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its equator, while its posterior two-thirds are enveloped by a loose membrane termed the fascia bulbi (O.T. capsule of Tenon).
The bulb of the eye is not quite spherical, being composed of the segments of two spheres, viz., an anterior, transparent, corneal segment, possessing a radius of 7 or 8 mm., and a posterior, opaque, scleral segment, with a radius of about 12 mm. (Fig. 677). The anterior or corneal segment, in consequence of its shorter radius, projects forwards, in front of the scleral portion, the union of the two parts being indicated, externally, by a slight groove, the sulcus scleræ. The central points of the anterior and posterior curved surfaces of the bulb constitute, respectively, its anterior and posterior poles, and a straight line joining the two poles is termed the optic axis; an imaginary line encircling the bulb, midway between the poles, is named the equator. The axes of the two bulbs are almost parallel, diverging only slightly in front; but the axes of the optic nerves converge behind, and, if prolonged backwards, would meet in the region of the dorsum sellæ of the sphenoid. The sagittal and transverse diameters of the bulb are nearly equal-about 24 mm.; its vertical diameter is about 23.5 mm. All three diameters are rather less in the female than in the male, but the size of the bulb is fairly constant in the same sex. What are popularly described as large eyes owe their apparent size to a greater prominence of the bulb and to a wider fissure between the eyelids.
At birth the bulb of the eye is nearly spherical and has a diameter of about 17.5 mm. By the age of puberty this has increased to 20 or 21 mm., after which it rapidly reaches its adult size.
Fascia Bulbi.-The fascia bulbi (O.T. capsule of Tenon) is a fibrous tunic enveloping the posterior two-thirds of the bulb of the eye, and separating the posterior part of the bulb from the surrounding orbital fat. It blends posteriorly with the sheath of the optic nerve and with the sclera around the lamina cribrosa; anteriorly it is continued into the ocular conjunctiva, and is also attached to the ciliary region of the bulb. It is pierced by the tendons of the ocular muscles, and is reflected on each as a tubular sheath. The sheath on the tendon of the obliquus superior surrounds the tendon as far as its pulley, to which it is attached; that on the obliquus inferior is prolonged as far as the floor of the orbit. The sheaths on the recti muscles are continuous posteriorly with the perimysium of those muscles, and each gives off an expansion. The expansion from the sheath of the rectus superior blends with the sheath of the levator palpebræ superioris, and that from the sheath of the rectus inferior is attached to the tarsus of the inferior eyelid. The expansions from the sheaths of the medial and lateral recti are strong, especially that from the latter muscle, and are attached to the lacrimal and zygomatic bones respectively; they are named the medial and lateral check ligaments, because they probably limit the action of the corresponding muscles. The portion of the fascia bulbi which lies inferior to the bulb of the eye has been named the suspensory ligament (Lockwood); it is expanded in the centre, and is slung like a hammock from side to side, its narrow ends being fixed to the lacrimal and zygomatic bones.
The bulb of the eye (Fig. 677) consists of three concentric tunics or coats, and contains three transparent refracting media. The three tunics are: (1) an outer fibrous tunic, consisting of an opaque posterior part, the sclera, and a transparent anterior portion, the cornea; (2) an intermediate vascular, pigmented, and partly muscular tunic, the tunica vasculosa oculi, comprising, from behind forwards, the chorioid, the ciliary body, and the iris; (3) an internal nervous tunic, the retina. The three refracting media are named, from before backwards, the aqueous humour, the crystalline lens, and the vitreous body.
TUNICA FIBROSA OCULI.
Sclera. The sclera is a firm, opaque membrane, forming approximately the posterior five-sixths of the outer tunic. Thickest posteriorly (about 1 mm.), it thins at the equator to 04 or 0.5 mm., and again increases to 0.6 mm. near the sulcus
scleræ. It is thinner in the child than in the adult, and presents a bluish appearance caused by the pigment of the chorioid shining through it; in old age it assumes a yellowish tinge. In front of the equator it gives attachment to the tendons of the ocular muscles, while its anterior part is covered by the conjunctiva. Its deep surface presents a brownish colour, and is loosely attached to the chorioid, except at the entrance of the optic nerve and in the neighbourhood of the sulcus scleræ. It is pierced, behind, by the optic nerve, the entrance for which is funnel-shaped, wide behind and narrow in front, and is situated 3 mm. to the nasal side and slightly below the level of the posterior pole. The fibrous sheath of the nerve blends with the outer part of the sclera, while the nerve bundles pass through a series of orifices; this perforated portion is named the lamina cribrosa scleræ. Around the entrance of the optic nerve are some fifteen to twenty small apertures for the passage of the ciliary nerves and short ciliary arteries. The two long posterior ciliary arteries pierce it, one on each side, some little distance from the entrance of the optic nerve; while a little behind the equator are four openings, two above and two below, for the exit of veins, called venæ vorticose; near the sulcus scleræ it is perforated by the anterior ciliary arteries. The deep surface of the sclera is lined
with flattened endothelial cells; and between it and the chorioid is an extensive lymph space, the spatium perichorioideale, which is traversed by the ciliary nerves and arteries just mentioned, and by an irregular meshwork of fine, pigmented, connective tissue, the lamina fusca, which loosely attaches the sclera to the chorioid. At the sclero-corneal junction the fibrous tissue of the sclera passes continuously into that of the cornea, and
Circular fibres of ciliary muscle
in the deeper part of this Parts of ciliary processes junction there is a circular canal, the sinus venosus FIG. 678.-SECTION OF IRIDIAL ANGLE. (Prof. Arthur Thomson.) sclera (O.T. canal of Schlemm) (Fig. 678) When seen in a meridional section of the sclero-corneal junction, the sinus venosus scleræ appears as a narrow cleft; its outer wall is formed by the compact tissue of the sclera, while its inner consists of a triangular mass of trabecular tissue; the apex of the triangle is directed forwards and is continuous with the posterior elastic lamina of the cornea. The sinus is lined with endothelium, and occasionally contains a few red blood corpuscles. It communicates, on the one hand, with the anterior ciliary veins, and on the other, through the spatia anguli iridis in the trabecular tissue, with the anterior chamber of the eye.
Meridional fibres of ciliary muscle
Structure. The sclera consists of bundles of white fibrous tissue, together with some fine elastic fibres, the bundles forming equatorial and meridional layers, which interlace with each other. Numerous spaces containing connective tissue cells and migratory cells exist between the fibres. Pigmented cells are plentiful in the lamina fusca, and a few are found also in the tissue of the sclera, near the entrance of the optic nerve, and in the region of the sclero-corneal junction. Vascular and Nervous Supply. The sclera receives its blood-supply from the short posterior ciliary and the anterior ciliary arteries, while its veins open into the venæ vorticosa and anterior ciliary veins. The cell spaces play the part of lymph vessels and communicate with the perichorioidal and suprascleral lymph spaces. Its nerves are derived from the ciliary nerves, which, after losing their medullary sheaths, pass between the fibrous bundles; their exact mode of ending is not known.
Cornea. The cornea is transparent and forms the anterior sixth of the outer tunic; its index of refraction is from 1:33 to 1:35; the thickness of its central part
is about 95 mm., of its peripheral part, about 1.19 mm. Its anterior surface is
cornea is almost circular, FIG. 679.-SECTION OF A PORTION OF THE BULB OF THE EYE SHOWING measuring 11 mm. vertically
THE SULCUS CIRCULARIS CORNEÆ.
and 119 mm. transversely;
that of the posterior surface is circular and has a diameter of 13 mm.
The tissue of the cornea is continuous posteriorly with that of the sclera, the line of union being known as the sclero-corneal junction. Directly in front of this junction the inner surface of the cornea projects in the form of a rounded rim; behind this rim, in the interval between the sclero-corneal junction and the attachment of the iris, is a groove, the sulcus circularis cornea (Arthur Thomson)1 (Fig. 679). The outer wall of this sulcus is composed of a thin stratum of trabecular tissue placed on the inner side of the sinus venosus scleræ.
Between this trabecular tissue and the front of the circumference of the iris is a narrow recess which on section appears as an acute angle; it is named the filtration angle or angle of the iris.
seen to be
FIG. 680.-VERTICAL SECTION OF CORNEA (magnified).
1. The epithelium corneæ is continuous with that covering the free surface of the conjunctiva and consists of six or eight strata of nucleated cells. Deepest of all is a single layer of perpendicularly arranged columnar cells, the flattened bases of which rest on the anterior elastic lamina, while their opposite ends are rounded and contain the nuclei. Superficial to this layer are three or four strata of polygonal cells, the majority of which exhibit finger-like processes joining with the corresponding processes of neighbouring cells; the more superficial layers consist of squamous cells. The thickness of this stratified epithelium is about 45 u at the centre, and about 80 μ at the periphery of the cornea.
1 The Ophthalmoscope, September 1910, and July 1911.
Structure. The cornea consists, from before backwards, of the following strata, viz., (Fig. 680):
1. Epithelium corneæ.
2. Anterior elastic lamina.
3. Substantia propria.
4. Posterior elastic lamina.
2. The anterior elastic lamina is from 19-20 μ thick, and is regarded merely as a differentiation of the anterior part of the substantia propria, from which it is with difficulty separated; it is not stained yellow by picrocarmine, thus differing from true elastic tissue. Its degree of development varies in different animals.
3. The substantia propria presents, in a fresh condition, a homogeneous appearance; but, with the assistance of reagents, it is seen to consist of modified connective tissue, with a few elastic fibres. An amorphous interstitial substance binds the fibres into bundles, and, in turn, cements the bundles into lamella which are flattened from before backwards. The fibres of any one lamella cross those of adjacent lamellæ almost at right angles, while the superimposed lamella are joined by sutural fibres and by amorphous substance. Between the lamellæ are found the cell spaces or lacunæ of the cornea-irregularly stellate in shape, and communicating freely with each other by means of fine canaliculi. The corneal cells or corpuscles are contained in these lacunæ, without, however, completely filling them, the remainder of the cavities being occupied by lymph. The cells are nucleated, flattened, and star-like, and their branched processes join those of neighbouring cells in the canaliculi. Migratory or lymph cells are also found in cell spaces.
After middle age a grayish opaque ring, 1.5 to 2 mm. in breadth, is frequently seen near the periphery of the cornea; it is termed the arcus senilis, and results from a deposit of fat granules in the lamellæ and corneal corpuscles.
4. The posterior elastic lamina is a clear homogeneous membrane, covering the posterior surface of the substantia propria and possessing a thickness of 6-8 u at the centre and 10-12 μ at the periphery of the cornea. Less firmly attached than the anterior elastic lamina, it may be stripped off, when it will be found to roll up with its attached surface inwards. Between the ages of twenty and thirty years small wart-like projections appear on its deep surface, near its periphery, and these increase in size and number as years advance, so that in old age the membrane may attain a thickness of 20 μ At the sclero-corneal junction the posterior elastic lamina splits into bundles of fine fibres which interlace and form the triangular area of trabecular tissue already referred to (p. 808), and which is usually spoken of under the name of the ligamentum pectinatum iridis. The meshes or spaces between the trabecula are termed the spatia anguli iridis (O.T. spaces of Fontana), and are lined with endothelium prolonged from the endothelium of the anterior chamber. They communicate internally with the filtration angle and externally with the sinus venosus sclera, and form important channels through which fluid may filter from the anterior chamber into the sinus and thence into the anterior ciliary veins. When the trabecular tissue of the ligamentum pectinatum iridis is followed backwards most of its fibres are seen to be attached to the anterior surface of an inwardly directed rim of scleral tissue; in a meridional section this rim appears as a triangular projection, and is named the scleral spur. A few fibres of the trabecular tissue are carried past the apex of the scleral spur on to the inner surface of the origin of the meridional fibres of the ciliary muscle, and, passing behind the filtration angle, are prolonged into the iris (Fig. 678), where they are directly continuous with the fibres of the dilatator pupillæ muscle (Arthur Thomson).1
5. The endothelium of the anterior chamber consists of a single stratum of nucleated, flattened, polygonal cells, which present a fibrillar structure and are continued as a lining to the spatia anguli iridis; this layer of endothelium is also reflected on to the anterior surface of the iris.
Vascular and Nervous Supply of the Cornea. In the foetus the cornea is traversed, almost as far as its centre, by capillaries; but in the adult it is devoid of blood-vessels, except near its margin. The capillaries of the conjunctiva and sclera pass into this marginal area for a distance of about 1 mm., where they terminate in loops. All the remainder of the cornea is nourished by the lymph which circulates in its cell spaces and canaliculi.
The nerves of the cornea are derived from the ciliary nerves. Around its periphery they form an annular plexus, from which fibres pass into the cornea, where, after a distance of 1 or 2 mm., they lose their medullary sheaths and ramify in the substantia propria, forming what is termed the fundamental or stroma plexus. Fibres extend from this plexus through the anterior elastic lamina and form a subepithelial plexus, from which fine filaments ramify between the epithelial cells as far as the superficial layers. From the annular and stroma plexuses fibrils pass to the substantia propria and come into close relation with the corneal corpuscles.
TUNICA VASCULOSA OCULI.
The middle, vascular, and pigmented tunic of the bulb of the eye comprises, from behind forwards, the chorioid, the ciliary body, and the iris (Fig. 681). The chorioid intervenes between the sclera and the retina, reaching as far forwards as the ora serrata of the retina (p. 815). It is dark brown or black in colour, and is thicker behind than in front; posteriorly it is pierced by the optic nerve, and is there firmly attached to the sclera. Its outer surface is flocculent and is connected to the sclera by the loose lamina fusca; its inner surface is smooth and is adherent to the outermost or pigmented layer of the retina.
The chorioid consists of a loose connective tissue, embedded in which are bloodvessels and branched pigment cells; from without inwards it consists of three
layers, viz.: (a) the lamina suprachorioidea; (b) the proper tissue of the chorioid; and (c) the lamina basalis (Fig. 681).
Vessels of iris
Vessels of ciliary process
Capillaries of chorioid
FIG. 681.-VERTICAL SECTION OF CHORIOID AND INNER PART OF SCLERA.
The lamina suprachorioidea resembles the lamina fusca of the sclera, and consists of a series of fine non-vascular lamellæ, each containing a delicate network of elastic fibres, amongst which are stellate, pigmented cells and amoeboid cells. The spaces between the lamina are lined with endothelium, and together form the spatium perichorioideale, already referred to (p. 808).
The proper tissue of the chorioid consists of blood vessels and numerous pigmented cells, supported by connective tissue, elastic fibres, and some nonstriped muscular fibres. Its outer part contains the larger blood-vessels, and is named the lamina its vasculosa, while inner portion is composed of a network of fine capillaries, and is termed the lamina choriocapillaris; these two laminæ are joined by a thin intermediate stratum. The arteries of the chorioid are derived from the short posterior ciliary vessels which pierce the sclera around the entrance of the optic nerve, and form a wide-meshed plexus in the lamina vasculosa. The circular muscular coats of the arteries are well developed, and longitudinal muscular fibres also are present in the larger branches. The veins,
Sinus venosus sclera
Long posterior ciliary artery
Short posterior ciliary artery
Linner vessels of optic sheath
-Central artery and vein of retina
FIG. 682.-DIAGRAM OF THE CIRCULATION IN THE EYE (Leber).