288 THE HORSE. 



Stead of being unconfined except by the walls of the chauiber in 

 which it lies, it is bound up in a network of transparent cells, 

 which give it the consistency and appearance of a delicate jelly. 

 Upon the perfect transparency and proper shape of these humors 

 depends the sight of the animal ; but in addition to the risk of 

 blindness from any defect in these parts, if the investing coats or 

 membranes are inflamed or disorganized, their functions are not 

 performed, and the sight is either impaired or destroyed. Thus 

 the rays of light may be fairly collected, so as to throw the impreS' 

 sion of every object within the sphere of vision upon the back of 

 the eye, and yet the horse may be blind, because the retina or 

 expansion of the optic nerve is disorganized by disease. When 

 inflammation attacks the coats of the eye, it generally extends to 

 the investments of the humors, and to the substance of the lens 

 itself, producing cataract or opacity of that part, but it is possible 

 to have the sight impaired from a mere defect of shape in the 

 anterior coat, so as to make the surface too convex and thus alter 

 the focus of the sight. This is the " buck-eye," which leads to 

 shying, and is perfectly incurable. The membranes a,ve, Jirst, the 

 cornea, a perfectly transparent coat, placed in front of the eye, and 

 inserted, like a watch glass, in the sclerotic coat covering the 

 posterior four-fifths of the globe. The latter is a white fibrous 

 membrane, strong and inelastic, so as to aff"ord protection to the 

 parts within it from external violence. This forms the white of 

 the eye, which, however, is only occasionally visible in the horse. 

 Beneath the sclerotic is the choroid coat, consisting of a network 

 of blood vessels, and lined with a black pigment, which again has 

 on its internal surface, at the part opposite the pupil, a greenish- 

 white iridiscent lining, called tapetum lucidum, or luminous carpet. 

 Lastly, within the whole of this surface is spread a beautiful 

 expansion of the optic nerve, called the retina, which receives the 

 impressions derived from the rays of light, forming a distinct figure 

 upon it exactly similar to the objects which are presented to it, 

 except in point of size, and in being inverted. Beyond these parts, 

 there is a provision made for moderating the rays of light according 

 to their intensity. This is effected by means of an opaque septum, 

 pierced with an oval hole — the former being called the iris, and 

 the latter the pupil. The substance of the iris itself is composed 

 of contractile tissue, which has the power of expanding or contract- 

 ing the pupil in obedience to the impression produced upon the 

 retina, and thus, if the eye is examined in a strong light, the pupil 

 will appear large when shaded by the hand, but contracts immedi- 

 ately on exposing the eye. The horse's iris is brown, varying 

 somewhat in shade in different individuals, and at the upper part 

 of the pupil it presents one or two little floating appendages, which 

 serve to moderate the sun's rays. Sometimes the brown color is 



