FOURTH SIXTH. 



539 



origin, by a transverse band of white fibres, which crosses the surface of the velum. 

 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. In 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 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. 



Fig. 276. Nerves of the Orbit and Ophthalmic Ganglion. Side view. 



Intental Caivtid Ay. 

 ft Ca/ratui Ple^ 



Moto 

 Jtoat 



SIXTH OK ABDUCENS NERVE. 



The SIXTH or ABDUCENS NERVE 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 ventricle. 

 The nerve pierces the dura mater, immediately below the posterior clinoid pro- 

 cess, 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 ophthalmic 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 then passes 

 between the two heads of the External rectus, and is distributed to that muscle 

 on its ocular surface. 



RELATIONS OF THE ORBITAL NERVES. 



The above-mentioned nerves, as well as the ophthalmic division of the fifth, as 

 they pass to the orbit, bear a certain relation to each other in the cavernous 



