The; Skull as a Whole. 77 



median point of attachment for the occipital muscles and the ligamentum 

 nuchae. 



The ventral wall of the cranial portion is the basal portion (basi- 

 cranium) of the entire skull. As indicated above (p. 31), its axial line, 

 the basicranial axis, continues, in general, that of the bodies of the 

 vertebrae, and its posterior portion is equivalent, morphologically, to 

 vertebral segments. It is formed by a linear series of three bones, 

 namely, the basilar portion of the occipital, the body of the posterior 

 sphenoid, and that of the anterior sphenoid (respectively basioccipital, 

 basisphenoid and presphenoid bones). Its extremely narrow, anterior 

 portion forms the roof of a deep groove which encloses the nasal portion 

 of the pharynx. As viewed from the ventral surface, it is seen to dis- 

 appear in the facial complex at some distance dorsal to the posterior 

 margin of the bony palatine bridge. Laterally, it is separated from the 

 orbit on either side by a vertical plate formed by the palatine bone, and 

 also by two downward projections of the posterior sphenoid, the medial 

 and lateral laminae of the pterygoid process (processus pterygoideus). 

 These structures enclose between them the pterygoid fossa (fossa 

 pterygoidea), the walls of which serve for the attachment of the external 

 and internal pterygoid muscles of the mandible. 



The lateral wall of the cranial portion forms anteriorly a large part of 

 the boundary of the orbit. The cranial wall of the orbit is partlv formed 

 by two upward projections of the basicranium, namely, the lesser wing 

 of the anterior sphenoid, or orbitosphenoid, and the greater wing of the 

 posterior sphenoid, or alisphenoid. The remaining portion is formed by 

 membrane elements, including the frontal bone of the roof of the skull 

 and the squamosal bone, the latter distinguishable as the support of the 

 posterior root of the zygomatic arch. Immediately behind the orbit, 

 the root of the zygomatic arch projects outward and then downward. 

 It is formed by a zygomatic process (processus zygomaticus) of the 

 squamosal bone, and the tip of this forms a vertical plate, which is united 

 by a horizontal suture with the zygomatic bone. On the ventral side of 

 the process, close to the cranial wall, is the mandibular fossa (fossa 

 mandibularis), for articulation with the mandible. On its dorsal side, 

 but more especially on the adjacent portion of the cranial wall, there is a 

 shallow, horizontal groove, lodging in the natural condition the 

 temporalis muscle of the mandible, and therefore representing a greatly 

 reduced temporal fossa (fossa temporalis). In the natural condition the 

 anterior portion of the groove is converted into a foramen through the 

 presence of a stout ligament extending from the posterior supraorbital 

 process to the base of the zygomatic arch. The dorsal boundarv of the 

 fossa is formed by a pronounced ridge, the temporal line flinea temporalis), 

 the latter forming also the lateral margin of the roof of the skull in this 

 region. 



Behind the posterior root of the zygomatic arch, the external surface 

 of the lateral wall is largely occupied by the swollen tympanic bulla 

 (bulla tympani), formed by the tympanic bone. It contains the capac- 

 ious tympanic cavity (cavum tympanicum) and certain skeletal structures 

 of the middle ear, namely, the auditory ossicles (ossicula auditus), the 



