MOVEMENTS OF THE LRIS. 99 



nerve are somewhat contradictory. Some experimenters have 

 noted that stimulation of these nerves produces immediate 

 contraction of the pupil ; but Bernard, Chauveau, and others 

 failed to observe this as a constant phenomenon, and assume 

 that, when contraction occurs, it is due to an extension of the 

 irritation, either to the ophthalmic branch of the fifth or to 

 the ciliary nerves passing to the iris from the ophthalmic 

 ganglion. 1 The experiments upon the inferior animals were 

 in the main confirmed in the human subject by Nuhn, in ob- 

 servations made upon the head of a decapitated criminal. 2 



Taking all the experimental facts into consideration, it is 

 certain that the third nerve has an important influence 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 immov- 

 able, 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 gan- 

 glion. 



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 certain 

 from experiments 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 



1 See vol. iv., Nervous System, p. 133. 



2 NUHN, Versuche an einem Enthaupteten nebst erlauternden Versuchen an 

 Thieren. Zeitschrift fur rationeUe Medecin, Heidelberg, 1853, Neue Folge, Bd. 

 iii., S. 135. 



