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sebaceous glands, named the tarsal glands. The superior tarsus is larger than the inferior, and of a half oval shape, with its greatest vertical diameter measuring about 10 or 11 mm. Its upper margin is thin and convex, and is continuous with the tendon of the levator palpebræ superioris muscle, while its lower edge is thick and straight. The inferior tarsus is a thin, narrow strip, with a nearly uniform vertical diameter of about 5 mm. The extremities of the two plates are continuous with the lateral palpebral raphe and the medial palpebral ligament. The lateral palpebral raphe is a narrow band attached to the zygomatic bone; it divides, at the lateral commissure, into superior and inferior pieces which are fixed to the margins of the respective tarsi. The medial palpebral ligament is a strong band attached to the frontal process of the maxilla, directly in front of the lacrimal
FIG. 698.-SAGITTAL SECTION THROUGH SUPERIOR EYELID.
groove; it divides at the medial commissure into two slips, one for each tarsus (Fig. 699).
The eyelids are further strengthened by membranous expansions, termed the superior and inferior palpebral ligaments, which extend into them from the margin of the orbit. The superior palpebral ligament is continuous, along the superior margin of the orbit, with the pericranium and with the periosteal lining of the orbit, and it blends, below, with the tendon of the levator palpebræ superioris. The inferior palpebral ligament is prolonged from the lower edge of the inferior tarsus to the inferior margin of the orbit, where it is continuous with the periosteum of the face and orbital floor. Laterally the two palpebral ligaments fuse to form the lateral palpebral raphe, while medially they become thinned, and, separating from the medial palpebral ligament, are attached to the lacrimal bone, behind the lacrimal sac. The superior and inferior palpebral ligaments form the septum orbitale, between the superficial and deep structures of the eyelids; this septum is perforated by the vessels and nerves, which extend from the orbital cavity to the face or scalp.
The skin covering the eyelids is thin and delicate, and is continuous, at their margins, with their conjunctival lining. It contains numerous small sudoriferous glands and fine hairs, the latter being provided with sebaceous follicles. Branched pigment cells are present in the cutis, and pigment exists also in the deep layers of the epidermis. The subcutaneous tissue is loose and devoid of fat, and in it are found the fibres of the orbicularis oculi muscle-a small separate bundle of which, termed the muscle of Riolan, occupies the margin of the lids behind the eyelashes.
The glandulæ tarsales (O.T. Meibomian glands) are elongated sebaceous glands with numerous lateral offshoots; they are embedded in the tarsi and are filled with cubical epithelium. There are from twenty-five to thirty in the superior eyelid, and from twenty to twenty-five in the inferior; they open by small ducts, about 1 mm. in length, along the lid margins, behind the eyelashes; the ducts are lined with stratified epithelium, placed on a basement membrane. Between the eyelashes and the muscle of Riolan are two or three rows of modified sudoriferous glands, termed the glands of Moll.
Tendon of levator
Palpebral branch of
Ligamentum palpebrale mediale
FIG. 699.-DISSECTION OF THE RIGHT EYELID. The orbicularis oculi has been completely removed.
H. Müller described a layer of non-striped muscle in each lid: in the superior extending from the tendon of the levator palpebræ superioris to the upper tarsus, and in the inferior connecting the inferior tarsus with the inferior oblique muscle.
The eyelashes are curved, silky hairs which project from the free margins of the lids; in the upper lid they are longer and more numerous than in the lower, and are curved upwards, while those of the lower lid are bent downwards.
Conjunctiva. The conjunctiva is the mucous membrane which lines the eyelids, and is continued, from them, on to the front of the bulb of the eye. The part on the eyelids is termed the tunica conjunctiva palpebrarum, that on the bulb of the eye the tunica conjunctiva bulbi; the lines of reflection of the membrane from the eyelids to the bulb are known as the superior and inferior conjunctival fornices. The conjunctiva palpebrarum is intimately adherent to the tarsi and presents numerous papillæ. It is covered with a layer of columnar epithelial cells, beneath the bases of which are small flattened cells; near the fornices a number of acino-tubular glands, much more plentiful in the upper than in the lower eyelid, open on its free surface. The conjunctiva bulbi is thinner than the conjunctiva palpebrarum, and is loosely attached to the sclera by submucous tissue. The plica semilunaris conjunctivæ has already been referred to (p. 821). On the cornea the conjunctiva is represented merely by the epithelium corneæ (p. 809).
Vessels and Nerves.-The chief arteries of the eyelids are the superior and inferior palpebral branches of the ophthalmic, which pierce the septum orbitale above and below the medial palpebral ligament, and run laterally in the corresponding lid near its free margin. On reaching the region of the lateral palpebral commissure they anastomose with each other and with twigs from the lacrimal, superficial temporal, and transverse facial arteries, and in this way an arch is formed in each lid. Secondary smaller arches are found, one above the primary arch in the upper lid, and another below that of the lower lid, while the upper lid receives branches also
from the supra-orbital and frontal arteries. The veins are arranged in two sets: (a) subconjunctival or retrotarsal, opening into the muscular tributaries of the ophthalmic veins, and (b) pretarsal, into the angular and superficial temporal veins. The lymph vessels, like the veins, form pre- and retrotarsal networks, which communicate with each other through the tarsal plates. The lymph is drained chiefly into the anterior auricular and parotid lymph glands, but partly, by vessels which accompany the anterior facial vein, into the submaxillary lymph glands. The sensory nerves of the eyelids are supplied by the trigeminal nerve-the upper lid chiefly by the supra-orbital and supra-trochlear branches of the ophthalmic; the lower, by the infraorbital branch of the maxillary. The region of the lateral commissure receives some filaments from the lacrimal nerve, that of the medial from the infra-trochlear. These sensory nerves form a marginal plexus behind the orbicularis oculi muscle. The levator palpebræ superioris muscle is supplied by the oculomotor nerve and the non-striped fibres of the eyelids by the sympathetic.
The lacrimal apparatus consists of: (1) the lacrimal gland, which secretes the tears; (2) the lacrimal ducts, by which the tears are drained from the front of the eye; and (3) the lacrimal sac and naso-lacrimal duct, which convey them into the nasal cavity.
FIG. 700.-DISSECTION TO SHOW THE LACRIMAL APPARATUS.
Glandula Lacrimalis.-The lacrimal gland is a flattened, oval body situated in the superior and lateral part of the orbital cavity; it consists of two portions -superior and inferior-imperfectly separated from each other by the expansion of the tendon of the levator palpebræ superioris muscle. The glandula lacrimalis superior is firm and much larger than the inferior; it measures transversely about 20 mm., and sagittally from 12 to 14 mm. It occupies the fossa lacrimalis on the medial surface of the zygomatic process of the frontal bone, and is fixed by fibrous bands to its periosteum, while its inferior surface is in contact with the levator palpebræ superioris and rectus lateralis muscles, which intervene between it and the bulb of the eye. The glandula lacrimalis inferior consists of small, loosely aggregated lobules; it lies below and in front of the orbital portion, and projects into the posterior part of the upper eyelid, where its deep surface is in contact with the conjunctiva. The ducts draining the glandula superior are from three to five in number; they pass between the lobules of the glandula inferior, and open at the upper and lateral part of the fornix conjunctivæ superior. The ducts of the glandula inferior number from three to nine; some of them join those from the glandula superior, while others open separately at the fornix conjunctivæ superior. The lacrimal gland has a structure resembling that of the parotid, and
is supplied by the sympathetic and lacrimal nerves and by the lacrimal artery, while its veins are drained into the ophthalmic vein.
Ductus Lacrimales.-The lacrimal ducts, two in number, commence in minute orifices, termed the puncta lacrimalia, at the apices of the papillæ lacrimales (p. 821), and are directed medialwards, along the medial parts of the margins of the eyelids, above and below the lacus lacrimalis. The superior duct at first ascends for a short distance and then inclines downwards; the inferior duct descends for a short distance and then runs horizontally; at the angle where it changes its direction each duct is dilated into an ampulla. The two ducts open close together into the lateral and front part of the lacrimal sac, a little below its middle; sometimes they open separately into a pouch-like dilatation of the sac, termed the sinus of Maier. The ducts are lined with stratified epithelium placed on a tunica propria, outside which is a layer of striped muscular fibres derived from the lacrimal part of the orbicularis oculi. These muscular fibres are arranged somewhat spirally around the ducts, but at the base of each papilla lacrimalis they are circular in direction and form a species of sphincter. On contraction they serve to empty the contents of the lacrimal ducts into the lacrimal sac.
The saccus lacrimalis and ductus nasolacrimalis together form the passage by which the tears are conveyed from the lacrimal ducts to the nasal cavity.
The lacrimal sac is the upper expanded part of the passage, and measures from 12 to 15 mm. in length, about 7 mm. antero-posteriorly, and from 4 to 5 mm. transversely. It lies in the groove formed by the lacrimal bone and frontal process of the maxilla, and ends above in a rounded, blind extremity or fundus, while it narrows below into the naso-lacrimal duct. At the junction with the duct a fold of mucous membrane, named the valve of Beraud, together with a laterally directed pouch, the sinus of Arlt, are sometimes present. Near its superior extremity it is crossed, in front, by the medial palpebral ligament (O.T. tendo oculi), from the upper and lower edges of which the orbicularis oculi takes origin; the lacrimal part of the orbicularis oculi muscle is behind it.
The naso-lacrimal duct averages about 18 mm. in length, and has a diameter of from 3 to 4 mm. Rather narrower in the middle than at its extremities, it is directed downwards and slightly backwards, and opens into the inferior meatus of the nose at the junction of its anterior fourth with its posterior three-fourths, i.e. a distance of 30 to 35 mm. from the posterior boundary of the nostril. Its lower orifice is somewhat variable in form and position, and is occasionally duplicated. It is frequently guarded by a fold of mucous membrane, termed the plica lacrimalis (Hasneri). Through this orifice the mucous lining of the duct is continuous with that of the nasal cavity. The mucous membrane of the duct is thrown into inconstant folds, several of which have been described as valves. Its epithelium is columnar and in part ciliated; opening into the lower part of the duct are numerous glands, similar to those in the nasal mucous membrane.
The nerves of the lacrimal ducts and sac are derived from the infra-trochlear branch of the naso-ciliary; their arteries from the inferior palpebral and nasal. The veins of the naso-lacrimal duct are large and numerous, forming a sort of erectile tissue similar to that in the nasal cavity.
DEVELOPMENT OF THE EYE.
The retina and optic nerve are developed from a hollow outgrowth of the fore-brain, termed the optic vesicle (see pp. 54 and 33). This extends towards the side of the head, and its connexion with the brain is gradually elongated to form the optic stalk. The ectoderm overlying the optic vesicle becomes thickened, invaginated, and finally cut off as a hollow island of cells, which is named the lens vesicle. This vesicle indents the outer and lower part of the optic vesicle, converting it into a cup (optic cup), lined by two layers of cells continuous with each other at the margin of the cup. The inner of these strata, thicker than the outer, is named the retinal layer, and becomes differentiated into the nervous and supporting elements of the retina; while the outer, named the pigmentary layer, forms the stratum pigmenti. The edge of the optic cup extends in front of the equator of the lens, and bounds the future aperture of the pupil. In front of the lens, and also opposite its equator, the retinal layer is thin, and represented only by a stratum of columnar cells which becomes closely applied to the pigmentary layer, the two forming the
pars ciliaris and pars iridica retina. The indentation of the optic cup extends as a groove for some distance along the postero-inferior aspect of the optic stalk, forming what is termed the chorioidal fissure (Fig. 701). Through this fissure mesoderm passes inwards between the lens and the retina to form a part of the vitreous body, while the arteria centralis retinæ also becomes enclosed in it and so gains its future position in the centre of the optic
FIG. 701.-SECTIONS THROUGH PORTIONS OF THE HEADS OF FETAL RABBITS, to illustrate the connexion of the optic cup with the fore-brain, and the invagination of the ectoderm to form the lens.
Outer layer of optic cup
The arteria centralis is prolonged forwards from the porus opticus through the vitreous body, as a cone of branches, as far as the back of the lens. By the fifth or sixth month all these branches have disappeared except one, the arteria hyaloidea, which persists until the last month of foetal life, when it also atrophies, leaving only the canalis hyaloideus to indicate its position.
The vitreous body is developed between the optic cup and the lens, and is derived partly from ectoderm and partly from mesoderm. It consists primarily of a series of fine protoplasmic fibres which project from the cells of the retinal layer of the cup and form a delicate Optic stalk reticular tissue. At first these fibres are seen in relation to the whole of the optic cup, but later they are limited to the ciliary region, where by a process of condensation Chorioidal they appear to form the zonula ciliaris. When the
mesoderm reaches the cup through the chorioidal fissure it unites with this reticular tissue to form the vitreous body.
FIG. 702.-OPTIC CUP AND LENS VIEWED
The lens, at first in contact with the ectoderm
FROM BEHIND AND BELOW, to show from which it is derived, is soon separated from it by
formation of chorioidal fissure and enclosure of the arteria centralis retinæ (from inodel by Ziegler).
mesoderm, and then consists of a rounded vesicle with epithelial walls. The anterior wall remains as a single layer of cells-the anterior lens epithelium of the adult; the cells of the posterior wall become elongated into lens fibres, and by the forward growth of these the cavity of the vesicle is obliterated. This elongation into lens fibres is greatest at the centre of the lens, while near the equator the fibres are shorter, and here the gradual transition between the anterior epithelium and the lens fibres is seen (Fig. 696). The lens becomes enveloped in a vascular tunic, which receives its vessels from the arteria centralis retina and from the vessels of the iris. The front part of this tunic forms the membrana pupillaris, and this, like the rest of the tunic, disappears before birth.
The hollow optic stalk becomes solid by the thickening of its walls and, acquiring nerve-fibres, is transformed into the optic nerve. These nerve-fibres are mostly centripetal, and are derived from the nerve-cells of the retina; but a few are centrifugal and have their origin in the brain. The further development of the retina resembles, in certain respects, that of the spinal medulla.
Cameron states (Journ. Anat. and Physiol., vol. xxxix.) that in the early stages of the development of the inner or retinal layer of the optic cup all the structures, described by His as being present in the spinal medulla of the human embryo, are to be found, viz., (a) spongioblasts, (b) germinal cells, and (c) neuroblasts.