THE SKELETON 37 



by the orbits into an anterior triangular portion and a posterior 

 quadrilateral portion, joined medially by a trough-like isthmus 

 and laterally by the zygomatic arches. The anterior surface 

 or hard palate forms the floor of the nasal chamber and lies 

 on a lower level than the basicranium. The basicranium, or 

 posterior portion, comprises the basioccipital, basisphenoid, 

 and presphenoid bones. On each side of the basioccipital a 

 large, rounded tympanic bulla projects boldly below the general 

 level of the surface. The presphenoid is likewise flanked by 

 two downward projections, the lateral lamince of the pterygoids, 

 which are terminated ventro-caudally by the hamular proc- 

 esses. These lamina? form the walls of a deep median fossa 

 which, in life, is covered by soft palate and constitutes the 

 nasopharynx. 



A number of foramina and depressions may be seen on the 

 ventral surface. The foramen magnum occipitale has already 

 been noted. The condyloid canal perforates the lateral margin 

 of the foramen magnum near the upper extremity of the con- 

 dyle. The hypoglossal foramen opens into the inner caudo- 

 medial margin of the jugular for a 7nen located at the caudomedial 

 angle of the tympanic bulla. The stylomastoid foramen and 

 the pit for the tympano-hyal at the caudolateral border of 

 the bulla. Craniad of these is the large external acoustic meatus. 

 In front of the bulla are the slit-like petrotympanic fissure, the 

 canal of Hugier, and the opening of the tuba auditiva or Eusta- 

 chian tube. On the lower surface of the zygomatic process of 

 the temporal bone is the mandibular fossa for the articulation 

 of the lower jaw. Mediad from this is the clearly defined 

 foramen ovale and in front of this on the margin of the orbit 

 is the somewhat smaller foramen rotundum. Above these is a 

 shallow groove, the lateral pterygoid fossa. Mediad of this is a 

 shorter, shallower groove, the scaphoid fossa, which passes 

 diagonally inward to within the lateral lamina of the ptery- 

 goid. Caudally the scaphoid fossa passes into the auditory 

 tube. Just within and anterior to the scaphoid fossa is 



