backwards, and slightly upwards on the superior 
surface of the petrous temporal to end in the hiatus Fallopii (a cleft opening into 
the aqueductus Fallopii), which gives passage to the large superficial petrosal branch 
from the geniculate ganglion on the seventh nerve, together with the small petrosal 
branch of the middle meningeal artery. Just external to the hiatus Fallopi 
there is another small foramen for the transmission of the lesser superficial 
petrosal nerve. Overhung by the posterior border of the lesser wing of the sphenoid 
is the sphenoidal fissure, the cleft which separates the lesser from the great 
sphenoidal wings, and which opens anteriorly into the hollow of the orbit ; through 
this pass the third, fourth, ophthalmic division of the fifth, and sixth nerves, 
together with the ophthalmic veins as well as the sympathetic filament to the 
lenticular ganglion. Just below its inner extremity is the foramen rotundum for 
the passage of the superior maxillary nerve to the spheno-maxillary fossa. Behind 
this, and between it and the foramen ovale, the foramen Vesalii may occasionally 
be seen, through which a vein passes to reach the pterygoid plexus. 
The lateral parts of the middle fossa are moulded in conformity with the 
convolutions of the temporal lobes, but towards their inner sides the splitting of 
the dura mater in the region of the cavernous sinus serves to separate the cranial 
base from the under surface of the cerebrum. As may be seen by transmitted light, 
the floor of the lateral parts of the fossa is thin as it overlies the temporal, 
zygomatic, and glenoid fosse. The grooves for the lodgment of the branches of the 
middle meningeal artery leading from the foramen spinosum are readily seen ; one, 
coursing backwards a little below the line of the squamoso-parietal suture, 1s 
specially well marked. Amongst other features may be noticed the depression for 
the lodement of the Gasserian ganglion overlying the summit of the petrous 
temporal; behind and to the outer side of the hiatus Fallopii, the arcuate 
eminence, indicating the position of the superior semicircular canal ; and immediately 
anterior and slightly to the outer side of this the tegmen tympani, which roofs in 
the cavity of the tympanum, the thinness of which can readily be demonstrated 
if light be allowed to fall through the external auditory meatus. 
The posterior fossa is larger and deeper than the others. In front it is limited 
by a line on either side leading backwards and outwards from each posterior clinoid 
process along the superior border of the petrous part of the temporal bone, where 
externally and posteriorly it becomes confluent with the superior lip of the 
transverse groove for the lateral sinus, ending posteriorly in the middle line at 
the internal occipital protuberance. Along the line thus indicated the process of 
dura mater called the tentorium cerebelli, which roofs in the posterior fossa, is 
attached. The floor of the fossa, in which the cerebellar lobes, the pons, and 
medulla are lodged, is formed by the petrous and mastoid portions of the temporal 
bone, with part of the body of the sphenoid and the basilar portion of the occipital 
bone wedged in between them. Above the mastoid temporal a small part of the 
posterior inferior angle of the parietal enters into the constitution of the outer 
wall of the fossa. Behind and within these the lateral parts and lower portions of 
the squamous occipital complete the floor. In the middle line the floor of the fossa 
is pierced by the foramen magnum, in which lies the lower part of the medulla, 
together with its membranes, and through which pass upwards the vertebral 
arteries and the spinal accessory nerves. On either side of the foramen magnum, 
and a little in front of a transverse line passing through its centre, is the opening 
of the anterior condylic foramen for the passage of the hypoglossal nerve, a small 
meningeal branch from the ascending pharyngeal artery and an emissary vein. 
Overhanging the opening of the anterior condylic foramen there is a thickened 
rounded bridge of bone, to the outer side of which is placed the irregular opening of 
the jugular foramen (foramen lacerum posterius). The size of this is apt to vary on 
_ the two sides, and the lumen is frequently subdivided by a spicule of bone which 
