THE SENSES. 267 



fleeted at the capsule of the lens over and upon the posterior surface of the 

 iris, to terminate at the margin of the pupil. 



The pigment is so much the darker, the farther it lies internally ; it is some- 

 times entirely wanting (in Albinos); it sometimes presents deposits of salts 

 (the so-called ossifications). In ruminants, also the dog and cat, remarkably 

 brilliant, it is called tapetum. 



500. 4. The Iris, 



a thin disc-shaped membrane, becoming thicker from without inwards, con- 

 sisting of contractile tissue, forms a vertical septum, perforated by a foramen, 

 between the anterior and posterior chambers of the eye. The foramen (pupilla) 

 is not placed quite in the centre, but rather to the inner side, is bounded by 

 the internal or free border of the Iris, is dilated or contracted according as the 

 last contracts or relaxes, and is closed in the foetus by the menibrana pupillaris. 

 The external, larger (ciliary) border of the Iris is loosely attached to the lig. 

 ciliare which on the anterior surface, and to the proc. ciliares, which on the 

 posterior surface project rather over it. The anterior surface of the Iris is 

 flat, speckled (blue, brown, &c.), striated, and forms the posterior wall of the 

 anterior chamber of the eye. Towards the pupillary margin the colour is 

 deeper than in the remaining two-thirds; we therefore distinguish a zona 

 iridis minor and major. The stria? pass from the external towards the inner 

 margin, converge and cross each other ; they are in the contracted pupil 

 straight, in the dilated wavy, like the vessels through which they arise. 



The posterior surface is covered with a thick layer of black pigment, on the 

 ciliary border by the proc. ciliares, and folded ; denuded of pigment, it is white 

 and smooth, like the Choroid, which, besides, is far thinner than the Iris. 



Structure. The Iris probably consists of contractile uniting tissue, accord- 

 ing to the older, now also the opinion supported by Valentin, of simple mus- 

 cular fibres, which pass like a sphincter on the pupillary margin. (In the ox 

 there are distinct circular fibres.) 



Vessels and Nerves : Art. ciliares anticce andposticce longcs (see art. ophthal.) 

 form a circulus arteriosus iridis major on the ciliary border, from which arises 

 a circ. art. irid. minor, on the pupillary margin. 



Veins: like the arteries; some open into the canalis Fontana. 



Nerves: N. ciliares, from ganglion ciliare, very numerous, pass through 

 lig. ciliare into the Iris. 



There was formerly distinguished an anterior lamina, the proper Iris, and 

 a posterior, uvea. 



c. The third layer of the coats of the globe of the eye. 



501. The Retina s. tunica nervea the nervous coat, 



a soft, yellowish white membrane, which is placed concentrically on the 

 internal surface of the Choroidea, covered with pigment, consists of several 

 layers, the most essential of which is a smooth expansion of the Optic nerve. 



