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A MANUAL OF PHYSIOLOGY 



whole of the optic nerve in front of the chiasma, would cause 

 complete blindness of the corresponding eye. Sometimes in disease 

 of the optic nerve vision is not totally destroyed in the eye to which 

 it belongs, but the field is narrowed by a circumference of blindness. 

 In this case the pathological change involves the circumferential 

 fibres of the nerve. When the chiasma is affected by disease, a very 

 frequent symptom is nasal hemianopia, blindness of the nasal halves 

 of the retinae, with loss of the outer or temporal half of each field of 

 vision. The optic nerve contains a few efferent fibres for the retina, 

 whose cell-bodies are situated in the anterior corpus quadrigeminum. 

 The third nerve, or oculo-motor, arises from a series of nuclei in 

 the floor of the Sylvian aqueduct below the anterior corpora quadri- 



gemina. The root-bundles 

 coming off from the most 

 anterior of the nuclei carry 

 fibres that have to do with 

 the mechanism of accom- 

 modation. The nuclei be- 

 hind these are connected 

 with fibres that cause con- 

 traction of the pupil when 

 light falls on the retina; 

 while, in dogs at least, the 

 posterior portion of the 

 series gives off fibres for the 

 muscles of the eye in the 

 following order from before 

 backwards : internal rectus, 

 superior rectus, levator pal- 

 pebrse superioris, inferior 

 rectus, inferior oblique. 

 Complete paralysis of the 

 third nerve causes loss of 

 the power of accommoda- 

 tion of the corresponding 

 eye, dilatation of the pupil 

 by the unopposed action of 

 the sympathetic fibres, dimi- 

 nution of the power of 

 moving the eyeball, ptosis, 

 or drooping of the upper 

 lid, external squint, and consequent diplopia, or double vision. 



In Fig. 258, optic nerve-fibres are represented as forming synapses 

 with cells in the anterior corpus quadrigeminum whose axons pass to 

 the nucleus of the third nerve and arborize around some of its cells. 

 It is possible that this is the path of the impulses which cause con- 

 traction of the pupil when light falls on the retina. 



The fourth or trochlear nerve arises from the posterior part of 

 the same tract of grey matter which gives origin to the third nerve. 

 It supplies the superior oblique muscle. Paralysis of the nerve 

 causes internal squint when an object below the horizontal plane is 



FIG. 258. SCHEME OF THE VISUAL PATH 

 (HALLIBURTON, AFTER SCHAFER). 



