866 



THE SENSE ORGANS AND SKIN OF THE HORSE 



ring (Orbiculus ciliaris) is the posterior zone, which is distinguished from the cho- 

 rioid mainly by its greater thickness and the absence of the chorio-capillaris. Its 

 inner face presents numerous fine meridional ridges, by the union of which the 

 ciliary processes are formed. The ciliary processes (Processus ciliares), more than 

 a hundred in number, form a circle of radial folds which surround the lens and fur- 

 nish attachment to the zonula ciliaris (or suspensory ligament of the lens). They 

 are small at their origin on the ciliary ring and become much thicker and higher 

 toward their central ends. The width of the circle formed by them is narrower 

 at the medial side than elsewhere. Their bases extend forward to the peri- 

 phery of the iris, and their central 

 ends are close to the margin of 

 the lens. They bear numerous 

 secondary folds (Plicae ciliares). 

 Their inner surface is covered by a 

 continuation of the lamina basalis 

 of the chorioid, on which there are 

 two layers of epithelial cells which 

 constitute the pars ciliaris retinae. 

 They consist of a rich network of 

 tortuous vessels supported in pig- 

 mented connective tissue. The cil- 

 iary muscle (M. ciliaris) (Figs. 689, 

 692, 696) constitutes the outer part 

 of the ciliary body, and lies between 

 the sclera and the ciliary processes. 

 It forms a circular band of unstriped 

 muscle, the fibers of which are for 

 the most part directed meridionally. 

 They arise from the inner surface of 

 the sclera and from the ligamen- 

 tum pectinatum iridis close to the 

 corneo-scleral junction, and run 

 backward along the sclera to be 

 inserted into the ciliary processes 

 and ring. When the muscle con- 

 tracts, it pulls the processes and 

 ring forward, thus slackening the 

 ciliary zone of the lens, and allowing the latter to become more convex. This is 

 the mechanism of accommodation for near objects. 



In man the muscle has the form of a prismatic ring which is triangular in meridional section, 

 the base being directed toward the periphery of the iris. It consists chieiiy of meridional fibers, 

 but a ring of circular fibers forms the inner angle of its base. In the horse the muscle is much 

 less developed, and has the form of a flat band; it does not contain distinctly circular fibers, but 

 the arrangement is rendered more or less plexif orm by the existence of oblique and equatorial fibers. 



3. The iris (Figs. 689, 694, 696) is a muscular diaphragm placed in front of the 

 lens, and is visible through the cornea. It is pierced centrally by an elliptical 

 opening, the pupil (Pupilla), which varies in size during life and determines the 

 amount of light admitted to act on the retina. In strong light the vertical diameter 

 of the pupil is very short-, but the opening is almost circular when the pupil is fully 

 dilated. The ciliary border (Margo ciliaris) is continuous with the ciliary body 

 and is connected with the corneo-scleral junction by strands of connective tissue 

 which constitute the ligamentum pectinatum iridis. The bundles of the ligament 

 interlace and enclose spaces (Spatia anguli iridis) which are lined with endothelium 

 and communicate with the anterior chamber. The pupillary border (Margo 



Fig. 696. — Vascul.^r Tunic of Eyeball of Horse; Front 



^'lEW. 



The cornea is removed and the sclera is reflected in flaps. 

 J, Sclera; 1', lamina fusca; ^, chorioidea; S', ciliary veins; 3, 

 ciliary muscle; 4< iris; 5, -5', granula iridis; 6, pupil, through 

 which the lens is visible. (After EUenberger, in Leisering's 

 Atlas.) 



