436 CEAK1AL NERVLS. 



is connected with the grey nucleus in the cms cerebri, the 

 corpora quadrigemina, and the valve of Vieussens. It is 

 distributed to the muscle which raises the upper eyelid, 

 and to all those of the eyeball except the superior oblique, 

 which is supplied by the fourth, and the external straight 

 muscle, which is moved by the sixth. In section or para- 

 lysis of this nerve, accordingly, the upper lid falls, and the 

 eye squints outward from the constant and unopposed 

 action of the external straight muscle. This nerve is con- 

 nected with the sympathetic through a ganglion in the 

 cavernous sinus, and with the fifth nerve at the ophthalmic 

 ganglion. Through one or both of these means it is sup- 

 posed to influence the iris, since the pupil generally dilates 

 when it has been divided. (See " Sympathetic Nerve.") The 

 intimate connection of the roots of this and the optic nerve 

 enables the animal instantaneously and instinctively to 

 adjust the axis of vision to luminous rays coming from any 

 direction. 



Pathetic Nerve. The fourth nerve, which is the motor 

 of the superior oblique muscle of the eye, arises from the 

 valve of Vieussens close to the corpora quadrigemina, so 

 that it is readily affected by impressions on the optic nerve. 

 When cut or paralyzed, the opposing muscle draws the 

 inner portion of the pupil upward, and from the light 

 falling on points of the two retinae which do not correspond, 

 a peculiar form of double vision is induced. 



A bducens. The sixth cranial nerve goes exclusively to 

 the external straight muscle of the eyeball, of which it is 

 the motor. Its section or paralysis is followed by squinting 

 of the eye inward, from the unopposed action of the inter- 

 nal straight muscle. 



Trifacial, Trigeminal. The fifth cranial nerve has two 

 roots a sensory and a motor which arise from the lateral 



