706 SPECIAL SENSES. 



The effort of converging the axes of the eyes, by looking at a very near ob- 

 ject, contracts the pupils ; and 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. 



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. Harless noted, in subjects dead of 

 various diseases, five to thirty hours after death, that the iris contracted un- 

 der the stimulus of light ; and he regarded this as probably due to direct 

 action upon its muscular tissue. It is not reflex, for the reason that the irri- 

 tability 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 con- 

 traction, 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 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 considered in connection with the physi- 

 ology 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 is admitted to the retina, the pupil con- 

 tracts, and the same result follows mechanical irritation of the optic nerves. 

 When the third pair of nerves has been divided, no such reflex phenomena 

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

 lower animals or their paralysis in the human subject produces permanent 

 dilatation of the pupil, the iris responding, only in the slow and gradual man- 

 ner already indicated, to the direct action of light. 



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 fila- 

 ments are derived from the third cranial nerve. If this nerve be divided, 

 the iris becomes permanently dilated and is immovable, except that it re- 

 sponds 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 ganglion. 



Most anatomists admit the existence of radiating muscular fibres in the 

 iris, the action of which is antagonistic to the circular fibres, and which dilate 



