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 (Plicxe  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 retine. 
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- 
Fic. 696.—VascuLar Tunic or Eyenart or Horse; Front tum pectinatum iridis close to the 
Vinw. corneo-scleral junction, and run 
The cornea is removed and the sclera is reflected in flaps. backward along the sclera to be 
1, Sclera; 1’, lamina fusca; 2, chorioidea; 2’, ciliary veins; 3, : : “ys 
Gigi ronecla; Ge insta ee eraulasinidis: Gupupilthrough inserted into the ciliary processes 
which the lens is visible. (After Ellenberger, in Leisering’ss and ring. When the muscle con- 
etles.) 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 chiefly 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 plexiform 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 eorneo-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 
