The =vascular tunic= (Tunica vasculosa oculi) lies internal to the
fibrous coat; it comprises three parts—the chorioid, the ciliary body,
and the iris.
1. The =chorioid= (Chorioidea) is a thin membrane which lies between the
sclera and retina. It is in general rather loosely attached to the
sclera by the lamina fusca, but is intimately adherent at the point of
entrance of the optic nerve and less closely in places where the ciliary
vessels and nerves pass through. The inner surface is in contact with
the layer of pigmented cells of the retina, which adhere so closely to
the chorioid that they were formerly regarded as a part of the latter.
The general color of the chorioid is dark brown, but an extensive
semilunar area a little above the level of the optic papilla has a
remarkable metallic luster, and is termed the =tapetum= of the chorioid
(Tapetum chorioideæ). The appearance here varies in different
individuals, but the prevailing colors in most cases are iridescent blue
and green in various nuances shading into yellow. Posteriorly the
chorioid is perforated by the optic nerve, and anteriorly it is
continuous with the ciliary body.
The chorioid consists of four layers, which from without inward are as
follows: (1) The =lamina suprachorioidea= consists of interlacing fine
lamellæ of fibrous tissue, each containing a network of elastic tissue.
Among these are large-branched, pigmented, connective-tissue cells. The
spaces between the lamellæ are lined with endothelium, and form a system
of lymph-clefts which together form the perichorioid space (Spatium
perichorioideale). (2) The =lamina vasculosa= is the outer part of the
proper tissue of the chorioid. It contains the larger blood-vessels,
which are supported by connective-tissue and elastic fibers. (3) The
=lamina choriocapillaris= consists of an extremely rich network of
capillaries embedded in an almost homogeneous matrix. Between it and the
lamina vasculosa is a layer of fibro-elastic tissue, the =tapetum
fibrosum=, which causes the metallic luster mentioned above. (4) The
=lamina basalis= is very thin and transparent. It is composed of an
inner homogeneous part and an outer elastic part.
2. The =ciliary body= (Corpus ciliare), the middle part of the vascular
coat, connects the chorioid with the periphery of the iris. In
meridional section it has the form of a narrow triangle, the base of
which is next to the iris. On its inner side are the ciliary processes
and on its outer side the ciliary muscle. It consists of three parts—the
ciliary ring, ciliary processes, and ciliary muscle. The =ciliary ring=
(Orbicularis ciliaris) is the posterior zone, which is distinguished
from the chorioid mainly by its greater thickness and the absence of the
choriocapillaris. 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 furnish
attachment to the zonula ciliaris or suspensory ligament of the latter.
They are small at their origin on the ciliary ring and become much
thicker and higher toward their inner ends. The width of the circle
formed by them is narrower at the inner side than elsewhere. Their bases
extend forward to the periphery of the iris, and their inner ends are
close to the margin of the lens. They bear numerous secondary folds.
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 retinæ. They consist of a rich network of
tortuous vessels supported in pigmented connective tissue. The =ciliary
muscle= (M. ciliaris) (Figs. 552, 555, 559) 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 ligamentum 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
contracts, it pulls the processes and ring forward, thus slackening the
ciliary zone or suspensory ligament of the lens, and allowing the latter
to become more convex. This is the mechanism of accommodation for near
objects.
[Illustration: