CORNEA (Plate II)

The cornea or horny body is the anterior part of the external tunic of the eye. It is a completely transparent circular structure made up of fibrous tissue arranged in layers and held together by a cement substance. The cornea forms the anterior 1/6 of the globe of the eye

Stratified Epithelium of Conjunctiva Membrane of Bownian

Substantia Propria

Membrane of Descemet Single Layer of Squamous Epithelium lining Descemèt's Membrane

Fig. 9. Section of Human Cornea Showing Different Layers.

and is practically the continuation forward of the selera, the point externally where the two parts meet being called the sclero-corneal junction. However, the selera overlaps somewhat, a faet which has suggested the comparison of the relation of the cornea to the sclera, with the watch glass on the face of the watch. Two surfaces are recognized-the external which is covered by epithelial tissue disposed in layers, and part of the posterior layer of the conjunctiva, and the internal surface which forms the anterior boundry of the anterior chamber of the eye containing the aqueous humor with which the posterior surface of the cornea is in contact. The cornea is convex on its external surface and concave on its posterior or internal surface. Its radius of curvature is somewhat less than that of the selera but this varies according to age, being slightly greater in youth than in advanced life. The transverse diameter average one-half inch (12mm.) is greater than the vertical 3% of an inch (10 mm.). Internally the cornea has a

diameter of 12 inch (12.5 mm.). The thickness of the cornea also varies, being thinnest in its center, 1/27 of an inch (0.95 mm.) and thickest at the edges, 1/20 of an inch (1.10 mm.). The difference between the internal and external diameters is due to the prolongation forward of the external fibers of the sclera over the external surface of the cornea.

The various layers of the cornea (Fig. 9.) are composed of five recognized strata which from before backward are divided into

1. Epithelium cornea continuous with the conjunctiva.

2.

Anterior elastic or Bowman's Membrane. 3. Substantia propria or cornea proper forming the greatest and thickest part of the cornea. 4. Posterior elastic layer Descemets or Demours membrane.

5 Endothelium or lining layer of flat cells. (1) The first layer consists of layers of cells usually about five deep and of various forms. These are columnar, pillar-like, polyhedral, having many faces or sides, and squamous, scaly or plate-like cells. The cells attached to the second or anterior limiting membrane are columnar, the middle rows are polyhedral whilst those immediately below the conjunctiva are squamous.

(3) The cornea proper consists of an aggregation of cells connective and elastic in nature, similar in structure to the sclera of which this strata is really a part composed of about sixty layers. The cells are formed into fibers which are bound into bundles by a cement substance, the bundles being arranged into layers, between these layers and imbedded in them are the corneal corpuscles (Fig. 10) which are flattened nucleated cells, provided with branching processes which entwine with the processes of other like cells, thus forming a continuous network through which the nutrition of the cornea is, to a large

extent, affected. White blood corpuscles and lymph cells are also found in the cell spaces.

(4) The posterior elastic membrane is an elastic tissue connected to the posterior surface of the cornea proper. It is the toughest of the corneal layers and is quite brittle. At its circumference it breaks up into bundles of fibers, which are partly continued into the ciliary processes and partly forward into the iris. These latter fibers constitute or are called the ligamentum pectinatum iridis. (Fig. 11 and Plate II.) Certain spaces occur in this ligament which are called the spatia anguli iridis or the spaces of Fontana. These form channels through which fluid may filter from the anterior chamber into the filtration angle of the iris, the canal of Schlemm and the anterior ciliary veins.

(5) The posterior endothelial layer (endothelium camaræ anterioris) or the endothelium of

the anterior chamber, is a single layer of flat, many angled cells forming an exceedingly thin membrane which covers the adjacent elastic lamina, then extends upwards and over to the anterior surface of the iris.

Blood supply. As has been noted, the cornea is perfectly transparent, and contains no blood vessels, as the presence of the red blood corpuscles would interfere with the proper refraction of the entering rays of light, small blood vessels or capillaries are found all around its margins, in the form of a plexus formed from the two anterior ciliary arteries.

Nerve supply. This is very plentiful in the cornea from 25 to 35 small shoots from the ciliary nerves forms a complex network around the cornea and passes into its substance for about 1/25 of an inch (1 mm.). Here the nerves lose their external sheaths and ramify in the substantia propria from which still other networks of nerves are formed and are distributed in the other layers.

Function. The cornea is one of the most important elements in the dioptric system of the eye and has a refractive power equal to a plus lens of over thirty-three diopters.

Tunica Vasculosa Oculi

The vascular or middle tunic of the eye, (Fig. 12 and 13) so named on account of the great number of blood vessels contained within its layers, lies internal to the sclera; it comprises, from behind forward, the chorioid, the ciliary body, and the iris, the three together forming the uveal

tract.

Chorioidea

The chorioid is a thin membrane situated between the sclera and retina, extending from the orra serata in front to the margin of the optic disc behind, forming seven-tenths of the middle tunic. It is rather loosely attached to the sclera by the lamina fusca, but is intimately adherent at the chorioidal fissure which is at the point of entrance of the optic nerve, being attached by a ring of connective tissue fibers; the attachment is less close in places where the ciliary vessels and nerves pass through. The thickness of the chorioid varies from 1/250 to 1/150 of an inch (.1 to 2 mm.). 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 in some of the lower animals a semilunar area a little above the level of the optic papilla has a metallic luster, and is termed the tapetum chorioideæ, the appearance of which varies in different animals; the prevailing colors in most cases being iridescent blue and green, variously

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 (Fig. 8) which from without inward are as follows: (1) The lamina suprachorioidea, corresponding to the lamina fusca, it consists of an interlacing fine layer of fibrous tissue, each containing a network of elastic fibers, covered by large flat cells, it contains also large-branched, pigmented cells, irregularly distributed. The spaces between the lamellæ are lined with endothelium, and form a system of lymph-spaces between the sclera and chorioid, 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 branches of the short ciliary arteries and venæ vorticosæ,

Sinus venosus scleræ Anterior ciliary-.

ciliary processes are anterior to the ciliary muscle. The ciliary body consists of three partsthe orbiculus ciliaris, ciliary processes, and ciliary muscle. The orbicularis ciliaris, annulus ciliaris, or ciliary ring, is the posterior zone, about 3 of an inch (4 mm.) in width and is directly continuous with the anterior part of the chorioid from which it is distinguished mainly by its greater thickness and the absence of the choriocapillaris. Its inner or retinal surface presents numerous fine meridional ridges, by the uniting of which the ciliary processes are formed. The ciliary processes, (Fig. 14) processus ciliares, about seventy in number, form a circle of radial folds by the plaiting of the chorioid and are received into corresponding folds of the zonula ciliaris or suspensory ligament which surrounds the lens. They are small at their origin on the ciliary

Anterior ciliary artery

Ciliary muscle

Vena vorticosa

Long posterfor ciliary artery

Vena vorticosa

which are supported by connective-tissue and elastic fibers. (3) The lamina chorio-capillaris or Tunica Ruyschiana, the internal layer consists of an extremely rich network of capillaries formed by the short ciliary vessels embedded in a finely granular tissue. Between it and the lamina vasculosa is a network of delicate elastic fibers, the intermediate stratum which contains almost no pigment cells. (4) The lamina basalis, or membrane of Bruch, is very thin and transparent. It is composed of an outer trellis-like network of fibers which attach it to the choriocapillaris and an inner smooth part in contact with the pigment layer of the retina.

Corpus Ciliars

The ciliary body, (Fig. 11, Plate II) the middle part of the vascular coat connects the chorioid with the circumference of the iris. In meridional section it is roughly of a narrow triangular shape, the base of which is next to the iris. The

ring and become thickened and more elevated toward their inner ends, the largest having a length of about 1/25 of an inch (1 mm.). The width of the circle formed by them is narrower at the inner side than elsewhere. Their bases extend forward to the margin of the iris. They bear numerous secondary folds. Their internal surface is covered by a continuation forwards of the retina, on which there are two layers of epithelial cells constituting the pars ciliaris retina. The processes consist of a rich network of tortuous vessels supported in pigmented connective tissue, in which are found glandular folds called the ciliary glands. These are presumed to assist in the secretion of the aqueous humor. The ciliary muscle, M. ciliaris, (Fig. 11) constitutes the outer part of the ciliary body and lies between the sclera and the ciliary processes. It forms a band of unstriped muscle, the fibers of which are directed meridionally and circularly. The meridi

passing to the interior of the eye. It has two margins and an anterior and posterior surface. The ciliary margin, margo ciliaris, is continuous with the ciliary body and is connected with the posterior elastic layer of the cornea by the ligamentum pectinatum iridis. The pupillary margin, margo pupillaris, forms the boundry of the pupil. The anterior surface is marked by delicate wavy lines, the plicæ iridis, which converge 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 is usually of a dark color and bears numerous fine radial lines except at the pupillary margin.

Fig. 13.

The Iris

The iris (Figs. 14 and 16, Plate II) is a muscular curtain situated in front of the lens, and visible through the transparent cornea. It measures about 1/2 an inch (11 mm.) across, and is about 1/75 of an inch (.4 mm.) thick, centrally slightly to the nasal side is an almost circular opening called the pupil, or pupilla, which varies from 1/25 to 1/3 of an inch (1-8 mm.) in size during life and is generally wider in young than in old persons. (The pupil is generally covered dur

Fig. 15. Segment of the iris, ciliary body and choroid-view from the internal surface (Testut from Garrish.)

Its central part is in contact with the anterior surface of the lens, but the two are separated, towards the ciliary margins, 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 dilator of the pupil. The spincter pupillæ is a ring-like muscle and forms the margins of the pupil, 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