SIXTH. 



609 



gray matter, close to the middle line of the floor of the Sylvian aquaeduct ; the 

 posterior one from a gray nucleus, at the upper part of the floor of the fourth 

 ventricle, close to the origin of the fifth nerve. The two nerves are connected 

 together at their origin, by a transverse band of white fibres, which crosses the 

 surface of the valve of Vieussens. The nerve winds round the outer side of 

 the crus cerebri, immediately above the pons Varolii, pierces the dura mater 

 in the free border of the tentorium cerebelli, near the posterior clinoid process, 

 above the oval opening for the fifth nerve, and passes forwards through the 

 outer wall of the cavernous sinus, below the third ; but, as it enters the orbit, 

 through the sphenoidal fissure, it becomes the highest of all the nerves. Iii 

 the orbit, it passes inwards, above the origin of the Levator Palpebrae, and 

 finally enters the orbital surface of the Superior Oblique muscle. 



In the outer wall of the cavernous sinus, this nerve receives some filaments 

 from the carotid plexus of the sympathetic. It is not unfrequently blended 

 with the ophthalmic division of the fifth ; and occasionally gives off a branch 

 to assist in the formation of the lachrymal nerve. It also gives off a recurrent 



Fig. 340. Nerves of the Orbit and Ophthalmic Ganglion. Side View. 



Internal Carotid As. 

 & Cwrotui Pl 



branch, which passes backwards between the layers of the tentorium, dividing 

 into two or three filaments, which may be traced as far back as the wall of the 

 lateral sinus. 



SIXTH NERVE. 



The Sixth Nerve (dbducens) supplies the External Rectus muscle. Its 

 apparent origin is by several filaments from the constricted part of the corpus 

 pyramidale, close to the pons, or from the lower border of the pons itself. 



The deep origin of this nerve has been traced, by Mayo, between the fasciculi 

 of the corpus pyramidale, to the posterior part of the medulla, where Stilling 

 has shown its connection with a gray nucleus in the floor of the fourth ven- 

 tricle. The nerve pierces the dura mater, immediately below the posterior 

 clinoid process, lying in a groove by the side of the body of the sphenoid bone. 

 It passes forwards through the cavernous sinus, lying on the outer side of the 

 internal carotid artery, where it is joined by several filaments from the carotid 

 plexus, by one from Meckel's ganglion (Bock), and another from the ophthal- 

 mic nerve. It enters the orbit through the sphenoidal fissure, and lies above 

 the ophthalmic vein, from which it is separated by a lamina of dura mater. It 

 39 



