THE OCCIPITAL BONE 



73 



occipital protuberance. More anteriorly is the foramen magnum, and on each 

 side of it, but nearer its anterior than its posterior part, the internal openings 

 of the anterior condylar foramen. On the superior aspect of the lateral portion 

 of the bone the jugular tubercle (tuberculum jugulare) is seen. This corresponds 

 to the portion of bone which roofs in the anterior condylar foramen. The 

 internal openings of the posterior condylar foramina are a little external and 

 posterior to the openings of the anterior condylar foramina, protected by a small 

 arch of bone. At this part of the internal surface there is a very deep groove in 

 which the posterior condylar foramen, when it exists, has its termination. This 

 groove is continuous, in the complete skull, with the transverse groove on the 

 posterior part of the bone, and lodges the end of the lateral sinus. In front 

 of the foramen magnum is the basilar process, presenting a shallow depression, 

 the basilar groove (clivus), which slopes from behind, upward and forward, and 

 supports the medulla oblongata and part of the pons (Varolii), and on each side 

 of the basilar process is a narrow channel, which, when united with a similar 

 channel on the petrous portion of the temporal bone, forms a groove (sulcus 

 petrosus inferior}, which lodges the inferior petrosal sinus. 



Borders. The superior border (margo lambdoideus] extends on each side 

 from the superior to the lateral angle, is deeply serrated for articulation with the 

 parietal bone, and forms, by this union, the lambdoid suture. 



The inferior border extends from the lateral to the inferior angle; its upper half 

 (margo mastoideus] is rough, and articulates with the mastoid portion of the tem- 

 poral, forming the masto-occipital suture; the inferior half articulates with the 

 petrous portion of the temporal, forming the petro-occipital suture ; these two por- 

 tions are separated from each other by the jugular process. In front of this 

 process is a deep notch, which with a similar one on the petrous portion of the 

 temporal forms the jugular foramen (foramen lacerum posterius). This notch 

 is occasionally subdivided into two parts by a small process of bone (processus 

 intrajugularis), and it generally presents an aperture at its upper part, the internal 

 opening of the posterior condyloid foramen. 



Angles. The superior angle is received into the interval between the posterior 

 superior angles of the two parietal bones; it corresponds with that part of the 

 skull in the fetus which is called the posterior fontanelle. 



The inferior angle is represented by the square-shaped surface of the basilar 

 process. At an early period of life a layer of cartilage separates this part of 

 the bone from the sphenoid, but in the adult the union between them is osseous. 



The lateral angles correspond to the outer ends of the transverse grooves, and 

 are received into the interval between the posterior inferior angles of the parietal 

 and the mastoid portion of the temporal. 



Structure. The occipital bone consists 

 of two compact laminae, called the outer 

 and inner tables, having between them 

 the diploic tissue; this bone is especially 

 thick at the ridges, protuberances, con- 

 dyles, and anterior part of the basilar 

 process; while at the bottom of the fossae, 

 especially the inferior, it is thin, semitrans- 

 parent, and destitute of diploe. 



Development (Fig. 41). At birth the 

 bone consists of four distinct parts a 

 tabular or squamous portion, which 

 lies behind the foramen magnum; two 

 condylic parts, which form the sides of 

 the foramen; and a basilar part, which 



lies in front of the foramen. The tabular portion is usually developed from four centres, though 

 the number may vary from one to eight ; two centres appear near the median line of the bone 



At birth 



the 4 pieces 



separate. 



4 for occipital 

 portion. 



1 for each condyloid 

 portion. 



1 for basilar portion. 



FIG. 41. Development of occipital bone. From seven 

 centres. 



