MOVEMENTS OF THE IRIS. 797 



Taking all the experimental facts into consideration, it is certain that the third nerve 

 has an important intiuence upon the iris. Filaments from the ophthalmic ganglion animate 

 the circular fibres, or sphincter, and these filaments derive their power from the third 

 cranial nerve. If this nerve be divided, the iris becomes permanently dilated and is im- 

 movable, except that it responds very 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 with regard to the nervous action upon the sphincter 

 of the pupil, this muscle being animated exclusively by filaments from the motor oculi 

 communis, coming through the ophthalmic ganglion. 



We admit, with most modern anatomists, 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 experi- 

 ments upon the sympathetic system. 



The effects of division of the sympathetic in the neck have been treated of fully in 

 connection with the general functions of these nerves. It will be sufficient for our present 

 purposes to state, in a general way, the influence of these nerves upon the movements of 

 the iris. There can be no doubt that the action of the sympathetic upon the pupil is 

 directly antagonistic to that of the third pair, the former presiding over the radiating, or 

 dilating muscular fibres ; and the only question to determine is the course taken by the 

 sympathetic filaments to the iris. Experiments on the influence of the fifth pair upon 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 observa- 

 tions upon different animals probably depend upon differences in the anatomical relations 

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

 mate the dilator fibres join the fifth nerve near the ganglion of Gasser and from this 

 nerve pass to the iris. 



There seem to be two distinct nerve-centres corresponding to the two sets of nerves 

 which regulate the movements of the iris. One of these centres presides over the reflex 

 contractions of the iris, and the other is the centre of origin of the nervous influence 

 through which the pupil is dilated. 



The mechanism of reflex contraction of the iris under the stimulus of light is suffi- 

 ciently simple. An impression is made upon the retina, which is conveyed by the optic 

 nerves to the centre of vision, and, in obedience to this impression, the sphincter of the 

 iris contracts. If the optic nerves be divided, so that the impression cannot be conveyed 

 to the centre, or if we divide the third pair, through which the motor stimulus is con- 

 veyed to the muscular fibres, no movements of the iris can take place. The centres 

 which preside over these reflex phenomena are situated in the tubercula quadrigemina. 

 In the remarkable experiments of Flourens upon the encephalic centres, it was shown 

 that the iris loses its mobility after destruction of the tubercula. This fact has been 

 repeatedly confirmed by later experimenters. In birds, in which the decussation of the 

 optic nerves is complete, this action is crossed, destruction of the tubercle upon one side 

 producing immobility of the iris upon the opposite side ; but in man, where the anatomi- 

 cal relations of the optic nerves upon the two sides are more complex, the crossed action 

 is probably not so complete. In man, the axes of both eyes are habitually brought 

 to bear upon objects, and it is well known that there is a physiological unity in the 

 action of the two eyes in ordinary vision. We also observe that, when one eye only 

 is exposed to light, the pupil becoming contracted under this stimulus, the pupil of 

 the other eye also contracts. There is, indeed, a direct contraction and dilatation 

 of the pupil of the eye which is exposed to the light, and an indirect, or "consen- 

 sual " movement of the iris upon the opposite side. The consensual contraction occurs 



