I2O 



VETERINARY PHYSIOLOGY 



two sets of non-striped muscular fibres first, the circular 

 fibres, a well-marked band running round the pupil, and called 

 the sphincter pupillce (Sph.P.) muscle ; second, a less well- 

 marked set of radiating fibres, which are absent in some 

 animals, and which constitute the dilator pupillce muscle (D.P.). 

 In the horse the pupil is elliptical, and from the edge of the 

 iris a process like a small bunch of grapes projects, and, when 



the pupil is contracted, nearly 

 occludes it. 



The membrana nictitans, lying 

 on the inner aspect of the orbit, 

 consists of a flexible plate of 

 elastic fibre - 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. 



That 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-like mucoid tissue, 

 the vitreous humour. The vitre- 

 ous humour is enclosed in a deli- 

 cate fibrous capsule, the hyaloid 

 membrane, and just behind the 

 ciliary processes this membrane 

 becomes tougher, and is so firmly 

 adherent to the processes that 

 it is difficult to strip it off'. It 

 passes forward from the processes as the suspensory ligament 

 (S.L.), and then splits to form the lens capsule. In this is 

 held the crystalline lens (L.), a biconvex lens, with its greater 

 curvature on its posterior aspect, and characterised by its great 

 elasticity. Normally it is kept somewhat pressed out and 

 flattened between the layers of the capsule, but if the suspen- 

 sory ligament is relaxed its natural elasticity causes it to bulge 

 forward. This happens when the ciliary muscle contracts and 

 pulls forward the ciliary processes with the hyaloid membrane. 



FIG. 52. Horizontal section through 

 the Left Eye. Cor., cornea ; Scl., 

 sclerotic ; Opt N., optic nerve ; 

 Choi:, choroid ; Cil.M., ciliary 

 processes with ciliary muscle ; 

 D. P. , dilator pupillse muscle ; 

 Sph.P., sphincter pupillae muscle ; 

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

 membrane forming suspensory lig- 

 ament and capsule of lens ; Ret. , 

 retina. The vertical line passing 

 through the axis of the eye falls 

 upon the central spot of the retina. 



