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destitute of muscular tissue, are superficial to the arteries; they are surrounded by perivascular lymph sheaths and converge to form whorls, which open into the venæ vorticosæ. In the tissue between the blood-vessels are numerous stellate, flattened, and pigmented cells.
The lamina choriocapillaris is composed essentially of small capillaries, which form an exceedingly close network, embedded in a finely granular or almost homogeneous tissue.
The intermediate stratum between the lamina vasculosa and lamina choriocapillaris consists of a network of delicate elastic fibres and contains almost no pigment cells; it is lined, next the lamina choriocapillaris, with a layer of endothelium.
The lamina basalis is transparent and nearly structureless. Its outer surface exhibits a trellis-like network of fibres which unite it to the lamina choriocapillaris, while its inner surface is smooth and is in contact with the pigmented layer of the retina.
Tapetum. In many animals a brilliant iridescence is seen on the postero-lateral part of the chorioid; to this the name tapetum is applied. Absent in man, it may be due, as in the horse, to a markedly fibrous condition of the stratum intermedium (tapetum fibrosum), or as in the seal, to the presence of some five or six layers of flattened iridescent cells lying immediately outside the lamina choriocapillaris (tapetum cellulosum).
Corpus Ciliare. The ciliary body connects the chorioid to the circumference of the iris (Fig. 683), and comprises three zones, viz.: (a) the orbiculus ciliaris, (b) the ciliary processes, and (c) the ciliary muscle.
Pars iridica retinæ
of ciliary muscle
Meridional fibres of
The orbiculus ciliaris is a zone of about 4 mm. in width immediately adjoining the chorioid; it exhibits numerous radially arranged ridges.
Processus Ciliares. -The ciliary processes, about seventy in number, form a circle of radial thickenings, each of a somewhat triangular shape; the base of the triangle is directed for
Pars ciliaris retina wards, towards the equator of the lens, while the apex is continuous behind with some three or four ridges of the orbiculus ciliaris. They vary in size, the largest having a length of 2-5
The structure of the orbiculus ciliaris and ciliary processes is similar to that of the chorioid, but the capillaries are larger and more tortuous, and there is no lamina The choriocapillaris.
FIG. 683.—SECTION THROUGH CILIARY REGION OF THE BULB OF THE EYE. deep surface of the ciliary
processes is covered by two strata of columnar epithelium, the anterior layer of which is pigmented; these two strata form a direct continuation forwards of the retina and constitute the
pars ciliaris retinæ; this epithelium is invaginated to form more or less tubular glands.
M. Ciliaris. The ciliary muscle is triangular on horizontal or vertical section, and consists of two sets of fibres-meridional and circular (Fig. 683). The meridional fibres arise from the scleral spur, already described, and radiate backwards, to be attached to the ciliary processes and orbiculus ciliaris. When they contract the chorioid is drawn forwards and the lens becomes more convex, owing to the relaxation of its suspensory ligament (see p. 810). The circular fibres form a triangular zone behind the filtration angle, close to the periphery of the iris. Considerable individual differences are found as to the degree of development of these two portions of the ciliary muscle; the meridional fibres are always more numerous than the circular fibres, the latter being absent or rudimentary in myopic eyes, but well developed, as a rule, in hypermetropic eyes.
Iris. The iris forms a contractile diaphragm in front of the lens, and is pierced, a little to the nasal side of its centre, by an almost circular aperture, the pupil, which, during life, is continually varying in size in order to regulate the amount of light admitted into the interior of the eye. It partially divides the space between the cornea and lens into two portions, which are filled by the aqueous humour, and are named, respectively, the anterior and posterior chambers of the eye. It is thinnest at its peripheral or ciliary margin which is directly continuous with the ciliary body, and, through the medium of the_ligamentum pectinatum iridis, with the posterior elastic lamina of the cornea. Its pupillary or free margin forms the circumference of the pupil, and rests upon the anterior surface of the capsule of the lens.
The distinctive colour of the eye, in different individuals, depends on the arrangement of the pigment in the iris; in the blue eye the pigment is limited to the posterior surface of the iris, but in the brown or black eye it is also scattered throughout its stroma; in the albino the pigment is absent.
The pupil is closed, during the greater part of foetal life, by a thin transparent vascular membrane, the membrana pupillaris, continuous with the pupillary margin of the iris. Its vessels are derived partly from the vessels of the iris and partly from those of the capsule of the lens; they converge towards the middle of the membrane, near which they form loops so as to leave the central part non-vascular. About the seventh month the vessels begin to be obliterated, from the centre towards the circumference; and this is followed by a thinning and absorption of the membrane, which becomes perforated by the aperture of the pupil. This perforation gradually enlarges, and at birth the membrane has entirely disappeared; in exceptional cases it persists.
On the anterior surface of the iris is a layer of flattened endothelium, placed on a basement membrane, and continuous with the endothelium of the anterior chamber. Depressions or crypts are seen here and there in which the endothelium and basement membrane are absent, and are, by some, regarded as stomata, through which the lymph vessels of the iris communicate with the cavity of the anterior chamber. The posterior surface of the iris is covered with a basement membrane, on which are placed two layers of columnar, pigmented epithelium, continuous with the pars ciliaris retina, and termed the pars iridica retina. stroma iridis, or proper tissue of the iris, consists of delicate connective tissue and elastic fibres, with pigmented cells, blood-vessels, nerves, and non-striped muscle.
The blood-vessels of the iris (Fig. 682) are derived from the long ciliary and the anterior ciliary arteries. The long ciliary arteries, two in number, pierce the sclera on the medial and lateral sides of the optic nerve respectively, and extend forwards, between the sclera and chorioid, towards the ciliary margin of the iris. There each divides into a superior and an inferior branch, and the resulting four branches anastomose in the form of a circle, termed the circulus arteriosus major. This circle is joined by a varying number of anterior ciliary arteries, derived from the lacrimal and muscular branches of the ophthalmic artery, and, after supplying the ciliary muscle, sends converging branches towards the aperture of the pupil, where a second circle, the circulus arteriosus minor, is formed. The veins proceed
towards the ciliary margin of the iris, and communicate with the veins of the ciliary processes and with the sinus venosus sclera. The convergence of the blood
vessels towards the aperture of the pupil gives to the anterior surface of the iris a striated appearance. The non-striped muscular fibres of the iris are arranged in two sets: (a) circular, (b) radial. The circular fibres form a band, the m. sphincter pupillæ, around the pupillary aperture; by
Circulus the contraction of these arteriosus fibres the size of the pupil
Circulus is lessened. The radial fibres constitute the m. dilatator pupillæ and extend outwards from the sphincter to the ciliary margin. Many anatomists regard the radial fibres, in man and most mammals, as being elastic and not muscular. In animals in which the radial fibres are muscular, the degree of their development varies considerably; they are feebly marked in the rabbit, but are well developed in the bird, and still more so in the otter.
Anterior ciliary arteries
FIG. 684.-BLOOD-VESSELS OF IRIS AND ANTERIOR PART OF CHORIOID, viewed from the front (Arnold).
Anterior ciliary artery Ciliary muscle
The nerves of the chorioid and iris (Fig. 685) are derived from the long and short ciliary nerves. The former, two or three in number, are branches of the nasociliary nerve; the latter, varying from eight to fourteen, are derived from the ciliary ganglion. Piercing the sclera around the entrance of the optic nerve, the ciliary nerves traverse the perichorioidal lymph space, where they form a plexus, rich in nerve-cells, Vena vorticosa from which filaments are supplied to the blood-vessels of the chorioid. In front of the ciliary muscle a second plexus, also rich in nerve-cells, is formed; this supplies the ciliary muscle and sends filaments into the iris, as far as its pupillary margin, for the
Long posterior ciliary artery
FIG. 685.-DISSECTION OF THE EYEBALL SHOWING THE VASCULAR
supply of its muscular fibres and blood-vessels. The sphincter pupilla is supplied by the oculo-motor nerve, the dilatator pupille by the sympathetic.
The retina, or nervous tunic of the eyeball, is a soft, delicate membrane, in which the fibres of the optic nerve are spread out. It consists of two strata, viz.: (a) an outer, pigmented layer, attached to the chorioid; and (b) an inner nervous lamina, the retina proper, in contact with the hyaloid membrane of the vitreous body, but attached to it only around the entrance of the optic nerve and in the region of the ciliary processes. Expanding from the entrance of the optic nerve,
the retina appears to end, a short distance behind the ciliary body, in a wavy border, the ora serrata (Fig. 686). There its nervous elements cease and the membrane becomes suddenly thinned, but a delicate continuation of it is prolonged over the posterior aspect of the ciliary body and iris. This continuation consists of the pigmented layer, together with a layer of columnar epithelium, and constitutes the pars ciliaris retina and pars iridica retinæ, already referred to (p. 813). The portion behind the ora serrata is termed the pars optica retinæ, and its thickness gradually
Pars ciliaris retinæ
from 0.4 mm. near the entrance of the optic nerve, to 0.1 mm. at the ora serrata. It presents, at the posterior pole of the eye, and therefore directly in the optic axis, a small, oval yellowish spot, the macula lutea. The greatest or transverse diameter of the macula measures from 2-3 mm.; its central part is depressed and is named the fovea centralis. About 3 mm. to the nasal side and slightly below the level of the posterior pole is a whitish, circular disc, the optic disc, which corresponds with the entrance of the optic nerve, and has a diameter of about 15 mm. The circumference of the optic disc is slightly raised and is named the papilla nervi optici, while its depressed central portion is termed the excavatio papillæ nervi optici (O.T. optic cup). The optic disc consists merely of nerve-fibres, the other layers of the retina being absent, and it constitutes the "blind spot."
Membrana limitans interna
FIG. 687.-DIAGRAMMATIC SECTION OF THE
HUMAN RETINA (modified from Schultze).
The nervous layer of the retina is transparent during life, but becomes opaque and of a grayish colour soon after death. If an animal is kept in the dark before the removal of its eyeball, the retina presents a purple tinge, due to the presence of a colouring matter named rhodopsin or visual purple, which is rapidly bleached on exposure to sunlight. This colouring matter is absent from the macula lutea, and absent also over a narrow zone, 3-4 mm. in width, near the ora serrata.
Structure of the Retina (Figs. 687, 688, 689). The nervous elements of the retina are supported by non-nervous or sustentacular fibres, and are arranged in seven layers, to which must be added the stratum pigmenti.
The layers from within outwards, i.e. from vitreous body to chorioid, are:
1. Stratum opticum or layer of nerve-fibres.
3. Inner molecular or inner plexiform layer.
8. Stratum pigmenti.
1. Stratum opticum or layer of nerve-fibres.-Most of the fibres of this stratum are centripetal, and are direct continuations of the axons of the cells in the
ganglionic layer; a few are centrifugal and end in branched clubbed extremities in the inner molecular or inner nuclear layers of the retina.
FIG. 688. PERPENDICULAR SECTIONS OF MAMMALIAN RETINA (Cajal). A. Layer of rods and cones; B, Outer nuclear layer; C, Outer molecular layer; D, Inner nuclear layer; E Inner molecular layer; F, Ganglionic layer; G, Stratum opticum; r, rods; c, cones, r.g, rod granules: c.g, cone granules; r.b, rod bipolars; c.b, cone bipolars; c.r, contact of rod bipolars with the spherules of the rod fibres; c. c, contact of cone bipolars with the branches of the cone fibres; ar, internal arborisation of cone bipolars; ar', internal arborisation of rod bipolars; c.n, centrifugal nerve fibre; h, horizontal cells; s.s, stratified spongioblasts; d.s, diffuse spongioblasts; s.g, stratified ganglion cell; M, Sustentacular fibre.
2. Ganglionic or nerve-cell layer. The cells of this stratum vary in size, are oval or piriform in shape, and form a single layer, except at the macula lutea, where several
strata are present. Each cell contains a large nucleus, and gives off, from its inner surface, an axon which is continued as a fibre of the stratum opticum. From the outer surface of each cell numerous dendrites arise, which form arborisations in the inner molecular layer. The cells may be divided into uni stratified, multi-stratified, and diffuse, according as their dendrites ramify in one or in several strata of the inner molecular layer, or extend throughout nearly its whole thickness.
3. Inner molecular or inner plexiform layer.-This is constituted chiefly by the interlacement of the dendritic arborisations of the cells of the ganglionic layer with those of the inner nuclear layer, and has been divided by Ramon y Cajal into five strata. It sometimes contains horizontal cells (spongio blasts), whose branched processes ramify in it.
4. Inner nuclear layer or layer of inner granules.This is the most complicated of the retinal strata, and consists of numerous cells which may be divided into three groups, viz.: (a) bipolar cells, (b) horizontal cells, and (c) spongioblasts, or amacrine cells.
(a) The bipolar cells, by far the most numerous, are fusi form and nucleated, and each gives off an external and an internal process. The internal processes terminate in flattened tufts, at different levels, in the inner molecular layer, while the external produce an abundant ramification in the external zone of the outer molecular layer. These bipolar cells are divided into rod bipolars, cone bipolars, and giant bipolars. The rod bipolars end peripherally in vertical arborisations around the button-like ends or spherules of the rod fibres, and, centrally, in branched extremities which mostly become applied to the cells of the ganglionic layer. The cone bipolars end peripher ally in flattened arborisations in the outer molecular layer, in contact with the ramifications of the foot-plates of the cone fibres, and, centrally, ramify in some one of the five strata of the inner molecular layer. The giant bipolars form, peripher ally, an extensive horizontally arranged arborisation in the outer molecular layer; centrally, they ramify in one or other of the strata of the inner molecular layer.
A, A cone and two rods from the human retina (modified from
Max Schultze); B, Outer part of rod separated into discs.