THE SKELETON. 



ing attachment to the tentorium cerebelli. 1 At the point of meeting of these 

 grooves is a depression, the torcular Uerophili* placed a little to one or the other 

 side of the internal occipital protuberance. More anteriorly is the foramen mag- 

 num, and on each side of it, but nearer its anterior than its posterior part, the 

 internal openings of the anterior condyloid foramen ; 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 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 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 laminse, called 

 the outer and inner tables, having 

 between them thediploic tissue; this 

 bone is especially thick at the ridges, 

 protuberances, condyles, and an- 

 terior part of the basilar process; 

 while at the bottom of the fossae, es- 

 pecially the inferior, it is thin, semi- 

 transparent, and destitute of diploe. 

 Development (Fig. 25). At birth 

 the bone consists of four distinct 



parts : a tabular or expanded portion, which lies behind the foramen magnum ; 

 two condylar 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 



1 Usually one of the transverse grooves is deeper and broader than the other ; occasionally, both 

 grooves are of equal depth and breadth, or both equally indistinct. The broader of the two transverse 

 grooves is nearly always continuous with the vertical groove for the superior longitudinal sinus. 



' l The columns of blood coming in different directions were supposed to be pressed together at this 

 point (torcular, a wine-press). 



At birth 



the 4 pieces 



separate. 



4 for occipital 

 portion. 



1 for each condyloid 

 portion. 



'1 for basilar portion. } r* 

 FIG. 25. Development of occipital bone. By seven centres. 



