166 



THE SKELETON. 



paratively 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 upward to the superior 

 angle of the bone ; it presents 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, 



Superior angle. 



Inferior angle. 

 FIG. 131. Occipital bone. Inner surface 



on the edge of which it becomes gradually lost ; this ridge, which is bifurcated 

 below, serves for the attachment of the falx cerebelli. It is usually marked by a 

 single groove, which commences at the back part of the foramen magnum and 

 lodges the occipital sinus. Occasionally the groove is double where two sinuses 

 exist. The transverse grooves pass outward to the lateral angles ; they are deeply 

 channelled, for the lodgment of the lateral sinuses, their prominent margins afford- 

 ing 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 mag- 

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



1 Usually one of the transverse grooves is deeper and lm>;ider 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. 



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

 point (torcular, a wine-press). 



