REMOVAL OF THE BRAIN 113 



of the inner layer of the dura mater which projects forwards, 

 between the lateral lobes of the cerebellum, from the internal 

 occipital crest (Figs. 33, 34). 



Sinus Transversus (O.T. Lateral). The horizontal part of 

 the transverse sinus has already been traced from the internal 

 occipital protuberance to the superior border of the petrous 

 part of the temporal bone, where it turns downwards to 

 the jugular foramen. At first the descending portion runs 

 downwards, on the inner surface of the mastoid part of the 

 temporal bone, and then forwards and again downwards 

 across the upper and anterior surfaces of the jugular process 

 of the occipital bone. On account of the sinuosity of its 

 course this part is called the sigmoid portion of the transverse 

 sinus (Fig. 36). 



Dissection. Open the sigmoid part of the sinus and find 

 the mouth of the mastoid emissary vein in its posterior border, 

 about half-way down. 



The dissectors should now obtain the basal part of a 

 macerated skull and should note the relation of the transverse 

 sinus to the outer surface. They will find that the position 

 of the sinus can be indicated on the external surface, by a line 

 which commences at the external occipital protuberance, passes 

 forwards, with a slight upward convexity, along the superior 

 nuchal line to the upper part of the mastoid part of the 

 temporal bone and then descends to the level of the lower 

 margin of the external meatus (Figs. 38, 204). 



Sinus Occipitalis. The occipital sinus is not uncommonly 

 absent. When it is present it commences in the right or left 

 transverse sinus or the confluens sinuum, and descends for a 

 short distance in the posterior border of the falx cerebelli. 

 It terminates below in two branches, which leave the falx 

 cerebelli and run along the borders of the foramen magnum, 

 between the layers of the dura mater, to terminate anteriorly 

 in the lower ends of the transverse sinuses. 



Sinus Petrosus Inferior. The inferior petrosal sinus lies 

 along the posterior border of the petrous part of the temporal 

 bone, extending from a point lateral to the opening for the 

 abducens nerve to the medial side of the opening in the dura 

 for the glossopharyngeal nerve of the same side. Lay the 

 sinus open. It opens anteriorly into the cavernous sinus, 

 from which it receives blood, and posteriorly it passes through 



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