616 PHYSIOLOGY 



Stimulation of the sympathetic in the neck causes maximal dilata- 

 tion 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 con- 

 clusions as to the functions of the nerves supplying the interior of 

 the eyeball : 



The third nerve supplies fibres which run through the lenticular 

 ganglion and the short ciliary nerves and cause constriction of the 

 pupil and contraction 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, immediately below the anterior corpora quadri- 



gemina. 



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 T 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 cilio-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 influ- 

 ence of tonic impulses which arrive at it along both sets of nerve 

 fibres, oculo-motor and sympathetic. Section therefore of the sym- 

 pathetic nerve causes constriction, and section of the third nerve 

 dilatation of the pupil. These tonic influences are probably reflex in 

 origin, since it is found that, after cutting off afferent impressions from 



