MOVEMENTS OF THE IKIS. 97 



in connection with the physiology of the third pair of nerves, 

 that the effort of converging the axes of the eyes by looking 

 at a very near object contracts the pupils. 1 We shall see, 

 also, that the effort of accommodation of the eye for near ob- 

 jects produces the same effect, even when the eyes are not 

 converged. This action will be fully considered under the 

 head of accommodation. 



One point relating to the anatomy of the iris is of great 

 importance in connection with the physiology of its move- 

 ments ; and that is the question of the existence of dilator 

 fibres. Upon this point there is some difference of opinion ; 

 but, as we stated in treating of the structure of the eye, the 

 weight of anatomical authority is decidedly in favor of the 

 existence of radiating fibres. 2 The physiology of the iris is 

 much more easily understood, if we assume the existence of 

 these fibres, and this is the view that we have adopted. 



Direct Action of Light upon the Iris. The variations in 

 the size of the pupil under different physiological conditions 

 are effected almost exclusively through the nervous system, 

 either by reflex action from variations in the intensity of 

 light, or by a direct influence, as in accommodation for dis- 

 tances ; but it is nevertheless true that the muscular tissue of 

 the iris will respond directly to the stimulus of light. In a 

 memoir presented to the French Academy of Sciences, in 

 1847, it was shown by Dr. Brown-Sequard, that, in some of 

 the lower animals, frogs, eels, etc., the iris continued to con- 

 tract under the stimulus of light many days, even, after death. 

 In frogs, the pupil was made to alternately contract and dilate, 

 from fifty to one hundred times in a minute, in the eye 

 extracted from the orbit. 3 Analogous phenomena have been 

 observed by Harless in the human subject after death. These 



1 See vol. iv., Nervous System, p. 134. 



2 See page 53. 



3 BROWN-SEQUARD, Eecherches experimentales sur ^influence excitatrice de la 

 lumiere, du froid et de la chaleur sur Viris. Journal de la physiologic, Paris, 

 1859, tome ii., p. 282. 



