OCCIPITAL BONE. 



135 



whilst its prominent extremity is marked by a quadrilateral rough surface, 

 covered with cartilage in the fresh state, and articulating with a similar surface 

 on the petrous portion of the temporal bone. On the outer side of each con- 

 dyle, near its forepart, is a foramen, the anterior condyloid ; it is directed down- 



89. Occipital Bone. Outer Surface. 



wards, outwards, and forwards, and transmits the hypoglossal nerve. This 

 foramen is sometimes double. Behind each condyle is a fossa, 1 sometimes 

 perforated at the bottom by a foramen, the posterior condyloid, for the trans- 

 mission of a vein to the lateral sinus. In front of the foramen magnum is a 

 strong quadrilateral plate of bone, the basilar process, wider behind than in 

 front ; its under surface, which is rough, presenting in the median line a tuber- 

 cular ridge, the pharyngeal spine, for the attachment of the tendinous raphe' 

 and Superior Constrictor of the pharynx ; and, on each side of it, rough depres- 

 sions for the attachment of the Eectus Capitis Anticus Major and Eectus Capitis 

 Anticus Minor muscles. 



The Internal or Cerebral Surface (Fig. 90) is deeply concave. The posterior 

 or occipital part is divided by a crucial ridge into four fossae. The two supe- 

 rior fossas receive the posterior lobes of the cerebrum, and present slight emi- 

 nences and depressions corresponding to their convolutions. The two inferior, 

 which receive the lateral lobes of the cerebellum, are larger than the former, 

 and comparatively smooth ; both are marked by slight grooves for the lodg- 

 ment of arteries. At the point of meeting of the four divisions of the crucial 

 riclge is an eminence, the internal occipital protuberance. It nearly corresponds 

 to that on the outer surface, and is perforated by one or more large vascular 



1 This fossa presents many variations in size. It is usually shallow ; and the foramen small ; 

 occasionally wanting, on one, or both sides. Sometimes both fossa and foramen are large, but 

 confined to one side only ; more rarely, the fossa and foramen are very large on both sides. 



