546 



NERVOUS SYSTEM. 



external part of its posterior hemisphere. It acts upon the eyeball from the 

 pulley at the upper and inner portion of the orbit as the fixed point and ro- 

 tates the eye upon an oblique, horizontal axis, from below upward, from 

 without inward and from behind forward. By its action, the pupil is di- 

 rected downward and outward. It is the antagonist of the inferior oblique, 

 the action of which has been described in connection with the motor oculi 

 communis. When the patheticus is paralyzed, the eyeball is immovable, as 

 far as rotation is concerned. When the head is moved toward the shoulder, 

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

 there is double vision. 



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 oblon- 



gata from the pons Varolii, from the up- 

 per 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, which is the larger and comes 

 from the corpus pyramidale ; and a supe- 

 rior 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 followed these fibres 

 to within about two-fifths of an inch (10 

 mm.) of the median line, but they could 

 not be traced beyond this point. It is 

 not known that the fibresof t he two sides 



trunk of the motor oculi communis, with decussate. From this origin the nerve 



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



externus, passing to the external rectus paSSCS into the Ol'blt by the SphenOldal 



muscle ; 9, filamentsof the motor oculi ex- -..-. ., , * i 



ternus, anastomosing with the sympathet- nSSUre and IS distributed exclusively to tiie 



ic ; 10, ciliary nerves. , , . , 



external rectus muscle ot 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 receives 

 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 ganglion ; and it was stated by Longet that 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 Externus. Direct experiments 



