CRANIAL NERVES. 521 



to the (2) ciliary muscle. The latter branches reach the eye by 

 a short twig from the inferior oblique branch, which goes to the 

 ciliary ganglion, and thence enter the ciliary nerves. 



The action of the orbital muscles is, in the main, under the 

 control of the will, though they afford good examples of peculiar 

 coordination and involuntary association of movements. The 

 contraction of the pupil by the action of the circular muscle 

 (sphincter pupillse) 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 accu- 

 rately 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. Contraction of the sphincter 

 pupillse and of the internal rectus is associated with the contrac- 

 tion 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 mus- 

 cle causes no very obvious impairment in the motions of the eye- 

 ball 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 



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