MOVEMENTS OF THE IRIS. 101 



if the third nerves have been extirpated ; 1 which makes it 

 evident that the influence of the fifth is direct, and not reflex 

 through the third. Bonders exposed and galvanized the 

 sympathetic in the neck, to demonstrate the continued action 

 of this nerve upon the iris. He then divided the fifth in the 

 cranial cavity ; and, after this division, in four out of eleven 

 experiments, galvanization of the sympathetic in the neck 

 failed to produce contraction of the pupil. 3 These experi- 

 ments would seem to show that the filaments from the sym- 

 pathetic which act upon the iris join the fifth, and that when 

 the fifth is divided, the communication between this nerve 

 and the radiating muscular fibres is cut off. It is somewhat 

 difficult to explain the slight contraction of the pupil follow- 

 ing galvanization of the sympathetic after division of the 

 fifth in seven out of the eleven experiments, except by the 

 fact, that, in many cases of division of the fifth in the cranial 

 cavity, the sympathetic filaments are not included in the 

 section. 



There seem to be two distinct nerve-centres correspond- 

 ing to the two sets of nerves which regulate the movements 

 of the iris. One of these centres presides over the reflex con- 

 tractions 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 sufficiently 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 conveyed to the muscular fibres, no move- 

 ments of the iris can take place. The centres which preside 



1 BERNARD, Lemons sur la physiologic et la patJwlogie du systeme nerveux, Paris, 

 1858, tome ii., pp. 208, 209. 



2 BONDERS, Anomalies of Accommodation and Refraction of the Eye, The 

 New Sydenham Society, London, 186^, p. 682. , 



