The Senses. 303 



Fig. 33). The eye all this time we must suppose to have 

 its accommodation relaxed, and to be looking into distant 

 space ; if now it be focussed on a near object, the lens 

 becomes convex, and the second image of the candle 

 advances and it becomes smaller and clearer. 



At the moment when the ciliary muscle acts the pupil 

 contracts, and when the muscle is relaxed it dilates. This 

 is due not only to the intimate connection existing between 

 the ciliary muscle and the iris, but also both are supplied 

 by the same nerve — viz., the third cranial. 



The Iris, or diaphragm, performs the same function as a 

 shutter : it regulates the amount of light entering the eye, 

 and, as pointed out in speaking of spherical aberration, it 

 cuts off' those rays entering at the circumference of the 

 lens, and so introduces a correction for spherical aberration. 



The movements of the iris are brought about by two sets 

 of muscles — a sphincter and a dilator (?). The former sur- 

 rounds the pupil, and by contracting closes it ; it is supplied 

 with nervous power through the third pair. The dilator by 

 contracting opens the pupil, and its nervous supply is fur- 

 nished chiefly by the sympathetic. The existence of a 

 dilator muscle, though very probable, is not absolutely 

 proved ; but it is certain that the contraction of the pupil 

 is under the control of the third pair of nerves, whilst its 

 dilatation is produced by the sympathetic. 



The ordinary contraction of the pupil due to light is a 

 well-marked reflex action. 



The iris would also appear to be affected by the action of 

 light. In a horse, for instance, just destroyed, the pupil 

 dilates considerably, and yet in a few hours' time it has 

 undergone extensive contraction. It is said that if the eyes 

 remain covered up the subsequent contraction does not 

 occur. It is important to note that the pupil of the horse 

 dilutes under artificial light, a point to be referred to 

 presently. 



The opening in the iris— termed ' the pupil' — is elliptical 

 in herbivora. The range of movement is extensive, from a 

 mere ring of iris, such as may be obtained under atropine, 



