THE CRANIUM. 



167 



internal openings of the anterior condyloid foramina ; the internal openings of the 

 posterior condyloid foramina are 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 termina- 

 tion. 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 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, 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 which lodges 

 the inferior petrosal sinus. 



Angles. The xnnerior 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 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 corre- 

 spond 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 border 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 por- 

 tion of the temporal, forming the masto-occipital suture ; the inferior half articu- 

 lates 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 por- 

 tion of the temporal, forms the foramen lacerum posterius or jugular foramen. 

 This notch is occasionally subdivided into two parts by a small process of bone, 

 and it generally 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 

 ;ui*l Inner tables, having between them the diploic tissue; this bone is espe- 

 cially thick at the ridges, protuberances, condyles, and internal part of the basilar 

 process : whilst at the bottom of the fossae, especially the inferior, it is thin, semi- 

 transparent, and destitute of diploe. 



Development. (Fig. 132). At birth the bone consists of four distinct parts : a 

 tabular or expanded portion, which lies behind the foramen magnum ; two con- 

 <liflar parts, which form the sides of the foramen ; and a basilar part, which lies in 

 front of the foramen. The number 

 of nuclei for the tabular part vary. 

 As a rule, there are four, but there 

 may be only one (Blandin) or as 

 many as eight (Meckel). They ap- 

 pear about the eighth week of foetal 

 life, and soon unite to form a single 

 piece, which is. however, fissured in 

 the direction indicated in the plate. 

 The basilar and two condyloid por- 

 tions are each developed from a 



single nucleus, which appears a lit- ^^ 



tie later. The upper portion of the ~ ^ 1 for basilar portion. 



tabular surface that is to say, the FIG. 132. Development of occipital bone. By seven centres, 

 portion above the transverse fissure 

 is developed from membrane ; the rest of the bone is developed from cartilage. 



4 for occipital 



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separate. 



