THE BRAIN AND ITS MENINGES 235 



part of the lateral wall of the cavernous sinus (p. 225). The 

 large (sensory) root is exposed by stripping the serous layer of 

 the dura mater backwards off the ganglion, and it is then divided. 

 The maxillary and mandibular nerves are cut through as far 

 away from their origin as possible, and the ganglion is then torn 

 out along with the ophthalmic nerve. As the latter lies in 

 contact with the thin endothelial wall of the cavernous sinus 

 and is closely related to the internal carotid artery and the 

 third, fourth, and sixth cerebral nerves (p. 225), few operators 

 attempt to divide it with the knife. As soon as the ganglion is 

 torn out severe hsemorrhage occurs, owing to the unavoidable 

 laceration of the wall of the cavernous sinus, but it is readily 

 controlled by pressure. It is seldom found possible to preserve 

 the small (motor) root, and, following the operation, the muscles 

 of mastication are paralysed on that side. In this condition 

 the mandible is permanently twisted over to the side of the lesion 

 by the unopposed action of the pterygoid muscles of the opposite 

 side (p. 181). Another common sequela is thrombosis of the 

 cavernous sinus with consequent proptosis, chemosis, and retinal 

 haemorrhage. Temporary blindness or permanent impairment of 

 vision may result, and there is usually some temporary paralysis 

 of the ocular muscles. The results of the alteration in the 

 sensibility of the conjunctiva are described on p. 204. 



Contents of the Posterior Cranial Fossa. The 

 Medulla Oblongata is the direct continuation of the spinal 

 medulla, and is continuous above with the Pons. Ventrally, 

 these two structures lie on the basilar portions of the occipital 

 and sphenoid bones ; dorsally, they are related to the cere- 

 bellum, which also overlaps them laterally. In the medulla 

 oblongata and the pons the central canal of the spinal medulla 

 expands to form the fourth ventricle, and the floor of this space 

 contains the important respiratory and cardiac centres. 



On each side the transverse fibres of the pons pass into the 

 cerebellum as the brachium pontis (middle peduncle), and the 

 region where the lower part of the brachium enters the cere- 

 bellum is known as the cerebello-p online angle. In this region 

 the facial and auditory nerves emerge at the lower border of 

 the pons. At its upper extremity the pons becomes con- 

 tinuous with the mid-brain, which sinks into the basal surface 

 of the cerebral hemispheres. 



Tumours in the region of the cerebello-pontine angle usually 

 involve the seventh and eighth cerebral nerves. They may 



