140 



VETERINARY PHYSIOLOGY 



from the sclerotic just behind the corneo- sclerotic junction, 

 and run backwards and outwards to be inserted into the 

 bases of the ciliary processes ; second, circular fibres which 

 run round the processes just inside the radiating fibres. The 

 choroid is continued forward in front of the ciliary processes 

 to the 2) if^pil as the iris, and in it are also two sets of nou- 



striped muscular fibres — first, 

 the circular fibres, a well- 

 marked band runninsf round 

 the pupil, and called the 

 sphincter 'piipillcti {&ph.P.) 

 muscle ; second, a less well- 

 marked set of radiatinsr fibres, 

 which are absent in some 

 animals, and which constitute 

 the dilator piipillcB muscle 

 {D.P.). In the horse the 

 pupil is elliptical, the long 

 axis beinof in the horizontal 

 plane, and from the edge of 

 tlie iris a process like a small 

 bunch of grapes projects, and, 

 Ghor., choroid; Cil.M., ciliary when the pupil is contracted, 



processes with ciliary muscle ; , . 



- nearly occludes it. 



The mernhrana nictttans, 

 Iving on the inner aspect of 

 the orbit, consists of a flexible 

 plate of elastic fibro-cartilage 

 covered with conjunctiva. 

 When the eye is retracted 

 the post- orbital fat is pushed 

 forwards and thrusts the membrane over the inner half of 

 the eyeball (p. 160). 



The part of the eye in front of the iris is filled by a 

 lymph-like fluid, the aqueous humour, while the part behind 

 is occupied by a fine jelly-Hke mucoid tissue, the vitreous 

 humour. The vitreous humour is enclosed in a delicate 

 fibrous capsule, the hyaloid membrane, and, just behind the 

 ciliary processes, this membrane becomes tougher, and is so 



Fig. 62. — Horizontal section through 

 the Left Eye. Cor., cornea; ScL, 

 sclerotic; Opt.N., optic nerve; 



D.P., dilator pupilUe muscle ; 

 Sph.P., sphincter pupilla; muscle ; 

 L., crystalline lens; S.L., hyaloid 

 membrane forming suspensory 

 ligament and capsule of lens ; 

 Bet., retina. The vertical line 

 passing through the axis of the 

 eye falls upon the central spot 

 of the retina. 



