OCCIPITAL BONE. 



137 



rior condyloid foramina; the internal openings of the posterior condyloid fora- 

 mina being a little external and posterior to them, protected by a small arch of 

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

 the posterior condyloid foramen, when it exists, has its internal termination. 

 This groove is continuous in the complete skull with that which separates the 

 upper from the lower fossae, and lodges the end of the same sinus, the lateral. 

 In front of the foramen magnum is the basilar process, presenting a shallow 

 depression, the basilar groove, which slopes from behind, upwards and forwards, 

 and supports the medulla oblongata ; and on each side of the basilar process is a 

 narrow channel, which, when united with a similar channel on the petrous por- 

 tion of the temporal bone, forms a groove, which lodges the inferior petrosal sinus. 



Angles. The superior angle is received into the interval between the poste- 

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

 of the skull in the foetus 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. 



Borders. The superior 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 is rough, and articulates with the mastoid 

 portion of the temporal, forming the masto-occipital suture: the inferior half 

 articulates with the petrous portion of the temporal, forming the petro-occipital 

 suture; these two portions are separated from one another 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 foramen lacerum posterius. This notch is occa- 

 sionally subdivided into two parts by a small process of bone, and presents an 

 aperture at its upper part, the internal opening of the posterior condyloid foramen. 



Structure. The occipital bone consists of two compact laminae, called the 

 outer and inner tables, having between them the diploic tissue: this bone is espe- 

 cially thick at the ridges, 



Fig. 91. Development of Occipital Bone. 



By 



centres 



{ -for occipital 



protuberances, condyles, and 

 anterior part of the basilar 

 process; whilst at the bot- 

 tom of the fossae, especially 

 the inferior, it is thin, semi- 

 transparent, and destitute of 

 diploe. 



Development (Fig. 91). The 

 occipital bone has/owr centres 

 of development : one for the 

 posterior or occipital part, 

 which is formed in mem- 

 brane; one for the basilar 

 portion, and one for each 

 condyloid portion, Avhich are 

 formed in cartilage. 



The centre for the occipital portion appears about the tenth week of foetal life; 

 and consists, according to Blandin and Cruveilhier, of a small oblong plate which 

 appears in the situation of the occipital protuberance. 1 The condyloid portions 



' Beclard considers this segment to have four centres of ossification, arranged in pairs, two 

 above and two below the curved lines, and Meckel describes eight, four of which correspond in 

 situation with those above described : of the other four, two are placed in juxtaposition, at the 

 upper angle of the bone, and the remaining two, one at each side, in the lateral angles. 



/ for each fonJyloi,tt 

 portuiii 



T oa,aiLa,r portion 



