FUNCTIONS OF THE THIED CRANIAL NERVE. 587 



Function. The optic nerve is the nerve of sight ; physiologically, it is excited 

 only by the transference of the vibrations of the ether to the rods and cones of the 

 retina ( 383). Every other form of stimulus, when applied to the nerve in its 

 course or at its centre, causes the sensation of light. Section or degeneration of 

 the nerve is followed by blindness. Stimulation of the optic nerve causes a reflex 

 contraction of the pupils, the efferent nerve being the oculomotorius or third cranial 

 nerve. If the stimulus be very strong, the eyelids are closed and there is a 

 secretion of tears. The influence of light upon the general metabolism is stated at 

 127, 9. 



As the optic nerve has special and independent connections with the so-called 

 visual centre ( 378, IV.), as well as with the centre for narrowing the pupil ( 345), 

 it is evident that, under pathological circumstances, there may be, on the one hand, 

 blindness with retention of the action of the iris, and on the other loss of the 

 movements of the iris, the sense of vision being retained ( Wernicke). 



Pathological. Stimulation of almost the whole of the nervous apparatus may cause excessive 

 sensibility of the visual apparatus (hyperaesthesia optica), or even visual impressions of the 

 most varied kinds (photopsia, chromatopsia), which in cases of stimulation of the visual 

 centre may become actual visual hallucinations ( 378, IV.). Material change in, and 

 inflammation of, the nervous apparatus are often followed by a nervous weakness of vision 

 (amblyopia), or even by blindness (amaurosis). Both conditions, however, may be the signs 

 of disturbances of other organs, i.e., they are "sympathetic" signs, due it may be to changes 

 in the movement of the blood-stream, depending upon stimulation of the vaso -motor nerves. 

 The discovery of the partial origin of the optic nerve from the spinal cord explains the 

 occurrence of amblyopia with partial atrophy of the optic nerve, in disease of the spinal cord, 

 especially in tabes. Many t poisons, such as lead and alcohol, disturb vision. There are 

 remarkable intermittent forms of amaurosis known as day-blindness or hemeralopia, which 

 occurs in some diseases of the liver and is sometimes associated with incipient cataract. [The 

 person can see better in a dim light than during the day or in a bright light. In night- 

 blindness or nyctalopia, the person cannot see at night or in a dim light, while vision is good 

 during the day or in a bright light. It depends upon disorder of the eye itself, and is usually 

 associated with imperfect conditions of nutrition.] 



345. HI. NEEVUS OCULOMOTORIUS. Anatomical. It springs from the oculomotorius 

 nucleus (united with that of the trochlearis), which is a direct continuation of the anterior 

 horn of the spinal cord, and lies under the aqueduct of Sylvius (fig. 428). [The motor nucleus 

 (fig. 427) gives origin to three sets of fibres, for (1) the most of the muscles of the eyeballs, (2) 

 the sphincter pupillse, (3) ciliary muscle. The nucleus of the 3rd and 4th nerves is also con- 

 nected with that of the 6th under the iter, so that all the nerves to the ocular muscles are thus 

 co-related at their centres.] 



The origin is connected -with the corpora quadrigemina, to which the intraocular fibres may 

 be traced, and also with the opposite half of the brain to the angular gyrus ( 378, I.) through 

 the pedunculus cerebri. Beyond the pons, it appears on the inner side of the cerebral peduncle, 

 between the superior cerebellar and posterior cerebral arteries (fig. 428, III). 



Function. It contains (1) the voluntary motor fibres for- all the external 

 muscles of the eyeballs except the external rectus and superior oblique and for 

 the levator palpebral superioris. The co-ordination of the movements of both eye- 

 balls, however, is independent of the will. (2) The fibres for the sphincter pupillm, 

 which are excited reflexly from the retina. (3) The voluntary fibres for the muscle 

 of accommodation, the tensor choroideae or ciliary muscle. The intrabulbar fibres of 

 2 and 3 proceed from the branch for the inferior oblique muscle, as the short root 

 of the ciliary ganglion (fig. 429). They reach the eyeball through the short ciliary 

 nerves of the ganglion. V. Trautvetter and others observed that stimulation of 

 the nerve caused changes in the eye similar to those which accompany near vision. 

 The three centres for the muscle of accommodation, the sphincter pupilloe, and the 

 internal rectus muscle, lie directly in relation with each other, in the most posterior 

 part of the floor of the third ventricle (ffensen and Vblckers). 



The centre for the reflex stimulation of the sphincter fibres by light was said 

 to be in the corpora quadrigemina, but newer researches locate it in the medulla 

 oblongata ( 379, 392). The narrowing of the pupil, which accompanies the act 



