254 DISEASES OF THE HORSE. 



racliated fibres draw up the iris somewhat irregularly, which, 

 adhering to the capsule of the opaque lens, the pupil remains 

 permanently open, as in fig. 4. p. 255. 



[Continually exposed to a changing light, — sometimes to the 

 glare of a summer's sun, at others to the chastened light of the 

 moon or stars, — the eye would be of little use, unless it possessed 

 the power of shutting out too great a glare, or of admitting the 

 utmost during the hours of darkness, or in the grey twilight. 

 This power is furnished by the iris or curtain of the eye, which 

 commences at the same place where the cornea begins, and 

 floats loosely in the aqueous humour, which it divides into two 

 parts, the anterior and the posterior chambers. The iris is com- 

 posed of two orders of fibres; — the one circular, which, by con- 

 tracting, diminishes, and even closes the aperture in the centre, 

 named pupil ; the other radiated, which, by contraction, opens 

 or enlarges the aperture. These muscular fibres are entirely 

 independent of the will, but sympathise intimately with the 

 retina, so as to lessen the pupil from the stimulus of light. 



This may be readily seen in the horse by alternately shading 

 the eye and exposing it to a strong light, under which operation 

 the pupil diminishes from the effects of light. Thus the iris 

 admits just light sufficient to make the object distinctly visible, 

 and excludes all superabundant rays. Light, however, produces 

 no immediate effect on the iris itself; for when the optic nerve 

 is paralysed, the iris is no longer affected by light or darkness. 

 The iris derives its name from its brilliancy, and ite colour de- 

 pends on a pigment secreted on its posterior surface, and whose 

 office is to prevent the rays of light from penetrating through it. 

 In the horse, this pigment renders the iris generally of a light 

 brown colour ; but, in some instances, the pigment is wanting, 

 and the iris appears white, and the animal is called wall-eyed. 

 This circumstance, though it by no means adds to the beauty of 

 the animal, is, however, no injury to the vision ; in fact, it neither 

 adds to nor diminishes sight. — Ed.] 



In the human eye the pupil is black, and of a circular shape, 

 and forms what is commonly called the apple of the eye. In the 

 horse it is of a dark blue colour, and of an oblong figure, with 

 its long diameter always ])arallel to the horizon, in whatever 

 position the head may be placed. This uniform parallelism of 

 the long diameter of the pupil with the horizon is effected by 

 means of the oblique muscles before described. 



When the horse stands in the stable in a moderate light, the 

 pupil is open or large, by the preponderant action of the radiated 

 fibres, as in fig. 1. ; but when taken out of the stable, the 

 circular fibres prevail, and the pupil becomes much smaller, as 

 in fig. 2. ; and if the eye be exi^osed to the sunshine, the pujjil 

 becomes smaller still, and scarcely perceptible. 



