u 4 HEAD AND NECK 



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 the 

 channels happen to be distended with blood the dissectors will 

 probably be unable to display the plexus (Fig. 36). 



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 

 of the various foramina its outer layer becomes continuous 

 with the periosteum on the outer surface of the cranium, 

 whilst at the margin of the foramen magnum the inner layer 

 becomes continuous with the single layer of dura mater which 

 surrounds the medulla spinalis ; and that, at the same level, the 

 arachnoid and pia mater of the brain become continuous with 

 the arachnoid and pia mater of the spinal medulla (O.T. spinal 

 cord). Before terminating the survey of the interior of the 

 cranium, the dissectors should revise their knowledge of the 

 blood vessels, and their relations to the dura mater ; and they 

 should remove the hypophysis (O.T. pituitary body) and in- 

 vestigate its naked-eye structure. 



Sinus Durse Matris. Four blood sinuses lie in the median 

 plane-, (i) the superior sagittal sinus, in the upper or attached 

 border of the falx cerebri ; (2) the inferior sagittal sinus, in the 

 free part of the lower border of the falx cerebri ; (3) the 

 straight sinus, along the line of attachment of the falx cerebri 

 with the tentorium cerebelli ; (4) the occipital sinus, in the 

 upper part of the attached border of the falx cerebelli. 



Two sinuses lie in a higher horizontal plane : they are the 

 spheno-parietal sinuses, which run along the posterior borders 

 of the small wings of the sphenoid bone. 



Six sinuses lie in a lower horizontal plane: (i) the two 

 cavernous sinuses, at the sides of the body of the sphenoid ; 

 (2) the two superior petrosal sinuses, along the upper borders 

 of the petrous parts of the temporal bones, in the anterior 

 parts of the attached border of the tentorium cerebelli ; (3) 

 the horizontal parts of the transverse sinuses, in the posterior 



