28O THE NERVOUS SYSTEM 



system; and whether this influence of the third nerve is 

 exerted directly upon the iris or indirectly, through the oph- 

 thalmic ganglion is a matter of some obscurity. The fact 

 that the action of the iris is not instantaneous strongly sug- 

 gests control by the sympathetic. 



The decussation under the aqueduct of Sylvius is evi- 

 denced by the reflex contraction of the pupil on the opposite 

 side when the central end of a divided optic nerve is stimu- 

 lated. 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 influencing movements of the iris. 

 Section of the sympathetic in the neck contracts the pupil, 

 even after section of the third. 



Fourth Nerve (Patheticus). 



Origin. This is a purely motor nerve. Its apparent 

 origin is behind the corpora quadrigemina from the valve of 

 Vieussens. The two nerves decussate above this valve. Its 

 deep origin is just below that of the third nerve beneath the 

 aqueduct of Sylvius. 



Course and Distribution. Emerging from the valve of 

 Vieussens the nerve winds around the superior peduncle of 

 the cerebellum and the crusta immediately above the pons, 

 and passes forward near the outer wall of the cavernous 

 sinus to find exit from the cranial cavity by the sphenoidal 

 fissure. Having entered the orbit, it runs forward to be dis- 

 tributed to the orbital surface of the superior oblique. In 

 the cavernous sinus it receives fibers from the ophthalmic 

 division of the fifth and from the sympathetic, and occasion- 

 ally gives off a branch to the lachrymal nerve. 



Function. It supplies motor power to the superior ob- 

 lique muscle alone. Remembering the origin and attachment 

 of this muscle it is not difficult to foretell the consequence of 

 lesions of the nerve. The action of the superior oblique is 

 to rotate the ball upon an oblique horizontal axis so that 



