684 SPECIAL SENSES 



detail to the experiments that have already been cited. The reflex 

 phenomena observed are sufficiently distinct When light is admitted 

 to the retina, the pupil contracts, and the same result follows mechanical 

 irritation of the optic nerves. When the third pair of nerves has been 

 divided, no such reflex phenomena are observed. It is well known, also, 

 that division of the third nerves in the lower animals or their paralysis 

 in the human subject produces permanent dilatation of the pupil, the iris 

 responding, only in the slow and gradual manner already indicated, to 

 the direct action of light. 



Taking all the experimental data into consideration, it is certain that 

 the third nerve has an important influence on the iris. Filaments from 

 the ophthalmic ganglion animate the circular fibres, or sphincter, and 

 these filaments are derived from the third cranial nerve. If this nerve 

 is divided, the iris becomes permanently dilated and is immovable, ex- 

 cept that it responds slowly to the direct action of light. The reflex 

 action by which the pupil is contracted under the stimulus of light 

 operates through the third nerve, and no such action can take place 

 after this nerve has been divided. In view of these facts, there can be 

 no doubt in regard to the nervous action on the sphincter of the pupil, 

 this muscle being animated exclusively by filaments from the motor oculi 

 communis, coming through the ophthalmic ganglion. 



Most anatomists admit the existence of radiating muscular fibres in 

 the iris, the action of which is antagonistic to the circular fibres, and 

 which dilate the pupil. That these fibres are subjected to nervous 

 influence, is rendered certain by experiments on the sympathetic system. 

 There can be no doubt that the action of the sympathetic on the pupil 

 is directly antagonistic to that of the third pair, the former presiding 

 over the radiating muscular fibres ; and the only question to determine 

 is the course taken by the sympathetic filaments to the iris. Experi- 

 ments on the influence of the fifth pair on the pupil have been somewhat 

 contradictory in different animals. In rabbits, section of this nerve in 

 the cranial cavity produces contraction of the pupil ; but in dogs and 

 cats, the same operation produces dilatation. In the human subject, of 

 course, it is impossible to determine this point by direct experiment ; 

 and the varying results obtained in observations on different animals 

 probably depend upon differences in the anatomical relations of the 

 nerves. It is probable, however, that the filaments of the sympathetic 

 that animate the radiating fibres join the fifth nerve near the ganglion 

 of Gasser and from this nerve pass to the iris. In observations on the 

 human subject it has been noted that division of the sympathetic in the 

 neck was attended with contraction of the pupil on the side of the in- 

 jury (Mitchell). Stimulation of the cervical sympathetic, in the head of 



