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(b) The horizontal cells are of two varieties: (1) small, flattened, star-like cells, lying immediately internal to the outer molecular layer, and sending a tuft of dendrites outwards, towards the bases of the cone fibres, while their axons are directed horizontally, for a variable distance; (2) large, irregular cells, lying internal to the above and ending in finger-like ramifications in the outer molecular layer. Their axons run horizontally for some distance, and end, in extensive varicose arborisations, under the spherules of the rod fibres.
(c) The spongioblasts are situated in the innermost part of the inner nuclear layer; their processes ramify in the inner molecular layer, it may be in one stratum (stratified spongioblasts) or in several strata (diffuse spongioblasts).
5. Outer molecular or outer plexiform layer. This is constituted by the interlacement of the dendrites of the bipolar and horizontal cells, just described, with the spherules of the rod fibres and the ramifications of the foot-plates of the cone fibres. It is divided into two strata: (a) external, indicating the contact of the rod bipolars with the spherules of the rod fibres; (b) internal, the line of contact between the cone bipolars and the branches of the cone fibres.
6. Outer nuclear layer or layer of outer granules.-This is made up of clear granules which somewhat resemble those of the inner nuclear layer, and are divisible into two kinds: (a) cone granules, (b) rod granules. The cone granules are the larger, and each contains an oval nucleus; they lie immediately inside the outer limiting membrane, through which they are continuous with the cones of the next layer. Each is prolonged internally as a straight fibre, which, on reaching the outer molecular layer, expands to form a footplate, from which several horizontal fibrils are given off. The rod granules are far more numerous than the cone granules, and each contains a small oval nucleus, which is transversely striated. Their outer processes are continuous, through the outer limiting membrane, with the rods of the next layer, while their inner processes pass into the outer molecular layer and end in free, unbranched spherules amongst the arborisations of the rod bipolars.
7. Layer of rods and cones.-This consists of two sets of structures, viz., rods and cones. Except at the macula lutea the rods are far more numerous than the cones, and assume the form of elongated cylinders, while the cones are shorter than the rods, and taper externally to fine points. Each rod and cone consists of two segments-inner and outer. The inner segment of the rod only slightly exceeds in diameter its outer segment, whereas the inner segment of the cone greatly exceeds its outer part. The inner segments of both rods and cones have an affinity for staining reagents, and consist of a basal homogeneous portion and an outer longitudinally striated part, the proportion of the latter to the former being greater in the cones than in the rods. The outer segments have not the same affinity for reagents, but tend to break transversely into numerous discs (Fig. 689, B). The colouring matter, rhodopsin, already referred to, is found only in the outer segments of the rods, the terminal parts of which extend into the layer of pigmented epithelium.
8. Stratum pigmenti.-This consists of a single stratum of cells which, on surface view, are hexagonal (Fig. 690), their outer flattened surfaces being firmly attached to the chorioid. When seen in profile the outer part of each cell contains
a large oval nucleus and is devoid of
FIG. 690. STRATUM
PIGMENTI OF HUMAN
RETINA (viewed from
Sustentacular fibres of the retina. These form a framework for the support of the nervous structures (Fig. 688, M). They begin at the inner surface of the nerve-fibre layer in single or forked expanded bases, by the apposition of which a delicate membrane, the membrana limitans interna, is formed. In the ganglionic layer they give off a few side branches, and, on passing through the inner nuclear layer, supply ramifications amongst the inner granules
for their support; in this part of each fibre there is an oval nucleus. In the outer nuclear layer they break up into a network of fibrils which surround the rod and cone granules and fibres, and end externally at the bases of the rods and cones in a delicate membrane, the membrana limitans externa.
Structure of the macula lutea and fovea centralis.-The yellow colour of the macula is due to the presence of pigment in the inner layers of the retina. At the circumference of the macula the nerve-fibre layer is greatly thinned and the rods are few in number; the ganglionic layer, on the other hand, is thickened and may contain from seven to nine strata of cells, while the outer granular layer also is thicker and its granules have an oblique direction. At the fovea centralis the retina is much thinned, since here its nerve-fibre and ganglionic layers are absent and its other strata greatly attenuated. The stratum pigmenti, on the other hand, is thicker and its pigmentation more pronounced. The cone nuclei are situated some distance internal to the outer limiting membrane, and thus the thin inner and outer granular layers are in apposition. There are no rods, and the cones, closely crowded together, are narrower and their outer segments more elongated than elsewhere, so that the line of their bases, indicated by the membrana limitans externa, presents a convexity directed forwards. The fovea centralis and macula lutea are spoken of by physiologists as the "region of distinct vision."
Structure of the ora serrata. Here the nervous layers of the retina suddenly cease; the layer of rods and cones ends a little behind the margin of the ora serrata; the other nervous strata persist as far as its margin. In front of the ora serrata the retina is prolonged over the ciliary processes in the form of two layers of cells: (a) an inner layer of columnar epithelium, and (b) an outer, consisting of the stratum pigmenti, the two forming the pars ciliaris retina. The same two layers are prolonged over the back of the iris, where both are pigmented and form the pars iridica retinæ.
Vessels of the retina (Fig. 692).—The retina is supplied by the arteria centralis retinæ, a branch of the ophthalmic artery, which pierces the sheath of the optic nerve about 2 cm. behind the bulb of the eye, and makes its appearance in the centre of the optic papilla. There it divides into an upper and a lower branch, and each of these again bifurcates into a medial or nasal, and a lateral or temporal, branch. The resulting four branches ramify towards the periphery of the retina, and are named the superior and inferior temporal
Superior nasal branch
Inferior nasal branch
-Superior temporal branch
Superior and inferior macular arteries
Inferior temporal branch
FIG. 692.-BLOOD-VESSELS OF THE RETINA.
and the superior and inferior nasal arteries. The temporal arteries pass laterally above and below the macula lutea, to which they give small branches; these do not, however, extend as far as the fovea centralis, which is devoid of blood-vessels. The macula also receives two small arteries (superior and inferior macular) directly from the stem of the arteria centralis. The larger vessels run in the nerve-fibre layer near the membrana limitans interna and form two capillary networks-an inner, in the nerve-fibre layer, and an outer, in the inner nuclear layer. The inner network arises directly from the arteries and sends numerous small branches to form the outer network, from which the veins take origin. The vessels do not penetrate deeper than the inner granular layer, nor do the arteries anastomose, except through the capillary plexuses. The veins follow the course of the arteries; they have no muscular coats, but consist merely of a layer of endothelial cells, outside which is a perivascular lymph sheath, surrounded by delicate retiform tissue.
Corpus Vitreum. The vitreous body is a transparent, jelly-like substance situated between the crystalline lens and the retina, and occupying the posterior four-fifths of the bulb of the eye (Fig. 677). In front it presents a deep concavity, the hyaloid fossa (O.T. fossa patellaris), for the reception of the posterior convexity of the lens. It is enclosed within a thin transparent membrane, the membrana hyaloidea, which is in contact with the membrana limitans interna of the retina, and is adherent to it at the entrance of the optic nerve. The portion of the membrana hyaloidea in front of the ora serrata is thickened and strengthened by radial fibres, and is termed the zonula ciliaris. Situated behind the ciliary body, the zonula is radially folded and presents a series of alternating furrows and elevations. The ciliary processes are received into, and are firmly adherent to, the furrows, with the result that, if removed, some of their pigment remains attached to the zonula. The elevations of the zonula are not attached to the interciliary depressions, but are separated by a series of lymph spaces named the recessus cameræ posteriores; these may be regarded as diverticula of the posterior chamber with which they communicate. As the zonula approaches the equator of the lens it splits into two chief layers, viz.: (a) a thin posterior lamina, which lines the hyaloid fossa; and (b) a thicker anterior layer, termed the suspensory ligament of the lens (Fig. 677), which blends with the front of the lens capsule a short distance from the equator of the lens. Scattered fibres of this ligament are also attached to the equator itself and to the regions immediately anterior and posterior to it. By this suspensory ligament the lens is retained in position, and its convexity varies inversely with the degree of tension of the ligament. The radial fibres of the ciliary muscle, by pulling forward the ciliary processes and the attached zonula ciliaris, relax the ligament, and thus allow the lens to become more convex. Behind the suspensory ligament, a sacculated lymph space surrounds the equator of the lens; it is named the spatia zonularia (O.T. canal of Petit), and may be easily inflated on introducing a fine blow-pipe through the suspensory ligament (Fig. 677). In the foetus a blood-vessel, termed the arteria hyaloidea, is continued from the arteria centralis retinæ forwards through the vitreous body, for the supply of the capsule of the lens. Its position, in the adult, is represented by a lymph channel, termed the canalis hyaloideus of Stilling (Fig. 677), the presence of which may be demonstrated by shaking up the vitreous body in a solution of picrocarmine, when some of the pigment may be seen to extend along
the canal (Anderson Stuart).
When the vitreous body is treated by a weak solution of chromic acid it presents a series of concentric, peripherally arranged striæ, together with numerous radial striæ. converging towards its centre. Between these the more fluid part lies, and it frequently contains vacuolated amoeboid cells scattered through it. The vitreous body consists of 98-4 per cent of water, having in solution. about 14 per cent of sodium chloride and traces of extractives and albumen.
FIG. 693.-THE SPATIA ZONULARIA DIS-
FIG. 694.-LENS HARDENED IN FORMALIN AND
Lens Crystallina.-The crystalline lens.
DISSECTED TO SHOW ITS CONCENTRIC LAMINE lies in front of the vitreous body and behind (enlarged). the iris, and is a biconvex, transparent body (Fig. 677). It is enclosed in a thin, transparent, homogeneous capsule, the capsule of the lens. The central points of its anterior and posterior surfaces are termed
respectively its anterior and posterior poles, a line joining which is known as its axis; its peripheral circumference is named the equator. Its axial measurement is 4 mm., and its transverse diameter from 9 to 10 mm. Its anterior surface is less curved than the posterior and on it rests the pupillary margin of the iris; the central part of the surface corresponds with the aperture of the pupil and is directed towards the anterior chamber; the peripheral part is separated from the iris by the aqueous humour of the posterior chamber. Its posterior surface, more convex than
FIG. 695.—DIAGRAMMATIC REPRESENTATION OF THE RADIAL the anterior, occupies the hyaloid
LINES OF THE FETAL LENS.
fossa of the vitreous body. curvatures of its surfaces, especi
A, Seen from the front; B, From behind.
ally that of the anterior, are constantly varying, during life, for the purpose of focussing near or distant objects on the retina.
The substantia lentis consists of a soft outer part, the substantia corticalis, easily crushed between the finger and thumb, and of a dense central part, the nucleus lentis. The refractive index of the substantia corticalis is about 14; that of the nucleus lentis about 1:45. Faint radial lines run from the anterior and posterior poles of the lens towards its equator. In the foetus they are three in number, and form angles of 120° with each other (Fig. 696). From the anterior pole one ray ascends vertically and the other two diverge downwards, while from the posterior pole one ray descends vertically and the other two diverge upwards. In the adult the rays may be increased to six or more. They represent the free edges of a corresponding number of septa which dip into the substance of the lens, and along which the extremities of the different groups of lens fibres come into contact, and are attached by a clear, amorphous substance. The lens, when hardened, exhibits a series of concentrically arranged laminæ (Fig. 694), superimposed like the scales of an onion and attached to each other by a clear, amorphous substance. Each lamina is split along the radiating lines, and consists of a series of hexagonal, riband-like fibres, the fibræ lentis, which are adherent to each other by their margins; those of the deeper laminæ are small and serrated, but nonnucleated; while those of the superficial coats are large and nucleated, but non-serrated. The fibres extend in a curved manner from the rays on the anterior surface to the rays on the posterior surface, but no fibre extends from pole to pole. Fibres which start at or near one pole end at or near the equator on the opposite surface, and vice versa, while the intervening fibres take up intermediate positions. Between the substantia lentis and the anterior part of the capsule there is a layer of nucleated columnar epithelial cells, the epithelium lentis. On being traced towards the equator its cells become gradually elongated and transformed into lens fibres, which, when fully formed, lose all trace of their nuclei, except in the more superficial laminæ. Each lens fibre represents, therefore, a greatly elongated columnar cell (Fig. 696).
In the foetus the lens is soft, of a pinkish colour, and nearly spherical; while in old age it becomes somewhat flattened, and assumes a yellowish tint.
Showing the gradual tran
sition of the epithelium into lens fibres (after Babuchin).
Camera Oculi (Fig. 677).-As already stated (p. 813), the space between the cornea and the lens is divided by the iris into two unequal parts, viz., the camera
oculi anterior, in front, and the camera oculi posterior, behind. These are filled with the aqueous humour, and, in the adult, communicate freely through the aperture of the pupil, but in the foetus are separated from each other by the membrana pupillaris. The camera oculi anterior or anterior chamber is bounded in front by the cornea, behind by the iris and lens, whilst peripherally it communicates with the spatia anguli iridis. The camera oculi posterior or posterior chamber is triangular on section, and is bounded in front by the iris, behind by the circumferential part of the lens and its suspensory ligament; the base of the triangle corresponds with the thick, anterior extremities of the ciliary processes. It communicates with the recessus cameræ posteriores and spatia zonularia. The aqueous humour has a refractive index of about 1:336, and consists of about 98 per cent of water, with 14 per cent of sodium chloride, and traces of albumen.
The eyelids are two movable curtains situated in front of the bulb of the eye, and named, from their positions, superior and inferior. The superior is the larger and more movable, being provided with a special elevator muscle, the m. levator palpebra superioris. The interval between the eyelids is termed the palpebral fissure, and measures transversely about 30 mm., but varies considerably in different individuals and in different races. When the eye is open the fissure is elliptical in shape, but when closed it assumes the form of a transverse slit, which lies on a level with the lower margin of the cornea. The two lids meet at the extremities of the fissure, and form the lateral and medial commissures. Their free margins are flattened and are surmounted by eyelashes, from the lateral commissure to a point about 5 mm. from the medial commissure-a point indicated by a small papilla, the papilla lacrimalis. Medial to this papilla the margins are rounded and destitute of eyelashes, and form the upper and lower boundaries of a triangular space, termed the lacus lacrimalis, which is occupied by a small pale red body, the caruncula lacrimalis. The caruncula consists of a small island of modified skin, and contains sudoriferous and sebaceous glands, and fine hairs. Posteriorly the lids are lined with mucous membrane, the conjunctiva, and are in contact with the bulb of the eye, except near the medial commissure, where, between the bulb of the eye and the caruncula lacrimalis, there intervenes a vertical fold of conjunctiva, the plica semilunaris conjunctivæ, which in many animals contains a plate of cartilage.
In each eyelid there exists a framework of condensed fibrous tissue, which gives consistence and shape to the lid, and is termed the tarsus. In front of the tarsus are the fibres of the orbicularis oculi muscle and the integument, while embedded in its posterior surface, and covered by the conjunctiva, are numerous modified
Margin of the upper eyelid with openings of ducts of tarsal glands
FIG. 697.-EYELID SLIGHTLY EVERTED TO SHOW THE CONJUNCTIVA (enlarged).