400 CHAPTER 57. 



according to these circumstances the pupillary opening is proportionately 

 enlarged. 



The opening in the centre of the Iris is called the pupil. Though 

 circular in man, it is transversely oblong in the horse and in many other 

 animals intended to seek safety in flight. The diameter of the pupil, or 

 opening formed by the iris, varies, as explained above, in an inverse ratio 

 to the amount of light to which the eye is exposed. 



The colour of the iris varies in different horses, but it is always bril- 

 liant. Its posterior surface, called the uvea, is in most cases brown or 

 nearly black. In wall-eyed horses this pigment is partly wanting, whilst 

 in albino or cream-coloured animals it is altogether absent. 



The iris is highly endued with nerves, in order to give it due suscepti- 

 bility to the ever-varying degrees of light to which the eye is subject. 

 It is also furnished with arteries, veins, and absorbents. 



From its situation, structure, and office of regulating the amount of 

 light admitted to the retina, from its connections with most of the opera- 

 tions of the organ of vision, and from its diseases, the iris is a structure 

 of great importance. 



829. The Corpora nigra. 



The Corpora nigra are little black pigmentary globular bodies, belong- 

 ing to the uvea or posterior lining of the iris and attached to its margin, 

 to which they fonii a kind of fringe. Three or four hang down from the 

 upper border of the iris, and usually one or two are attached to the lower 

 margin. Their use, as indicated by their colour, appears to be to 

 absorb and modify the rays of light entering the eye. They may be said 

 to be auxiliaries to the iris to some extent, and perhaps partly supply the 

 want of eyebrows in the horse. 



830. The Optic nerve. 



The Optic nerve, which is given off directly from- the brain, enters the 

 eye by a round cribriform aperture at the inner inferior and posterior 

 part of the globe. Its expansion forms the chief part of the retina. 



831. Other nerves of the Eye. 



The various structures of the eye are, as we might suppose, very highly 

 endued with nervous power. The chief peculiarity of its nervous system 

 is the intimate connection between the two eyes. Each part of each 

 eye is supplied with branches of the nerve which supplies the correspond- 

 ing part of the other eye. Hence in amaurosis in one eye the pupil may 

 contract, when the other eye, which is not so affected, is exposed to a 

 strong light. The retina of the blind eye of course does not feel the 

 effect of the light, but its iris is moved in connection with the motion of 

 the iris of the other eye, which does feel the effect of the light, but not 

 quite to the same extent. Hence also disease of the internal structures, 

 such as specific ophthalmia in one eye, is apt to affect the other eye ; and 



