THE ABDUCENT NERVE. 1249 



removed or the sensory root resected. The skull is opened in the temporal region 

 and the unopened dura (unless unavoidably torn) is separated inward from the floor 

 of the skull until the ganglion, lying on the apex of the petrous portion of the 

 temporal bone between the two layers of the dura, is exposed and removed. The 

 middle meningeal artery is especially exposed to rupture as it comes through the 

 foramen spinosum. A possible source of even more dangerous hemorrhage, how- 

 ever, is the cavernous sinus, with which the ganglion is intimately associated. 

 Trophic changes in the eye are liable to occur from damage to the first division of 

 the nerve. 



The lingual nerve is sometimes divided in painful conditions of the tongue, as 

 in cancer. It is easily reached in the floor of the mouth as it is passing forward to 

 the tongue, just under the mucous membrane. The incision is made about midway 

 between the tongue and the alveolus of the lower jaw. 



Paralysis of the sensory branches of the fifth nerve, nontraumatic in origin, is rare, 

 and when it does occur involves usually only individual branches, and these often 

 only in a part of their distribution. When implicating all the divisions of the fifth 

 nerve and associated with pain, it should suggest a tumor of the Gasserian ganglion. 



A paroxysmal cough may occur in some patients in whom the respiratory 

 organs are perfectly normal, from irritation of the terminal branches of the trigeminal 

 nerve in the nose, pharynx and external auditory meatus. 



THE ABDUCENT NERVE. 



The sixth or abducent nerve (n. abducens) is exclusively motor and supplies the 

 external rectus muscle of the eyeball, its deep origin is from the abducent nucleus 

 (nucleus n. abducentis) (Fig. 933), a rounded cluster of multipolar neurones which 

 lies in the dorsal part of the tegmentum of the pons and under the gray matter of the 

 floor of the fourth ventricle. It is situated anterior to the striae acusticae, beneath the 

 eminentia teres and ventral to and within the loop formed by the fibres of the facial 

 nerve. Leaving the nucleus on its inner aspect, the root-fibres form several fasciculi 

 which pass backward and ventro-laterally, lying- to the inner side of the superior 

 olive. Arriving at the ventral portion of the pons, the major portion of the fibres 

 passes to the outer side of the pyramidal group, a few fasciculi traversing them to 

 reach the surface. The superficial origin (Fig. 1046) lies in the sulcus which 

 demarcates the lower edge of the pons from the medulla, a little lateral to the 

 pyramid. 



Central and Cortical Connections. — As in the case of the third and fourth nerves, the 

 nucleus of the sixth receives, by way of the posterior longitudinal fasciculus, some of the 

 fibres of the pedicle of the superior olive, thus completing the establishment of a refiex-path 

 between the auditory apparatus and the centres for the nerves controlling the eye-muscles. A 

 second connection is effected by means of the posterior longitudinal fasciculus with the oculo- 

 motor nucleus of the opposite side. Finally, the abducent nucleus is brought into relation with 

 the motor area of the cortex by way of the pyramidal tract of the opposite side. 



Course and Distribution. — After leaving the surface of the brain-stem, the 

 nerve, which at its superficial origin is flat and often represented by several strands, 

 becomes consolidated and rounded, and bends forward to follow for about 15 mm., 

 the lower surface of the pons. It then pierces the dura mater over the sphenoid bone 

 at a point medial and slightly posterior to the opening for the fifth nerve (Fig 1052). 

 Thence it runs forward through a notch beneath the posterior clinoid process and 

 passes to the outer side of the inferior petrosal sinus and over the apex of the petrous 

 portion of the temporal bone to enter the cavernous sinus. Here it lies somewhat 

 below and to the outer side of the internal carotid artery and, eventually reaching the 

 outer wall of the anterior portion of the sinus, enters the orbit through the sphenoidal 

 fissure, lying above the ophthalmic vein and below the third, fourth and ophthalmic 

 nerves. Leaving the fissure, it passes between the heads of the external rectus 

 muscle, which, after entering its ocular surface, it supplies. 



The communications of the sixth nerve are : (i) as it traverses the cavernous 

 sinus, filaments from the carotid plexus of the sympathetic and (2) as it enters the 

 orbit, a small sensory filament from the ophthalmic nerve. 



79 



