CRANIAL NERVES. . 837 



double vision that occurs when the patient, in going down, looks 

 at his steps. 



To overcome this diplopia he gives to his head a position that 

 is quite characteristic. He holds his head bent forward and directed 

 to the ground. This position overcomes the necessity of moving the 

 eyeballs from above downward and so minimizes the liability to 

 diplopia. 



SIXTH PAIR, OR ABDUCENT NERVE. 



This nerve arises from a collection of cells seated beneath the 

 floor of the fourth ventricle below the striae acustica?. The loop of 

 the facial incloses it. The abducent emerges between the summits 

 of the pyramidal bodies of the medulla oblongata and the pons. As 

 a threadlike nerve it goes through the cavernous sinus and through 

 the sphenoidal foramen to the external rectus. The nucleus of the 

 abducent has a connection with the posterior longitudinal bundle of 

 fibers to the opposite oculomotor nucleus, thus permitting associated 

 movements of the eyeball. The pontal olives are connected by fibers 

 with the oculomotor nucleus. And these olives are also connected 

 with the auditory nuclei and these nuclei are connected with the 

 cerebellum, so that there is an association between the motor nerves 

 of the eye, the auditory nerves, and the cerebellum. 



Physiology. The sixth nerve is exclusively motor. It has for 

 its only aim to excite the external rectus. When the nerve is 

 strongly galvanized the eyeball deviates outward. Its section, on 

 the contrary, produces an internal strabismus. It is especially 

 adapted for seeing objects placed to one side. In general, the 

 abducent is but one of the elements for the exercise of perfect 

 vision. 



Pathology. Paralysis is the most common manifestation in the 

 sixth pair. A considerable concussion of the orbital cavity, espe- 

 cially when it is upon the external side, will particularly paralyze 

 the abducent. Unilateral paralyses of this nerve are usually of peri- 

 pheral origin. Bilateral paralysis is generally due to central dis- 

 turbance. The most prominent symptom of this affection is an 

 internal or convergent strabismus. The eye is held inward by the 

 tonus of the rectus internus, so that not more than one part of 

 the cornea is perceived. 



