THE SENSES 533 



by the liberal application of pigment, which covers the inner 

 surface of the choroid coat, with the exception of a surface 

 above the optic nerve, which is brilliant and iridescent in ap- 

 pearance and is known as the tapetum lucidum. Between the 

 two humours a diaphragm is situated, known as the iris, which 

 regulates the amount of light passing into the eye, and behind 

 this is a focussing arrangement or lens. The cornea, lens, and 

 humours constitute the refracting apparatus of the eye. By 

 means of the muscles of the eyeball the globe is given a con- 

 siderable range of movement, and, in addition, it can be re- 

 tracted within the orbital cavity. These muscles also afford 

 protection to the optic nerve. In eyes placed laterally the range 

 of vision is considerable. The movements of the head are of 

 even more importance than the movements of the eyeball muscles 

 in securing a wide range of vision. 



The similarity in construction between the eye and the appa- 

 ratus known as a camera is very marked ; both have a refract- 

 ing surface placed anteriorly, a diaphragm to cut off superfluous 

 rays of light, an arrangement for focussing, and a dark chamber in 

 which a sensitised surface is placed, and on which a reduced and 

 inverted image of the picture is impressed. 



The Cornea in most animals is circular in outline ; in the horse 

 it is somewhat oval. When viewed from the front and divided 

 into two halves by a vertical line, it is distinctly larger on its 

 nasal than on its temporal side. It is a very tough, non- vascular 

 membrane, richly supplied with nerves, and nourished by lymph 

 which circulates freely in it. It may be regarded as the chief 

 refractive apparatus of the eye. When viewed from the side, the 

 cornea is seen to be convex. Measurement shows that in the 

 majority of horses the curvature of the cornea taken in its 

 horizontal and vertical meridians is not exactly the same, which 

 it would be supposing its surface were accurately spherical. 

 The excess of curvature of one meridian of the cornea over that 

 of the surface at right angles to it produces a defect in vision 

 which is known as astigmatism. The meridian in the horse 

 which is nearly always the flattest is the horizontal. 



The Lens is composed of various onion-like layers of different 

 refractive powers. In shape it is bi-convex, the convexity of its 

 posterior face being greater than that of the anterior. It is 

 held in its place by a capsule which really suspends the lens in 

 the eye, the capsule receiving attachment to some long processes 

 behind the iris known as the ciliary processes. In the horse the 

 lens is in contact with the ciliary processes ; in most other 

 animals there is a small space between the two. The lens 

 possesses inherent elasticity, which admits of its surface under- 

 going an alteration in shape, so as to be flatter at one time, more 



