The; Bones of the Skull. 85 



B. THE BONES OF THE SKULL. 

 1. THE OCCIPITAL BONE. 



The occipital bone (os occipitale) is the first of the basicranial seg- 

 ments as numbered from the occipital articulation forward. It forms 

 the posterior boundary of the skull, and establishes the connection of 

 the latter with the vertebral column. Its external surface is identifiable 

 for the most part with the nuchal surface, but a portion of it falls in 

 the horizontal plane of the basis cranii. The internal surface is partly 

 exposed to the cranial cavity, and forms in this relation the posterior, 

 dorsal, and ventral boundaries of the posterior cranial fossa. The re- 

 maining portion is excluded from the cranial cavity, being applied 

 instead to the broad posterior surfaces of the petrotympanic bones. 



The occipital bone is divisible into four portions, namely, the basilar 

 portion (pars basilaris), or basioccipital, the paired lateral portions 

 (partes laterales), or exoccipitals, and the squamous portion (squama 

 occipitalis), or supraoccipital. ' All four portions take part in the forma- 

 tion of the foramen magnum. In the young animal (Fig. 11) they are 

 represented by separate elements, formed in a continuous mass of 

 cartilage, and united for a time by synchondroses, but in the course of 

 growth they become fused to form a single occipital bone. 



The basioccipital is that portion lying below and in front of the 

 foramen magnum. Its main surfaces are respectively dorsal and ven- 

 tral. Its anterior margin is united with the posterior margin of the 

 basisphenoid by a thin, transverse cartilage union, the sphenooccipital 

 synchondrosis (synchondrosis sphenooccipitalis). Posteriorly its dorsal 

 and ventral surfaces come together in a thin concave edge which forms 

 the ventral boundary of the foramen magnum. Laterally it is bounded 

 by the petrotympanic bone and by the lateral portion of the occipital. 

 The dorsal surface bears a median groove, deeper in its middle portion, 

 where the lateral margins of the bone are raised to form a pair of rounded 

 bosses for articulation with the petrotympanic. The groove represents 

 the sloping porjtion or clivus of the occipital, and lodges in the natural 

 condition, as described above, the ventral portion of the medulla 

 oblongata. The ventral surface presents a similar groove, in the pos- 

 terior portion of which there is a small ridge-like elevation, the pharyngeal 

 tubercle (tuberculum pharyngeum). 



The exoccipital is directed dorsad from the basioccipital in such a 

 way that it falls in the plane of the nuchal surface. It is applied to the 

 posterior surface of the petrotympanic bone, and also extends down- 

 ward beyond the latter as the jugular process. The occipital condyle is 

 borne on the exoccipital, with the exception, however, of its ventral tip, 

 which belongs to the basioccipital. The portion of the occipital bone con- 

 necting the basioccipital and exoccipital contains the jugular fossa and 

 the apertures representing the hypoglossal canal. Its anterior margin • 

 bears a jugular incisure (incisura jugularis), forming the occipital 



