THE EYE 497 



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 humours, and to the substance of the lens itself, produc- 

 ing cataract or o^Dacity 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 are, first, the cornea, a perfectly transptarent 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 afibrd 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 iridescent 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 j^oint 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 j^upil. The 

 substance of the iris itself is composed of contractile tissue, which has the 

 power of expanding or contracting 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 

 immediately on exposing the eye. The horse's iris is brown, varying some- 

 what 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 colour is absent, and the iris is either 

 partially white or light blue, in which case it is called a " wall eye " ; but 

 though this is considered unsightly, it does not interfere with vision. The 

 iris is stretched across the chamber of the aqueous humour, and is thus 

 enabled to act freely. There are many other delicate structures worthy 

 of being examined, but want of space must prevent any further allusion to 

 them. 



The appendages of the eye are: 1st. The conjunctiva or membrane 

 protecting the exposed surface of the eye. 2nd. The eyelids. 3rd. The 

 membrana nictitans or haw. 4th. The muscles of the eye. 5th. The 

 lachrymal apparatus. 17ie conjunctiva covers the whole front of the eye, 

 being thin, and perfectly transparent in a healthy state, but on the occur- 

 rence of inflammation speedily becoming red and puffy. It is reflected from 

 this face to the inside of the eyelids, and the whole membrane is extremely 

 liable to inflammation from any external irritation. The eyelids have 

 nothing very remarkable about them, being merely cartilaginous shutters 

 covered with fine skin, and lined with conjunctiva, and I'aised and lowered 

 by muscles peculiar to them. The membrane nictitans or haw is a cartilage 

 lying just within the inner corner of the eye, but capable of being thrust 



