132 



OSTEOLOGY 





upper surface of the jugular process is a deep groove which curves medialward 

 and forward and is continuous with the jugular notch. This groove lodges the 

 terminal part of the transverse sinus, and opening into it, close to its medial 

 margin, is the orifice of the condyloid canal. 



Basilar Part (pars basilaris) . The basilar part extends forward and upward 

 from the foramen magnum, and presents in front an area more or less quadrilateral 

 in outline. In the young skull this area is rough and uneven, and is joined to the 

 body of the sphenoid by a plate of cartilage. By the twenty-fifth year this cartil- 

 aginous plate is ossified, and the occipital and sphenoid form a continuous bone. 

 Surfaces. On its lower surface, about 1 cm. in front of the foramen magnum, 

 is the pharyngeal tubercle which gives attachment to the fibrous raphe of the pharynx. 

 On either side of the middle line the Longus capitis and Rectus capitis anterior 

 are inserted, and immediately in front of the foramen magnum the anterior 

 atlantooccipital membrane is attached. 



The upper surface presents a broad, shallow groove which inclines upward 

 and forward from the foramen magnum; it supports the medulla oblongata, and 

 near the margin of the foramen magnum gives attachment to the membrana 

 tectoria. On the lateral margins of this surface are faint grooves for the inferior 

 petrosal sinuses. 



Foramen Magnum. The foramen magnum is a large oval aperture with its long 

 diameter antero-posterior; it is wider behind than in front where it is encroached 

 upon by the condyles. It transmits the medulla oblongata and its membranes, 

 the accessory nerves, the vertebral arteries, the anterior and posterior spinal 

 arteries, and the membrana tectoria and alar ligaments. 



Angles. The superior angle of the occipital bone articulates with the occipital 

 angles of the parietal bones and, in the fetal skull, corresponds in position with the 

 posterior fontanelle. The inferior angle is fused with the body of the sphenoid. 

 The lateral angles are situated at the extremities of the grooves for the transverse 



sinuses: each is received into the interval 

 between the mastoid angle of the parietal 

 and the mastoid part of the temporal. 



Borders. The superior borders extend 

 from the superior to the lateral angles: 

 they are deeply serrated for articulation 

 with the occipital borders of the parietals, 

 and form by this union the lambdoidal 

 suture. The inferior borders extend from 

 the lateral angles to the inferior angle; 

 the upper half of each articulates with 

 the mastoid portion of the corresponding 

 temporal, the lower half with the petrous 

 part of the same bone. These two por- 

 tions of the inferior border are separated 

 from one another by the jugular process, 

 the notch on the anterior surface of which 

 forms the posterior part of the jugular 

 foramen. 



Fio. 131. Occipital bone at birth. 



Structure. The occipital, like the other cranial 

 bones, consists of two compact lamellae, called 



the outer and inner tables, between which is the cancellous tissue or diploe; the bone is especially 

 thick at the ridges, protuberances, condyles, and anterior part of the basilar part; in the inferior 

 fossae it is thin, semitransparent, and destitute of diploe. 



Ossification f (Fig. 131). The planum occipitale of the squama is developed in membrane, 

 and may remai'n separate throughout life when it constitutes the interparietal bone; the rest of 



Planum 

 occipitale 



Planum 

 nuchale 



Kerclcring's 

 center 



Lateral 

 part 



Basilar part 



