The eyelids (palpebrae) are moveable portions of integument, strengthened toward their margins by a thin lamina of dense fibrous tissue. A mucous membrane lines their inner surface, and is reflected thence in the form of a pellucid covering on the surface of the eyeball. This is named the conjunctival membrane or conjunctiva.
The upper lid is larger and more moveable than the lower, all the transparent part of the globe being covered by it when the eye is closed ; it is chiefly by the elevation of this lid that the eye is opened, the movement being effected by a muscle (levator palpebrae) devoted exclusively to this purpose. At the outer and inner angles {canthi) of the eye the eyelids are united. The interval between the angles - fissura palpebrarum - varies in length in different persons, and, according to its extent, gives the appearance of a larger or a smaller eye, the size of the globe being nearly the same. The greater part of the edge of each eyelid is flattened, but towards the inner cauthus it is rounded off" for a short space, at the same time that it somewhat changes its direction ; where the two differently formed parts join, there exists on each lid a slight conical elevation - papilla lachrymalis - the apex of which is pierced by the aperture or punctum of the corresponding lachrymal canalicule.
In the greater part of their extent the lids are applied to the surface of the eyeball; but at the inner canthus, opposite the puncta lachrymalia, there intervenes a vertical fold of conjunctiva, the plica semilunaris, which rests on the eyeball; whilst, occupying the recess of the angle at the border of this fold, is a spongy-looking reddish elevation, formed by a group of sebaceous glands which open into the follicles of very fine hairs. It is named the caruncula lachrymalis. The plica semilunaris is the rudiment of the third eyelid (membrana nictitans) found in many animals. It contains a small amount of plain muscular tissue (H. Müller).
Structure of the eyelids
The skin covering the eyelids is thin and delicate, and covered with excessively fine, downy hairs; at the line of the eyelashes it joins the conjunctival mucous membrane which lines the inner surface of the lids. The cutis vera is remarkable for containing ramified pigment cells. Beneath the skin, and between it and the conjunctiva, the following structures are successively met with, viz. :
The fibers of the orbicularis muscle; loose connective tissue ; the so called tarsal cartilages, together "with a thin fibrous membrane, the palpebral ligament, which attaches them to the margin of the orbit ; and, finally, the Meibomian glands. In the upper eyelid there is, in addition, the insertion of the levator palpebral superioris, in the form of a fibrous expansion attached to the upper or anterior surface of the tarsal cartilage.
The orlicularis muscle is closely adherent to the skin by fine connective tissue entirely devoid of fat, but glides loosely on the tarsal cartilages. A marginal fasciculus lies within the line of the eyelashes, separated by the bulbs of the lashes from the other fibres, and constituting the musculus ciliaris Riolani.
The tarsal cartilages (tarsi) are two thin elongated plates formed of dense connective tissue, without, according to most observers, any inter- mixture of cartilage-cells. They are placed one in each lid, and serve to give shape and firmness to those parts. The upper cartilage, the larger, is half oval in form, being broader near the center and narrowing towards the angles of the lids. The lower is thinner, much narrower, and more nearly of a uniform breadth throughout. Their free or ciliary edge, which is straight, is thicker than any other part. At the inner canthus they are fixed by fibrous slips of the tendon of the orbicularis muscle; and at the outer angle are attached to the malar bone by a fibrous band belonging to the palpebral ligament, and named the external tarsal ligament.
The palpebral ligament is a fibrous membrane placed beneath the orbicularis muscle, attached peripherally to the margin of the orbit, and internally to the tarsi, with which its tissue is continuous. The membrane is thickest at the outer part of the orbit.
On the ocular surface of each lid are seen from twenty to thirty parallel vertical rows of yellow granules, lying immediately under the conjunctival mucous membrane, and known as the Meibomian glands. They are compound sebaceous glands, imbedded in grooves at the back of the tarsi; and they open on the free margin of the lids by minute orifices, generally one for each. The glands consist of nearly straight tubes, closed at the end, with numerous small caecal appendages projecting from the sides. The tubes are lined for some distance by stratified epithelium continuous with that of the skin: the glandular recesses have a lining of cubical epithelium and are filled with the fatty secretion. According to Colosanti the glands have a basement membrane, and a muscular layer outside this: he further describes a network of fine nervous fibrils amongst the epithelium cells.
A layer of unstriped muscular tissue is contained in each eyelid; that of the upper arising from the under sui-face of the levator palpebrae, that of the lower from the neighborhood of the inferior oblique muscle, and each being inserted near the margin of the tarsus. A few fibers are also to be found in the plica semilunaris (H. Müller). It may also be mentioned in this place that the same writer describes a layer of unstriped muscle crossing the spheno-maxillary fissure, corresponding to a more largely developed layer found in the extensive aponeurotic part of the orbital wall of various mammalia. This set of fibres has been more particularly described by Turner.
The eyelashes (cilia) are strong short curved hairs, arranged in two or more rows along the margin of the lids, at the line of union between the skin and the conjunctiva. The upper lashes are more numerous and longer than the lower, and are curved in an opposite direction. Near the inner canthus the hairs are weaker and more scattered. Immediately within the eyelashes, between them and the muscle of Riolan, is a row of large modified sweat-glands, which open into the mouths of sebaceous glands (not the Meibomian).
Structure of the conjunctiva
The conjunctiva consists of the palpebral part, with which may be included the plica semilunaris and caruncula, and of the ocular part or conjunctiva bulbi, in which may be distinguished the sclerotic and corneal portions : each of these parts presents distinctive characters. The epithelium of the conjunctiva varies somewhat at different parts, but is mainly columnar, with smaller cells between the fixed ends of the columnar cells. Near the skin and cornea it shades off into the stratified epithelium which covers these parts.
The palpebral portion of the conjunctiva is thicker and more vascular than any other part of the membrane, and presents numerous fine papillae freely supplied with nerves. It passes through the puncta lachrymalia into the canaliculi, and is continuous with the lining membrane of the lachrymal sac. Although closely united to the tarsi, it exhibits, nevertheless, numerous small creases or folds, which are visible with a lens. A layer of small racemose or tubulo-racemose glands is found on the ocular surface of the lids, immediately under the conjunctiva, and be- yond the ends of the Meibomian glands (Sappey, W, Krause). Their minute ducts open near the line of reflection of the conjunctiva upon the globe of the eye (fornix conjunctivae).
The sclerotic portion
The conjunctiva changes its character at the line of reflection from the eyelids, becoming thinner and losing its papillary structure : it is loosely connected to the eyeball by submucous tissue. It is transparent and a few blood-vessels are generally visible in it in the healthy condition, but under the influence of inflammatory congestion a copious network of vessels very irregularly disposed comes into view. These vessels are derived from the palpebral and lachrymal arteries.
Another set of vessels exists on the surface of the sclerotic, and are seen when congested. These are entirely sub-conjunctival and adherent to the sclerotic coat ; they are less tortuous than the conjunctival set, and are derived from the muscular and anterior ciliary branches of the ophthalmic artery: they remain immoveable on pressure of the eyelid, whereas the conjunctival vessels of course shift with that membrane. These sclerotic vessels dip in near the cornea, and appear to unite with a deeper minute network disposed in closely set straight lines, which radiate from the margin of the cornea, and the gorged condition of which is well known to ophthalmic surgeons as characteristic of sclerotitis.
The corneal conjunctiva consists almost entirely of epithelium, any underlying membrane being extremely thin, transparent, and adherent to the anterior layers of the cornea, in connection with which it will be again referred to. Around the circumference vessels lie between it and the cornea, and form a circle of anastomotic capillary loops. This plexus of vessels extends farther inwards in the fetus.
A well-developed network of lymphatic exists throughout the sclerotic and palpebral portions of the conjunctiva : but at the margin of the cornea a sudden diminution takes place in the size of the meshes and diameter of the vessels, which become irregular, and come into connection with ramified cell-spaces in the cornea.
The nerves in the membrane, as far as the cornea, seem to have the same arrangement as in the skin. Their mode of ending has not been traced with certainty, but according to Krause many of them terminate in end-bulbs.
The mucous membrane of the palpebral conjunctiva contains, especially at its back part, a large quantity of lymphoid tissue. Lymphoid follicles have also been described in the conjunctiva (Bruch), but their existence in man is doubtful (Waldeyer).
