DISSECTION OP THE EYEBALL. 269 



outline according to its size. When much contracted, the pupil is a 

 very elongated ellipse, the long axis of which is in the line joining 

 the nasal and temporal canthi of 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 de- 

 pendent~Mllsof~the uvea, 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 pig- 

 mented. 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 pupil, 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 contracted ; 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 Epithelium, or Uvea, comprises several layers of cells 

 similariy 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. Occasionally in the horse and dog also it is white 

 owing to the absence of pigment, 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 membrana pupillaris, 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 

 oircumferent 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 



