

THE OCCIPITAL BONE 131 





one side of it (generally the right) is a deep groove, the sagittal sulcus, which lodges 

 the hinder part of the superior sagittal sinus; to the margins of this sulcus the falx 

 cerebri is attached. The lower division of the cruciate eminence is prominent, 

 and is named the internal occipital crest; it bifurcates near the foramen magnum 

 and gives attachment to the falx cerebelli; in the attached margin of this falx 

 is the occipital sinus, which is sometimes duplicated. In the upper part of the 

 internal occipital crest, a small depression is sometimes distinguishable; it is 

 termed the vermian fossa since it is occupied by part of the vermis of the cerebellum. 

 Transverse grooves, one on either side, extend from the internal occipital protuber- 

 ance to the lateral angles of the bone; those grooves accommodate the transverse 

 sinuses, and their prominent margins give attachment to the tentorium cerebelli. 

 The groove on the right side is usually larger than that on the left, and is 

 continuous with that for the superior sagittal sinus. Exceptions to this condition 

 are, however, not infrequent; the left may be larger than the right or the two 

 may be almost equal in size. The angle of union of the superior sagittal and trans- 

 verse sinuses is named the confluence of the sinuses (torcular Herophili 1 ), and its 

 position is indicated by a depression situated on one or other side of the 

 protuberance. 



Lateral Parts (pars lateralis"). The lateral parts are situated at the sides of 

 the foramen magnum; on their under surfaces are the condyles for articulation 

 with the superior facets of the atlas. The condyles are oval or reniform in shape, 

 and their anterior extremities, directed forward and medialward, are closer together 

 than their posterior, and encroach on the basilar portion of the bone; the posterior 

 extremities extend back to the level of the middle of the foramen magnum. The 

 articular surfaces of the condyles are convex from before backward and from 

 side to side, and look downward and lateralward. To their margins are attached 

 the capsules of the atlantooccipital articulations, and on the medial side of each 

 is a rough impression or tubercle for the alar ligament. At the base of either 

 condyle the bone is tunnelled by a short canal, the hypoglossal canal (anterior 

 condyloid foramen) . This begins on the cranial surface of the bone immediately 

 above the foramen magnum, and is directed lateralward and forward above the 

 condyle. It may be partially or completely divided into two by a spicule of bone ; 

 it gives exit to the hypoglossal or twelfth cerebral nerve, and entrance to a meningeal 

 branch of the ascending pharyngeal artery. Behind either condyle is a depression, 

 the condyloid fossa, which receives the posterior margin of the superior facet of 

 the atlas when the head is bent backward; the floor of this fossa is sometimes 

 perforated by the condyloid canal, through which an emissary vein passes from the 

 transverse sinus. Extending lateralward from the posterior half of the condyle 

 is a quadrilateral plate of bone, the jugular process, excavated in front by the jugular 

 notch, which, in the articulated skull, forms the posterior part of the jugular fora- 

 men. The jugular notch may be divided into two by a bony spicule, the intra- 

 jugular process, which projects lateralward above the hypoglossal canal. The 

 under surface of the jugular process is rough, and gives attachment to the Rectus 

 capitis lateralis muscle and the lateral atlantooccipital ligament; from this 

 surface an eminence, the paramastoid process, sometimes projects downward, and 

 may be of sufficient length to reach, and articulate with, the transverse process 

 of the atlas. Laterally the jugular process presents a rough quadrilateral or tri- 

 angular area which is joined to the jugular surface of the temporal bone by a plate 

 of cartilage; after the age of twenty-five this plate tends to ossify. 



The upper surface of the lateral part presents an oval eminence, the jugular 

 tubercle, which overlies the hypoglossal canal and is sometimes crossed by an 

 oblique groove for the glossopharyngeai, vagus, and accessory nerves. On the 



1 The columns of blood coming in different directions were supposed to be pressed together at this point (torcular, 

 a wine press) . 



