CRANIAL NERVES. 523 



coordination and involuntary association of movements. The 

 contraction of the pupil by the action of the circular muscle 

 (sphincter pupil he) is a bilateral reflex act, the afferent impulse 

 of which originates in the retina, passes along the optic nerves, 

 and is transmitted, probably in the corpora quadrigemina, to both 

 the third nerves. The central extremities of the third nerves 

 must have an intimate connection with each other and with the 

 optic nerves, for the diminution in size of the pupils follows 

 accurately the increase in intensity of the light to which even 

 one of the retinae is exposed. In retinal blindness and after 

 section of the optic nerve the pupil is dilated. The action 

 of the ciliary muscle may be said to be voluntary, since we 

 can voluntarily focus our eyes for near or far objects. Con- 

 traction of the sphincter pupillse and of the internal rectus 

 is associated with the contraction of the ciliary muscle in 

 accommodation. 



Injury or disease of the third nerve within the cranium gives 

 rise to the following group of phenomena: (1) Drooping of the 

 upper lid (Ptosis). (2) Fixation of the eye in the outer angle 

 (Luscitas). (3) Dilatation and immobility of pupil (Mydriasis). 

 (4) Inability to focus the eye for short distances. 



IV. THE TROCHLEAR NERVE. 



This thin, nervous filament arises under the Sylvian aqueduct, 

 and passes into the superior oblique muscle, to which it carries 

 voluntary impulses, which are involuntarily associated with those 

 of the other muscles moving the eyeball. Paralysis of this muscle 

 causes no very obvious impairment in the motions of the eyeball 

 when the head is held straight, but it is accompanied by double 

 vision, so there must be some displacement of the eyeball. When 

 the head is turned on one side the eye follows the position of the 

 head instead of being held in its primary position. In paralysis 

 of this nerve a double image is seen only when looking down- 

 ward, and the image on the affected side is oblique and below 

 that seen by the sound eye. 



