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 



