DIOPTRIC MECHANISMS OF THE EYEBALL 553 



muscular fibres forming the ciliary muscle, the sphincter, and the dilatator 

 iridis can be influenced, viz. the third nerve, the fifth nerve, and the sym- 

 pathetic nerve. On exciting the root of the third nerve we obtain : 



(a) Constriction of the pupil. 



(b) Contraction of the ciliary muscle, i.e. spasm of accommodation. 

 The same effects are produced by stimulating the lenticular ganglion 



or the short ciliary nerves. 



Excitation of the long ciliary nerves of the ophthalmic division of the 

 fifth nerve, or of the Gasserian ganglion, causes dilatation of the pupil, but 

 is without influence on the ciliary muscle. 



Stimulation of the sympathetic in the neck causes maximal dilatation 

 of the pupil accompanied by constriction of the vessels of the iris and the 

 eyeball generally. If the superior cervical ganglion be extirpated so as to 

 cause degeneration of all the sympathetic fibres passing up to the eye, it will 

 be found a fortnight later that stimulation of the Gasserian ganglion has no 

 longer any influence on the size of the pupil. We may therefore come to 

 the following conclusions as to the functions of the nerves supplying the 

 interior of the eyeball : 



The third nerve supplies fibres which run through the lenticular gan- 

 glion and the short ciliary nerves and cause constriction of the pupil and con- 

 traction of the ciliary muscle. These fibres arise in the oculo-motor nucleus, 

 which is situated at the back part of the floor of the third ventricle, immedi- 

 ately below the anterior corpora quadrigemina. 



The sympathetic nerve sends fibres which pass to the eye along two 

 routes. A certain number which run on the external carotid artery in the 

 cavernous sinus pass by the sympathetic root to the ganglion and by the 

 short ciliary nerves to the eyeball and cause contraction of the blood-vessels. 

 Other fibres pass from the superior cervical ganglion to the Gasserian ganglion 

 of the fifth nerve, along the nasal branch of its first division and then along 

 the long ciliary nerves to the eyeball. These fibres carry impulses which 

 dilate the pupil. The sympathetic fibres to the eyeball arise in the cord, 

 probably from cells of the lateral column in the lower cervical or uppermost 

 dorsal region. They leave the cord by the first two dorsal anterior roots, 

 pass through the stellate ganglion, the ansa Vieussenii, and up the cervical 

 sympathetic to the superior cervical ganglion where they terminate. New 

 relays of fibres start in this ganglion and travel direct to their destination 

 in the eyeball. Excitation of the cervical spinal cord easily evokes dilatation 

 of the pupil, and it was on this account that Budge located in this part of the 

 cord a cih'o-spinal centre. 



The fibres derived from the fifth nerve itself must be looked upon as 

 chiefly afferent or sensory in function. Some observers have ascribed to 

 them a dilatator effect on the blood-vessels of the eye, but confirmation for 

 this view is wanting. The ciliary muscle is normally at rest and is only set 

 into activity as a result of volitional or reflex efforts to direct the gaze to 

 near objects. The iris is under the influence of tonic impulses which arrive 

 at it along both sets of nerve fibres, oculo-motor and sympathetic. Section 



