THE SKULL AS A WHOLE. 133 
the point at which the internal carotid artery enters the tympanic 
bone, or the external carotid foramen (foramen caroticum 
externum), being visible as a rounded aperture lying on the postero- 
medial surface of the tympanic bulla. The second, lateral aper- 
ture communicating with the foramen lacerum is that of the 
auditory (Eustachian) tube (tuba auditiva). It leads into the 
tympanic cavity, and in the natural condition the tube places this 
cavity in communication with the nasal portion of the pharynx. 
Associated with the mastoid process is a small aperture, the stylo- 
mastoid foramen (foramen stylomastoideum), the external 
opening of the facial canal, which serves for the passage of the facial 
nerve. A slit-like aperture, the jugular foramen (foramen 
jugulare), lies in the jugular fossa, between the posterior ventral 
margin of the tympanic bulla and the occipital condyle. It 
transmits the first portion of the internal jugular vein from the 
transverse sinus of the dura mater, and also the ninth, tenth, and 
eleventh cranial nerves. Finally, immediately in front of the 
dorsal portion of the condyle, the occipital segment is perforated 
by several small apertures together representing the hypoglossal 
canal (canalis hypoglossi), and serving for the transmission of the 
hypoglossal nerve. 
The roof of the cranial portion is largely formed by two pairs of 
thin membrane elements, the frontal and parietal bones. The 
former occupy a general position between the orbits, while the latter 
are interposed between the frontal bones and the occipital segment. 
A small portion of the roof is formed posteriorly, however, by a small 
lozenge-shaped element, the interparietal bone, and by the shield- 
shaped projection, described above, which is part of the occipital 
bone. 
The space enclosed by the cranial portion of the skull is the 
cranial cavity (cavum cranii). Its form depends on the external 
configuration of the brain. It is divisible into three portions, 
known as the cranial fossae. The anterior cranial fossa (fossa 
cranii anterior) is a small division lodging in the natural condition 
the olfactory bulbs of the brain. The middle cranial fossa, 
the largest division of the cavity, lodges the enlarged cerebral 
hemispheres. The posterior cranial fossa is a small division 
extending backward to the foramen magnum and lodging in the 
