262 THE ANATOMY OF THE HORSE. 



the eyelids; but when it is extremely dilated, the ellipse approaches the 

 circular in form. Appearing at the upper margin of the pupil, there are 

 generally two or three little sooty masses termed the corpora nigra. 

 These are little dependent balls of the iivea, or pigmentary layer covering 

 the back of the iris. 



Structure. — This comprises a connective-tissue stroma, muscular tissue, 

 and an anterior and a posterior epithelium. 



The Stroma is a framework of connective-tissue, the fibres having a 

 radial arrangement, and the corpuscles being branched and pigmented. 

 The pigment varies in shade from yellow to dark brown or almost black. 

 The Muscular Tissue is of the non-striated variety, and its fibres are 

 arranged in two sets, viz., (1) the sphincter of the pupil, a narrow band 

 around the pupil, and close to the posterior surface of the curtain ; (2) 

 the dilator of the 2iupil, whose fibres begin at the attached edge of the 

 curtain, and extend radially inwards to end in the sphincter. The size 

 of the pupil is regulated by the state of contraction of these two muscles. 

 When the action of the sphincter preponderates, the aperture is con- 

 tracted ; when that of the dilator preponderates, the pupil is dilated. 



The Anterior Epithelium is continuous at the attached edge of the 

 iris with the posterior epithelium of the cornea. It is a single layer of 

 pigmented cells. 



The Posterior Ejnthelium, or Uvea, comprises several layers of cells 

 similarly pigmented ; and, as before stated, the corpora nigra are small 

 dependent portions of it. In the eyes of albinos the iris is devoid of 

 pigment ; and occasionally in the horse and dog the pigment is only 

 present in spots, and the animal is then said to be " wall-eyed." 



Vessels. — The arteries of the iris are derived from the ciliary branches 

 of the ophthalmic. They form at the circumference of the iris a larger 

 circle, from which radial vessels pass inwards and form around the 

 pupil a smaller circle. The veins have a similar disposition, and termin- 

 ate in those of the choroid. 



In the foetus the pupil is closed by a vascular transparent membrane 

 — the memlnrina 2iupillaris, which disappears before birth. 



The Ciliary Muscle. This is a zone of non-striated muscular tissue 

 which forms the outer layer of the ciliary body, and lies behind the 

 circumferent edge of the iris. It consists (1) of an outer radiating 

 set of fibres, which arise from the inner surface of the sclerotic close 

 behind its line of junction with the cornea, and pass backwards to 

 be inserted into the choroid and ciliary processes ; and (2) of an inner 

 circular set, which surround the rim of the iris. When the radiating 

 fibres contract, they pull forward the choroid coat and ciliary pro- 

 cesses, and allow the lens to bulge forwards by slackening its tense 

 suspensory ligament. This is the mechanism by which the eye is 

 accommodated for near objects. 



