72 



SPECIAL ANATOMY OF THE SKELETON 



divisions of the crucial ridge is an eminence, the internal occipital protuberance (pro- 

 tuberantia occipitalis internd). It nearly corresponds to that on the outer surface, 

 though it is often on a slightly higher level, 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, 

 the sagittal sulcus (sulcus sagittalis), for the superior saggittal sinus. The 

 margins of the groove give attachment to the falx cerebri. The inferior division, 

 the internal occipital crest (crista occipitalis internal), runs to the posterior margin 

 of the foramen magnum, on the edge of which it becomes gradually lost; this 



Superior angle. 



Lateral 

 Angle. 



Inferior angle. 

 FIG. 40. Occipital bone. Inner surface. 



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. A transverse groove (sulcus transversus] passes 

 outward on each side to the lateral angle. The grooves are deep channels for 

 the lodgement of the lateral sinuses, their prominent margins affording attachment 

 to the tentorium. 1 At the point of meeting of these grooves is a depression, 

 the torcular 2 (confluens sinuum), placed a little to one or the other side of the internal 



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 sagittal sinus. 



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

 (torcular, a wine press). 



