The cornea is removed and the sclera is reflected in flaps. _1_,
Sclera; _1′_, lamina fusca; _2_, chorioidea; _2′_, ciliary veins;
_3_, ciliary muscle; _4_, iris; _5_, _5′_, granula iridis; _6_,
pupil, through which the lens is visible. (After Ellenberger, in
Leisering’s Atlas.)
]
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 circular fibers, but the arrangement is rendered more
or less plexiform by the existence of oblique and equatorial fibers.
3. The =iris= (Figs. 552, 557, 559) 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) or periphery of the iris
is continuous with the ciliary body and is connected with the
corneo-scleral junction by the ligamentum pectinatum iridis. The
ligament consists of strands of connective tissue which are attached
externally to the corneo-scleral junction. Its bundles interlace and
inclose spaces (Spatia anguli iridis) which are lined with endothelium
and communicate with the anterior chamber. The =pupillary border= (Margo
pupillaris) surrounds the pupil. Its upper part bears in its middle
several black masses of variable size, termed the =granula iridis= or
corpora nigra; similar, but much smaller, projections may be seen on the
lower margin of the pupil. The =anterior surface= (Facies anterior)
determines the color of the eye, which is dark brown usually. It is
marked by delicate concentric lines (Plicæ iridis), which fade out near
the pupil. The smooth, narrow, central part is termed the pupillary zone
or annulus iridis minor, while the much broader plicated part is the
ciliary zone or annulus iridis major. The =posterior surface= (Facies
posterior) is usually black. It bears numerous fine radial lines except
at the pupillary margin. Its central part is in contact with the
anterior surface of the lens, but peripherally the two are separated by
a narrow space termed the posterior chamber. The iris consists chiefly
of the =stroma iridis=, a delicate framework of connective tissue, which
supports numerous blood-vessels, and contains branched pigmented cells.
The muscular tissue (unstriped) consists of a sphincter and a dilator of
the pupil. The =sphincter pupillæ= lies in the posterior part around the
pupil, with which the fibers are largely concentric. The =dilatator
pupillæ= consists of fibers which radiate outward from the sphincter to
the ciliary border. The anterior surface of the iris is covered by a
continuation of the endothelium of the cornea. Beneath this is a
condensation of the stroma, in which the cells are close together and
are full of pigmented granules.[219] There appear to be minute clefts
here by which the lymph-spaces of the stroma communicate with the
anterior chamber. The posterior surface is covered by a pigmented
epithelium, the stratum pigmenti iridis, which is a continuation of that
of the ciliary body.
The =arteries= of the vascular tunic come from the ciliary branches of
the ophthalmic artery. The arteries of the chorioidea are derived
chiefly from the short posterior ciliary arteries. These perforate the
sclera around the posterior pole, run forward in the lamina vasculosa,
and form the rich capillary network of the choriocapillaris. The two
long ciliary arteries perforate the sclera obliquely near the optic
nerve; they run forward in the lamina suprachorioidea in the horizontal
meridian, one on the inner, the other on the outer, side of the eyeball.
On reaching the ciliary body each divides into diverging branches; the
subdivisions of these unite with each other and with twigs of the
anterior ciliary arteries to form a circular anastomosis, the =circulus
arteriosus major=. From this branches go to the ciliary muscle and
processes and to the iris. The two anterior ciliary arteries, dorsal and
ventral, form an episcleral plexus around the corneo-scleral junction,
and give off branches which perforate the sclera. These supply twigs to
the ciliary muscle and recurrent branches to the chorioid, and assist in
forming the circulus arteriosus major.
The blood is carried away from the vascular tunic chiefly by four or
five venous trunks, the =venæ vorticosæ=, which are formed by the
convergence in whorls of numerous veins, coming not only from the
chorioid, but also from the ciliary body and iris. The venæ vorticosæ
perforate the sclera about at the equator and join the veins of the
ocular muscles.
The =nerves= come from the long and short ciliary nerves. They form a
plexus in the lamina suprachorioidea, which contains ganglion cells, and
sends numerous non-medullated fibers chiefly to the blood-vessels of the
chorioid. At the ciliary muscle a second plexus (P. gangliosus ciliaris)
is formed, which supplies the muscle and sends fibers to the iris. The
sphincter pupillæ is supplied by fibers derived from the oculomotor
nerve, while the dilatator pupillæ is innervated by the sympathetic.