614 



NERVOUS SYSTEM. 



with the motor oculi communis. When the patheticus is paralyzed, the eyeball is im- 

 movable, as far as rotation is concerned ; and, when the head is moved toward the shoul- 

 der, the eye does not rotate to maintain the globe in the same relative position, and we 

 have double vision. 



FIG. 197. Distribution of the patheticus. (Hirschfeld.) 

 I, olfactory nerve ; II, optic nerves ; III, motor oculi com- 

 munis; IV, patheticus, ~by the xide of the ophthal- 

 mic branch of the fifth, and passing to the superior 

 oblique muscle ; VI, motor oculi externus ; 1, gan- 

 glion of Gasser ; 2, 3, 4, 5, 6, 7, 8. 9. 10, opththalmic 

 division of the fifth nerve, with its branches. 



FIG. 198. Distribution of the motor oculi externus. 

 (Hirschfeld.) 



1, trunk of the motor oculi communis, with its branches 

 (2, 3, 4, 5, 6, 7) ; 8, motor oculi externus, passing 

 to the external rectus muscle; 9, filaments of 'the 

 motor oculi externus anastomosing with, the sym- 

 pathetic; 10, ciliary nerves. 



Motor Oculi Externus^ or Abducens (Sixth Nerve). 



Like the patheticus, the motor oculi externus is distributed to but a single muscle, the 

 external rectus. 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 

 which separates the anterior corpus pyramidale of the medulla oblongata from the pons 

 Varolii, 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 rs 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 ven- 

 tricle. Vulpian has followed these fibres to within about two-fifths of an inch of the 

 median line, but they could not be traced beyond this point. 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 eye- 

 ball. 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 some anatomists that this nerve occasionally sends a 

 small filament to the ophthalmic ganglion ; and it is supposed by Longet that this branch, 

 which is exceptional, exists in those cases in which paralysis of the motor oculi com- 

 munis, 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 have 

 shown that the motor oculi externus is entirely insensible at its origin, its stimulation 



