The Skull as a Whole. 79 



process presents three foramina, of which the largest, anterior, and 

 medial one, the anterior sphenoidal foramen, serves for the transmission 

 of the internal maxillary artery, while the remaining two, the middle and 

 posterior sphenoidal foramina, transmit muscular branches (masseterico- 

 temporal and pterygobuccinnator) of the mandibular nerve. On the 

 medial side of the base of the medial lamina of the pterygoid process 

 there is a shallow longitudinal groove, representing the pterygoid canal 

 (canalis pterygoideus) of the human skull. Immediately in front of the 

 tympanic bulla, on the ventral surface of the skull, an irregular aperture, 

 the foramen lacerum, leads directly into the cranial cavity. It is 

 incompletely divided into two parts by a slender bony splint. It 

 contains, in addition to the foramen lacerum, the foramen ovale of the 

 typical mammalian skull, and serves to transmit the mandibular portion 

 of the fifth nerve and the internal carotid artery. Looking into the aper- 

 ture from the front, it is seen to communicate not only with the 

 cranial cavity, but also with two apertures in the anterior portion of 

 the auditory complex. One of these — that toward the middle line — is 

 the internal carotid foramen (foramen caroticum internum). It is the an- 

 terior end of a canal transmitting the internal carotid artery; the pos- 

 terior end of this canal, 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 

 posteromedial surface of the tympanic bulla. The second, lateral 

 aperture 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 com- 

 munication with the nasal portion of the pharynx. Associated with the 

 mastoid process is a small aperture, the stylomastoid 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 trans- 

 verse 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 configura- 

 tion of the brain. It is divisible into three portions, known as the 

 cranial fossae. The anterior cranial fossa (fossa cranii anterior) is a 



