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. 
