60 OSTEOLOGY. 



or occipital part is divided by a crucial ridge into four fossae. The two superior, 

 the smaller, receive the posterior lobes of the cerebrum, and present slight emi- 

 nences and depressions corresponding to their convolutions. The two inferior, 

 which receive the lateral lobes of the cerebellum, are larger than the former, and 

 comparatively smooth ; both are marked by slight grooves for the lodgment of 

 arteries. At the point of meeting of the four divisions of the crucial ridge is an 

 eminence, the internal occipital protuberance. It nearly corresponds to that on 

 the outer surface, and is perforated by one or more large vascular foramina. 

 From this eminence, the superior division of the crucial ridge runs upwards to 

 the superior angle of the bone ; it presents occasionally a deep groove for the 

 superior longitudinal sinus, the margins of which give attachment to the falx 

 cerebri. The inferior division, the internal occipital crest, runs to the posterior 

 margin of the foramen magnum, on the edge of which it becomes gradually lost: 

 this ridge, which is bifurcated below, serves for the attachment of the falx cere- 

 belli. The transverse grooves pass outwards to the lateral angles; they are 

 deeply channelled, for the lodgment of the lateral sinuses, their prominent margins 

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

 grooves is a depression, the " Torcular Herophili,"* placed a little to one or the 

 other side of the internal 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 condyloid foramina ; the internal openings 

 of the posterior condyloid foramina being a little external and posterior to them, 

 protected by a small arch of bone. 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 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 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 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 

 portion of the temporal, forms the foramen lacerum posterius. This notch is 

 occasionally 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 especially 



1 Usually one of the transverse grooves is deeper and broader than the other ; this seems in 

 nearly equal proportion on the two sides; 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, and occupies the corre- 

 sponding side of the median line. 



2 The columns of blood coming in different directions were supposed to be pressed together at 

 this point. 



