796 SPECIAL SENSES. 



Movements of the Iris. 



The movements of the iris are sufficiently simple, as well as the physiological con- 

 ditions under which they take place ; and it is only when we come to study the exact 

 mechanism of the production of these movements through the nervous system, that the 

 subject becomes complex, and, to a certain extent, obscure. As regards the movements 

 themselves, the simple facts are as follows : 



There are two physiological conditions under which the size of the pupil is modified : 

 The first of these depends upon the amount of light to which the eye is exposed. When 

 the quantity of light is small, the pupil is widely dilated, so as to admit as much as pos- 

 sible to the retina. 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 character- 

 istic 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, upon the voluntary action of muscles. 

 We have already seen, 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. We shall see, also, that the effort of accommodation of the eye for near 

 objects 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 movements ; 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. 



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 distances ; but it is nevertheless true that the muscu- 

 lar tissue of the iris will respond directly to the stimulus of light. Harless noted, in sub- 

 jects dead of various diseases, from five to thirty hours after death, that the iris con- 

 tracted under the stimulus of light ; and he justly remarks that this is probably due to 

 direct action upon its muscular tissue, and that it is not reflex, for the reason that the 

 irritability of the nerves in warm-blooded animals disappears certainly in twenty hours 

 after death. The experiments of Harless were made upon the two eyes, one being 

 exposed to the light, while the other was closed. The contraction, however, took place 

 very slowly, requiring an exposure of several hours. This mode of contraction is very 

 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 

 undoubtedly depends upon the direct action of light upon the muscular fibres. 



Action of the Nervous System upon the Iris. This subject, as far as it relates to the 

 third pair, has been pretty fully considered in connection with the physiology of these 

 nerves ; and it is unnecessary to refer again in detail to the experiments which have 

 already been cited. The reflex phenomena observed are sufficiently distinct. When light 

 admitted to the retina, the pupil contracts, and the same result follows mechanical 

 irritation of the optic nerves. When the third pair of nerves has been divided, no such 

 eflex phenomena are observed. It is well known, also, that division of the third nerves 

 m the lower animals or their paralysis in the human subject produces permanent dilata- 

 tion of the pupil, the iris responding, only in the slow and gradual manner already indi- 

 cated, to the direct action of light. 



