140 Anatomy of the Rabbit. 



dorsal margin the alveoli of the lower teeth. The mandibular 

 ramus forms a broad plate, the lateral surface of which is occupied 

 in the natural condition by the masseter muscle, while the medial 

 surface forms an area of insertion for the external and internal 

 pterygoids. The surface of the ramus is greatly increased in its 

 posteroventral portion through the expansion of the bone to 

 form the angle (angulus mandibulae), or angular apophysis. 

 The elongated articular surface is borne at the end of a vertical, 

 or slightly oblique condyloid, process (processus condyloideus). 

 The nerve and vessels of the mandible enter at the mandibular 

 foramen (foramen mandibulare), the latter being situated on the 

 medial surface of the bone immediately behind the last cheek-tooth. 



B. THE BONES OF THE SKULL. 



1. THE OCCIPITAL BONE. 



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

 segments 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 remaining 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 formation of the foramen magnum. 

 In the young animal (Fig. 12) 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. 



