MOTOR OCULI EXTERNUS, OR ABDUCENS 



501 



MOTOR OCULI EXTERNUS, OR ABDUCENS (SIXTH NERVE) 



Like the patheticus, the motor oculi externus is distributed to but 

 a single muscle. Its uses, therefore, are apparent from a study of its 

 distribution and properties. 



Physiological Anatomy. The apparent origin of the sixth nerve 

 is from the groove separating the anterior corpus pyramidale of . the 

 medulla oblongata from the pons Varolii, from the upper portion of the 

 medulla and from the lower portion of the pons next the groove. Its 

 origin at this point is by two roots : 

 an inferior root, which is the larger 

 and comes from the corpus pyrami- 

 dale ; and a superior root, sometimes 

 wanting, which seems to come from 

 the lower portion of the pons. All 

 anatomists are agreed that the deep 

 fibres of origin of this nerve pass to 

 the gray matter in the floor of the 

 fourth ventricle. It is not known that 

 the fibres of the two sides decussate. 

 From this origin the nerve passes into 

 the orbit by the sphenoidal fissure 

 and is distributed exclusively to the 

 external rectus muscle of the eyeball. 

 In the cavernous sinus it anastomoses 

 with the sympathetic through the 

 carotid plexus and receives a filament 

 from Meckel's ganglion. It also re- 

 ceives sensory filaments from the 

 ophthalmic branch of the fifth. It is thought by some anatomists that 

 this nerve occasionally sends a small filament to the ophthalmic gan- 

 glion; and this branch, which is exceptional, exists in those cases in 

 which paralysis of the motor oculi communis, which usually furnishes 

 all the motor filaments to this ganglion, is not attended with immobility 

 of the iris. 



Properties and Uses of the Motor Oculi Extermis. Direct experi- 

 ments have shown that the motor oculi externus is insensible at its 

 origin, its stimulation producing contraction of the external rectus 

 muscle and no pain. The same experiments illustrate the action of the 

 nerve, inasmuch as its stimulation is followed by contraction of the 

 muscle and deviation of the eye outward. Division of the nerve in 

 the lower animals or its paralysis in the human subject is attended with 



Fig. 120. Distribution of the motor oculi ex- 

 ternus (Hirschfeld). 



i, trunk of the motor oculi communis, with 

 its branches (2, 3, 4, 5, 6, 7) ; 8, motor oculi 

 externus, passing to the external rectus mus- 

 cle ; 9, filaments of the motor oculi externus, 

 anastomosing with the sympathetic ; 10, ciliary 

 nerves. 



