DISSECTION OF 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 balls of 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 

 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. 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 dis- 

 position, and terminate in those of the choroid. 



In the foetus the pupil is closed by a vascular transparent mem- 

 brane — the membrana papillaris^ 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 



