822 



TEE APPARATUS OF THE SENSES. 



Fig. 385. 



other on the surface of a structureless laminas ; the cells are provided with a 

 nucleus, and contain pigment-granules which exclusively occupy their anterior 

 moiety.' (On the choroid this cell formation is single, but on the iris and ciliary 

 processes there are several layers. A very delicate membrane— mem&mwe 

 of Bnich—h&s been described as lining the inner surface of the choroid, and 

 retaining the pigment in its place ; this membrane may be seen on the 

 posterior surface of the iris, and it probably prevents 

 the pigment being removed by the aqueous humour.) 

 The use of the choroid membrane is to convert the 

 ocular globe into a veritable darkened chamber. (The 

 pigment absorbs the rays of light virhieh pass through 

 the retina, and thus prevents their becoming reflected 

 and confusing the vision. The brilliant metallic- 

 coloured layer named the tapetum is more particularly 

 observed in nocturnal animals, and especially in the 

 Carnivora ; it is due to the presence of a thick layer 

 of vpavy fibrous tissue outside the choroidal epithe- 

 lium. By reflecting the rays of light a second time 

 through the retina, it probably enables the animal 

 to see better at night. It is the cause of the glare 

 perceived in the eyes of Cats and other creatures in 

 the dark.) 



The ciliary ligament is a contractile body, being 

 composed of unstriped muscular fibres which are arranged in orbicular 

 fasciculi, or extend backwards (and are lost in the choroid, behind the ciliary 

 processes). These fibres are intermixed in the plexus of ciliary nerves, on 

 whose track small ganglia are formed. By its contractions, the ciliary 

 muscle (or ligament) plays an important part in accommodating the eye to 

 the perception of objects at different distances. (In Birds, the muscular 

 fibres are striped.) 



The ciliary body or processes are formed by intercrossed fasciculi of con- 

 nective tissue, vessels, and some unstriped fibres ; their inner surface is 

 covered by pigment, like that of the choroid zone. 



CELLS FROM PIGMENTUM 



NIGRUM. 

 a, Pigmentary granules 



concealing the nucleus; 



6, The nucleus distinct. 



4. The Iris. (Figs. 383/; 386, 6.) 



The iris forms in the interior of the eye, at the anterior opening of the 

 sclerotic, and in front of the crystalline lens, a veritable diaphragm pierced 

 with a central opening — the pupil — which contracts or dilates according 

 to the intensity of the light and the distance of the objects to which the 

 vision is directed. This diaphragm divides the space between the cornea 

 and the anterior face of the lens and internal extremities of the ciliary 

 processes, into two compartments or chambers of unequal size : the anterior 

 being the largest, and the posterior having only a virtual existence, the iris 

 being close to the crystalline lens. 



In shape, the iris is elliptical, like the cornea and the sclerotic aperture. 



Its anterior face is flat or very slightly convex, and has very marked 

 circular furrows and radiating striae, noticeable only at the outer cir- 

 cumference of the membrane. It is diversely coloured, not only according 

 to species, but also in individuals. In Solipeds, it has nearly always a 

 brownish-yellow hue ; though sometimes it is nearly white or bright grey, 

 when the animal is said to be " wall-eyed." 



The posterior face, in relation with the lens and ciliary processes, is 



