THE SKULL AS A WHOLE 167 



dible, are united by immovable articulations, so that they produce 

 the effect of a continuous mass. In this, the sutures between com- 

 ponent bones appear as fine lines of varying distinctness. The 

 mandible is a more or less independent structure, articulated with 

 the main body of the skull by a typical movable joint. 



The posterior, cranial portion of the skull has a somewhat 

 conical shape, its apex being directed forward. It is separated from 

 the anterior, facial portion by a depression on either side of the 

 skull, the orbital cavity (orbita), which serves for the accommoda- 

 tion of the optic bulb. Unlike the nasal and auditory sense-organs, 

 the eye is not included within the skull-wall. The two portions 

 are united both medially and laterally, the lateral connection being 

 established by the zygomatic arch (arcus zygomaticus), which 

 bridges the lateral portion of the orbit. The facial portion has 

 also a somewhat conical shape, its apex being formed by the an- 

 terior extremiity of the upper jaw and the incisor teeth. Its base is 

 formed in part by the connection with the cranial portion, as 

 already described, and also by the anterior walls of the orbits. 



The cranial portion exhibits an extensive posterior surface, the 

 nuchal surface (planum nuchale), situated in general at right 

 angles to the cervical portion of the vertebral column and also to 

 the dorsal, lateral, and ventral walls of the skull. This surface 

 includes the external aspect of the occipital bone, with the ex- 

 ception, chiefly, of the basilar part of the latter. Its dorsal portion 

 forms an area of attachment for the spinal and special occipital 

 musculature. Its ventral portion is perforated by a large aper- 

 ture, the foramen magnum occipitale, for the passage of the central 

 nervous system from the cranial cavity into the vertebral canal. 

 On either side of this is a smooth, ridge-like projection, the oc- 

 cipital condyle (condylus occipitalis), for articulation with the 

 superior articular pits of the atlas. The single occipital bone of 

 the adult is formed by the fusion of a dorsal, median supraoccipital, 

 a ventral, median basioccipital, and paired, lateral erxoccipital bones, 

 the last of these bearing the main parts of the occipital condyles. At 

 a little distance lateral to the occipital condyle, the nuchal surface 

 is continued downward through the medium of a somewhat tri- 

 angular, pointed jugular or paramastoid process (processus 

 jugularis) of the exoccipital bone. This structure is separated 



