MOTOR OCULI EXTERNTTS. 137 



corpus pyramidale of the medulla oblongata from the pons 

 Yarolii, and from the upper portion of the medulla and the 

 lower portion of the pons next the groove. Its origin at 

 this point is by two roots : an inferior, which is the larger, 

 and comes from the corpus pyramidale ; 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. Vulpian has followed 

 these fibres to within about two-fifths of an inch of the me- 

 dian line, but could not trace them beyond this point. 1 It 

 is not known that the fibres on the two sides decussate. 



From this origin, the nerve passes into the orbit by the 

 sphenoidal fissure, and is distributed exclusively to the ex- 

 ternal rectus muscle of the eyeball. In the cavernous sinus, 

 it anastomoses with the sympathetic through the carotid 

 plexus and Meckel's ganglion. It also receives sensitive 

 filaments from the ophthalmic branch of the fifth. It is 

 stated by Longet, a Sappey, 8 and others, that this nerve occa- 

 sionally sends a small filament to the ophthalmic ganglion ; 

 and it is supposed by Longet that this branch, which is ex- 

 ceptional, exists in those cases in which paralysis of the mo- 

 tor oculi communis, which usually furnishes all the motor 

 filaments to this ganglion, is not attended with immobility 

 of the iris. 



Properties and Functions of the Motor Oculi Externus. 

 Direct experiments, the most satisfactory being those of 

 Longet 4 and of Chauveau, 6 have shown that the motor oculi 

 communis is entirely insensible at its origin, its stimulation 

 producing contraction of the external rectus muscle and no 



1 YULPIAN, Essai sur Vorigine de plusieurs paires des nerfs rachidiens, These, 

 Paris, 1853, p. 29. 



8 LOXGET, Traite de physiologic, Paris, 1869, tome in., p. 561. 



3 SAPPET, Traite cTanatomie descriptive, Paris, 1852, tome ii., p. 249. 



4 LOXGET, op. tit., tome iii., p. 560. 



6 CHAUVEAU, op. cit. Journal de la physiologic, Paris, 1862, tome v., p. 275. 



