THE CRANIAL NERVES 279 



or dropping of the upper lid ; for the lid is kept open by the leva- 

 tor palpebrae superioris. (2) There is external strabismus, 

 because the external rectus is not supplied by this nerve and is 

 unopposed by the internal rectus, the action of which is paralyzed. 

 Diplopia is the consequence. (3) There is inability to turn the 

 ball except in an outward direction because the muscles producing 

 movements on the vertical and horizontal axes are deprived of in- 

 nervation. (4) There is inability to rotate the eye in certain direc- 

 tions on the antero-posterior axis. The antagonist of the inferior 

 oblique is the superior oblique, the tendency of which latter is 

 to rotate the globe so as to make the pupil look downward and 

 outward. When the inferior oblique is paralyzed the superior 

 oblique is unopposed, it is impossible to rotate the ball as is 

 usual in sidewise movements of the head, and double vision is the 

 result. (5) There is slight protrusion of the whole ball from re- 

 laxation of the muscles. (6) The pupil is dilated and move- 

 ments of the iris are interfered with. Stimulation of the third 

 nerve contracts the pupil, but when it is cut the pupil does not 

 respond to light. The ciliary nerves controlling the movements 

 of the iris come from the ophthalmic ganglion of the sympa- 

 thetic; to this ganglion goes a branch from the third nerve. It is 

 known that the action of the sympathetic cannot be divorced 

 from that of the cerebro-spinal system; and whether this influ- 

 ence of the third nerve is exerted directly upon the iris or in- 

 directly through the ophthalmic ganglion is a matter of some 

 obscurity. The fact that the action of the iris is not instanta- 

 neous strongly suggests control by the sympathetic. 



The decussation under the aqueduct of Sylvius is evidenced by 

 the reflex contraction of the pupil on the opposite side when the 

 central end of a divided optic nerve is stimulated. The impulse 

 is reflected through the third nerve. It is not to be understood, 

 however, that the motor oculi is the only nerve capable of influ- 

 encing movements of the iris. Section of the sympathetic in 

 the neck contracts the pupil, even after section of the third. 



