216 HEAD AND NECK 



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

 course this part is called the sigmoid portion of the transverse 

 sinus. Open this 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. 



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 lateral 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 angle of the petrous part of the temporal 

 bone extending from a point lateral to the opening for the 

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

 for the ninth nerve of the same side. Lay the sinus open. 

 It opens anteriorly into the cavernous sinus, from which it 

 receives blood, and it passes posteriorly through the jugular 

 foramen to join the upper end of the internal jugular vein. 



Plexus Basilaris. The two inferior petrosal sinuses are 

 connected together across the upper surface of the basilar 

 part of the occipital bone by a plexus of small venous channels, 

 to which the term basilar plexus is applied. Unless these 

 channels happen to be distended with blood the dissectors will 

 probably be unable to display this plexus. 



The dissectors should note that the dura mater is much 

 more firmly attached to the bones of the base than it was to 

 the bones of the vertex, a fact which should have attracted 

 their attention as they removed the membrane from the floor 

 of the middle fossa. They should note also that it gives 

 sheaths to the nerves which pierce it, and that at the margins 



