THE BRAIN AND ITS MENINGES 233 



scissors and the bone is broken across its base. A similar flap 

 of the dura mater is turned down, and the clot can then be 

 removed. 



The Trigeminal or Fifth Cerebral Nerve possesses 

 a large (sensory) and a small (motor) root, both of which pierce 

 the serous layer of the dura mater near the apex of the petrous 

 portion of the temporal bone. The Semilunar (Gasserian) 

 Ganglion, which is situated on the large (sensory) root as it lies 

 on the petrous temporal, is enclosed between the serous and 

 fibrous layers of the dura mater. The Ophthalmic, Maxillary, 

 and Mandibular nerves arise from the anterior border of the 

 ganglion. 



1. The Ophthalmic Nerve at once enters the lateral wall 

 of the cavernous sinus (p. 225) and is conducted to the orbit 

 (Fig. 63). 



2. The Maxillary Nerve passes forwards to the foramen 

 rotundum and enters the upper part of the pterygo-palatine 

 (spheno-maxillary) fossa. Here it is connected to the spheno- 

 palatine (Meckel's) ganglion, which distributes sensory nerves 

 to the mucous membrane of the nose and palate. It then 

 reaches the infra-orbital groove, in the floor of the orbit, and, 

 after supplying the upper teeth, appears on the face as the 

 infra-orbital nerve (p. 172). 



As it lies in the pterygo-palatine fossa, the nerve may be 

 reached by a needle passed medially from the surface. Advan- 

 tage is taken of this fact in tic douloureux to destroy the nerve 

 by injecting absolute alcohol or some other corrosive fluid into 

 its sheath. The needle is inserted 4 cm. in front of the external 

 acoustic meatus and immediately below the zygomatic arch. 

 As the coronoid process of the mandible may obstruct the 

 instrument or even break it, if the patient suddenly opens his 

 mouth during the operation, it is better to keep the mouth open 

 by means of a gag. The needle is then thrust medially through 

 the masseter, temporal, and external pterygoid muscles to a 

 depth of 5 cm. from the surface. If the point of the needle is 

 not obstructed, as it may be by the lateral pterygoid lamina 

 (external pterygoid plate) before it reaches this depth, it will 

 have passed through the pterygo-maxillary fissure into the 

 pterygo-palatine fossa. The contents of the syringe are injected 

 at this point, and, even although they do not actually pass into 

 the nerve sheath, the action of the destructive agent is sufficient 

 to destroy the nerve and its branches. 



