373 VETERINARY LECTURES 



the horse it is generally dark brown. The opening in the centre of 

 the iris is called the pupil, and in horses is of elliptical shape. The 

 muscular fibres of the iris are arranged in two directions, circular 

 and radiating, the former contracting, the latter dilating the pupil. 

 Some horses have a white, unpigmented iris, and are called ' wall- 

 eyed ' or '■albinos.'' 



629. The Corpora Nigra {Plate XXXIX., A 7) are the small 

 black bodies, of a globular shape, to be observed in front of the 

 papillary opening, and generally seen on the upper fringe of the iris. 

 Their use is supposed to be to modify the rays of light. 



630. The Humours of the eye are the aqueous, the crystalline, 

 and the vitreous. They act as refractive media. The aqueous 

 {Plate XXXIX., B 6 and 12) is in front, between the interior surface 

 of the cornea and the crystalline lens. It is composed chiefly of 

 water, and has an alkaline reaction. The crystalline lens 

 {Plate XXXIX., B 8) is bi-convex, more flattened in front than 

 behind. It is made up of concentric layers, like an onion, and is 

 enclosed in a capsule. It is situated behind the iris, having the 

 aqueous humour in front and the vitreous behind, and is held in 

 position by a suspensory ligament. It is transparent, its use being 

 to concentrate the rays of light to a focus. The vitreous humour 

 {Plate XXXIX., B 7 7) occupies the posterior chamber, which con- 

 stitutes about four-fifths of the interior of the sac. It is an albu- 

 minous semi-fluid of a jelly-like consistency, and is enclosed in a 

 structure termed the hyaloid membrane. 



63 t. The Retina (Plate XXXIX., B 5) is the expansion of the 

 optic nerve. It is composed of several layers, consisting of rods, 

 cones, etc., and is the essential part of the organ of vision. The 

 optic nerve {Plate XXXIX., B 16 16) enters the eyeball, in 

 company with the ophthalmic artery and vein, at the back of the 

 eye. 



632. The muscles of the eye are the retractor, abductor, 

 adductor, depressor, levator, and superior and inferior obliques. 

 These muscles turn the eye in the various directions required. 



