MOVEMENTS OF THE IRIS 683 



fectly corrected by placing cylindrical glasses before the eyes, it is 

 impossible, in the majority of cases, to construct glasses that will 

 correct entirely what is called irregular astigmatism. 



MOVEMENTS OF THE IRIS 



There are two physiological conditions under which the size of the 

 pupil is modified : The first of these depends on the degree of illumina- 

 tion to which the eye is exposed. When the illumination is dim, the 

 pupil is widely dilated. When the eye is exposed to a bright light; the 

 retina is protected by contraction of the iris. The muscular action by 

 which the iris is contracted is characteristic of the smooth muscular 

 fibres, as can be readily seen by exposing an eye, in which the pupil is 

 dilated, to a bright light. Contraction does not take place instantly, but 

 an appreciable interval elapses after the exposure, and a more or less 

 gradual diminution in the size of the pupil is observed. This is seen 

 both in solar and in artificial light. The second of these conditions 

 depends indirectly on the voluntary action of muscles. The effort of 

 converging the axes of the eyes, in looking at a very near object, con- 

 tracts the pupils ; and accommodation of the eye for near objects pro- 

 duces the same effect, even when the eyes are not converged. This 

 action will be fully considered under the head of accommodation. 



Direct Action of Light on the Iris. The variations in the size of the 

 pupil under different physiological conditions are effected almost ex- 

 clusively through the nervous system, either by reflex action from 

 variations in the intensity of light, or by a direct influence, as in accom- 

 modation for distances ; but it is nevertheless true that the muscular 

 tissue of the iris will respond directly to the stimulus of light. Harless 

 noted, in subjects dead of various diseases, five to thirty hours after 

 death, that the iris contracted under the stimulus of light; and he 

 regarded this as probably due to direct action on its muscular tissue. 

 The experiments of Harless were made on the two eyes, one being 

 exposed to the light, while the other was closed. The contraction, how- 

 ever, took place slowly, requiring an exposure of several hours. This 

 mode of contraction is different from the action of the iris during life ; 

 but it is precisely like the contraction observed after division of the 

 motor oculi communis, which is slow and gradual and depends on the 

 direct action of light on the muscular fibres. 



Action of the Nervous System on the Iris. This subject, so far as it 

 relates to the third pair, has been considered in connection with the 

 physiology of these nerves ; and it is unnecessary to refer again in 



