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bounded by the inner surfaces of the internal pterygoid plates, whilst above, the 
outer side of the arch is formed by the vaginal processes of the same plate; intern- 
ally they are separated by the thin vertical posterior border of the vomer, whilst 
above the everted ale of the same bone form the inner sides of the arch. The 
plane of these apertures is not vertical but oblique, corresponding usually to a 
line drawn from the bregma above through the last molar tooth of the upper 
jaw below. ‘Their size varies considerably, but the height is usually equal to twice 
the width. 
. The region of the cranium which hes external to the superior maxilla and 
_ external pterygoid plate corresponds to the zygomatic fossa, which has been already 
described as seen from the side (norma lateralis, p. 152). Viewedjfrom below, the 
zygomatic fossa is bounded in front by the posterior surface of the body of the 
superior maxilla and the internal surface of the malar bone. The roof, which is 
t 
; 
NORMA BASALIS OF THE SKULL 159 
_ traversed by the spheno-squamosal suture, is formed in front by the under surface 
_ of the great wing of the sphenoid, and behind by a small triangular surface of the 
under side of the squamous part of the temporal bone immediately in front of the 
eminentia articularis. 
. Circumscribed externally and behind by the anterior root of the zygoma, which 
_ curves forward to become continuous in front with the infra-temporal crest crossing 
the external surface of the great wing of the sphenoid, the roof of the fossa is 
separated from its anterior wall by the spheno-maxillary fissure, which is so inclined 
that with its fellow of the opposite side it forms an angle of 90°. Superiorly the 
zygomatic fossa communicates freely with the temporal fossa beneath the zygo- 
matic arch, though the student must bear in mind the fact that when the inferior 
maxilla is in position the external limits of the space are very much reduced 
Bp. 153). 
¥ The under surface of the great wing of the sphenoid is here V-shaped. The 
angle corresponds to the spine, the outer limb to the spheno-squamosal suture, 
whilst the imner limb corresponds to a narrow cleft, the fissura spheno-petrosa, 
_ which separates it from the petrous portion of the temporal bone to which it is 
united in the recent condition by a synchondrosis. Along the line of this latter 
fissure the edges of the adjacent bones (sphenoid and petrous temporal) are 
_ bevelled so as to form a groove, which extends from the root of the inner pterygoid 
_ plate internally to the inner side of the base of the alar spine externally, where 
_ the groove ends by entering an osseous canal. In the groove (sulcus tubs auditive) 
is lodged the cartilaginous part of the Eustachian tube, whilst the osseous canal 
includes the bony part of the same tube, together with the tensor tympani muscle, 
_ which is lodged in a separate compartment immediately above it. The anterior 
extremity of the cartilaginous part of the Eustachian tube is supported by the 
posterior edge of the internal pterygoid plate, which is often notched for its recep- 
tion. Between the root of the external pterygoid plate and the alar spine there 
are two foramina, which he immediately in front of the suleus tube auditive. 
Of these, the larger and anterior is the foramen ovale, through which pass the 
‘Motor root and inferior maxillary division of the fifth nerve, together with the 
small meningeal artery. The smaller, which from its position immediately in 
front of the alar spine is called the foramen spinosum, transmits the middle menin- 
geal artery and sympathetic plexus surrounding that vessel. The lesser superficial 
petrosal nerve here passes through the base of the skull to join the otic ganglion 
either through a small foramen (canalis innominatus) placed between the foramen 
ovale and the foramen spinosum, or through the foramen ovale or through the 
spheno-petrosal fissure. ‘The position of the suture between the basioccipital and 
-basisphenoid corresponds to a line connecting the tips of the pterygoid tubercles at 
the root of the internal pterygoid plates. 
Occasionally in the centre of this line there is a small pit with a foramen leading from it. This 
probably represents the lower end of the cranio-pharyngeal canal. 
The under surface of the basioccipital (pars basilaris) stretches between the 
body of the sphenoid in front and the anterior margin of the foramen magnum 
behind ; projecting from its centre is a slight elevation, the pharyngeal tubercle 
