CHAPTER XV 

 THE SENSES 



Section i. 



Sight. 



The delicate structures composing the eye receive a very thorough 

 protection by the anatomical arrangement of the parts. The 

 orbital cavity, for example, is nearly surrounded by incomplete 

 bony walls, and the layers of fat within it assist the muscles in 

 protecting the globe and the optic nerve. The eyelids sweep the 

 cornea and protect the part from dust and exposure ; the tears 

 keep the face of the cornea brilliant ; the membrana nictitans 

 removes particles of solid matter which would otherwise produce 

 irritation ; and the eyeball can be retracted to a considerable 

 extent to assist it in withdrawing from injury. The size of the 

 orbit is such that ordinary blows inflicted upon the eye are 

 expended on the margin of the orbital cavity, and not on the 

 eyeball itself, so that the risk of serious injury is far less from 

 large than from small bodies. The shape of the eyeball is not 

 (in the horse) quite spherical : the vertical axis is greater than 

 the horizontal, and the posterior face of the eyeball is distinctly 

 flatter than the anterior. 



Structure of the Eye. — Issuing from the back of the eyeball 

 very low down, and inclined to the temporal side of the globe, 

 is the optic nerve, which, after describing a peculiar curve up- 

 wards, runs in the substance of the retractor muscle to enter 

 the cranium through the optic foramen. This curve in the optic 

 nerve (Fig. 158) is necessitated by the horizontal movements of 

 the eyeball ; when the eye looks backwards, the curve is in- 

 creased, whereas when it looks forwards the ' slack ' is taken out 

 of the nerve and the curve entirely disappears. 



The globe of the eye is anteriorly made up of a transparent 

 convex surface, known as the cornea, whilst the remainder of 

 its walls are opaque, and formed by the sclerotic, choroid, and 

 retina. The sclerotic is the tunic on which the strength of the 



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