234 HEAD AND NECK 



THE LATERAL PART OF THE MIDDLE 

 CRANIAL FOSSA. 



The structures contained within the middle cranial fossa 

 may now be examined or re-examined. In carrying out this 

 dissection, the head should be supported on a block so that 

 the floor of the cranial cavity looks upwards. The follow- 

 ing are the structures which must be displayed : 



1. Cavernous venous sinus. 



2. Internal carotid artery. 



3. Middle meningeal artery. 



4. Accessory meningeal artery. 



5. The two roots of the Trigeminal nerve, with the Semilunar 



ganglion and the three main divisions of the trigeminal 

 nerve. 



6. Oculo-motor nerve (3rd cerebral). 



7. Trochlear nerve (4th cerebral). 



8. Abducent nerve (6th cerebral). 



9. Internal carotid plexus of the sympathetic. 



10. Greater superficial petrosal nerve. 



11. Lesser superficial petrosal nerve. 



Dissection. The dura mater has already been removed from 

 one half of the middle cranial fossa (pp. 109, no), and on that 

 side it is necessary only to differentiate again the structures 

 which lie in the cavernous sinus ; on the other side the dura 

 mater must be stripped from the medial part of the lateral 

 portion of the middle cranial fossa. Enter the knife at the 

 anterior clinoid process, and carry it backwards to the apex of 

 the petrous bone. This incision must go no deeper than is 

 necessary to divide the dura mater, and must be made immedi- 

 ately to the lateral side of the openings in the membrane through 

 which the oculo-motor, the trochlear, and trigeminal nerves 

 pass. It is very important to preserve those apertures intact, 

 so that the proximal ends of the nerves may be held in position 

 during the dissection. The incision through the dura mater 

 may now be carried backwards and laterally along the upper border 

 of the petrous bone in the line of the superior petrosal sinus, and, 

 forwards and laterally, along the posterior margin of the small wing 

 of the sphenoid. After the incisions are made, raise the dura 

 mater with great care, for it is intimately connected with the 

 nerves which lie subjacent to it. Thus, where it forms the 

 lateral wall of the cavernous sinus, it is closely applied to the 

 oculo-motor and trochlear nerves, and it is firmly attached to 

 the ophthalmic division of the trigeminal nerve, whilst over the 

 petrous bone it is united to the surface of the semilunar ganglion, 

 and the greater and lesser superficial petrosal nerves are immedi- 

 ately beneath it. The edge of the knife, therefore, must be kept 

 close to the membrane, and a small portion of the membrane 

 may be left upon the nerves. The part which is left can be 

 removed afterwards as the nerves are defined. 



