56 



THE SKELETON 



The foramen magnum is oval in shape, with its long axis in a sagittal direc- 

 tion. It transmits the medulla oblongata and its membranes, the accessory- 

 nerves (spinal portions), the vertebral arteries, the anterior and posterior spinal 



Fig. 70. — The Occipital with a Separate Interparietal. 





arteries, and the tectorial membrane (occipito-axial ligament). It is widest 

 behind, where it transmits the medulla, and is narrower in front, where it is 

 encroached upon by the condyles. 



Fig. 71. — Skull showing a Pre-interparietal Bone (P.I.)- 



Occasionally a facet is present on the anterior margin, forming a lliird occipital condyle foi 

 articulation with the dens. Between the condyles and behind the margin of the foramen mag- 

 nurn the posterior occipito-atlantal ligament obtains attachment. 



Blood-supply. — The occipital bone receives its blood-supply from the occipital, posterior 

 auricular, nii<l<llcnieiiingeal, vertebral and the ascending pharyngeal arteries. 



Articulations. ^The occipital bone is connected by suture with the two parietals, the two 

 temporals, and the sphenoid; the condyles articulate with the atlas, and exceptionally the occip- 

 ital articulates with the dens of the epistropheus by means of the third occipital condyle. 



