The great use of the orbicularis palpebrarum is to close the eyelids; but in effecting this it acts at a disadvantage, inasmuch as its action on the eyelids is not direct, but oblique ; there fore they are brought together only by being drawn horizontally inwards, though it is the lower eyelid alone which yields to this latter movement. NVe may imitate in some degree the mode of action of the orbicularis, but in an opposite direction, by pressing the skin imme diately outside the outer canthus, towards the temple.
Levator palpebra superioris muscle.—This is the antagonist of the upper part of the orbicu laris. It is a weak slender muscle, but then it has the advantage of exerting its action in a di rect manner. It extends from the bottom of the orbit to the superior tarsal cartilage, lying im mediately underneath the roof of the orbit. It is the longest of all the muscles of the orbit. Thin and triangular, it rises by its apex, which is a short tendon, from the upper edge of the optic foramen. The fleshy body of the muscle gradually increases in breadth as it proceeds forwards; then bending downwards over the eyeball, its insertion takes place by a broad thin tendinous expansion, the base of the triangle, into the upper margin and anterior surface of the superior tarsal cartilage, being incorporated at the same time with the so-called tarsal liga ment. It is by the action of this muscle that the upper eyelid is drawn up and retracted within the orbit.
having thus described the skeleton and mus cles of the eyelids, it remains to consider their investments and appendages. The investment of their inner surface, the palpebral conjunctiva, will be described farther on, along with the rest of the conjunctiva. The skin of the eyelids lies over the fibres of the orbicularis palpebra rum. It is very fine and destitute of hairs, but contains minute sebaceous follicles. The latter are sometimes, especially in the lower eyelid, enlarged, and give out a morbid secretion, which is hard, and forms those horny excrescences occasionally met with in old persons.
Cellular tissue of the eyelids.—The conjunc tiva investing the inner surface of the tarsal car tilages adheres without the intermedium of any cellular tissue. The connection between the two structures is immediate and intimate, as in the compound membranes called fibro-mucous,. The rest of the palpebral conjunctiva adheres by cellular tissue. The palpebral and ciliary portions of the orbicularis muscle are connected on the one hand to the tarsal cartilages and other subjacent parts, and on the other to the superjacent skin, by a laminar cellular tissue, which, like that in some other parts of the body, is not combined with the adipose tissue. Being rather loose, the cellular tissue of the eyelids becomes readily infiltrated by effused fluids, as in edema and emphysema. It is not
unfrequently the seat of abscess.
Roots of the eyelashes. From the anterior edge of the free margins of the eyelids, the eyelashes spring. They are inserted three or four deep, especially in the middle. The cap sules of the bulbs of the eyelashes lie close on the tarsal cartilage under the cilia/is muscle a ' skin, extending to the depth of about o eighth of an inch. One of the operations trichiasis is to extirpate the roots of the e lashes, but it is very difficult to remove th all, the oozing of blood is generally so gr When the part has healed after the operati and the case seems doing well, a hair or t will often be found here and there sprou • out again.
Connected with the roots of the eyelash as with other hairs, are small sebaceous glan consisting of minute but distinct lobules grains closely surrounding the capsule, i which they send one or more excretory ducts lileibomian glands. Glandula Mei mianee s. palpebrarum sebacex ; — Fr. glandes de Meiboni Le glandule bomiane ; -Germ. Die Meibonischen Drii These are elongated more or less compound follicles, secreting a peculiar sebaceous matter intended as an ointment to protect the delicate integument of the margins of the eyelids from any irritation which might result from friction, or the frequent contact of the tears, and also to preserve to it that peculiar degree of sensibility which, like all other transition structures from skin to mucous membrane, it possesses. The Meibomian glands lie imbedded in the sub stance of the tarsal cartilages. They are ar ranged close and parallel to each other, and ge nerally speaking in a direction at right angles to the cil iary margin of the eyelids, where they open in that row of minute apertures already mention ed. There arebetween thirty and forty Meibomian glands in the upper eyelid, but not so many in the lower, in which also they are shorter in consequence of the difference in breadth be tween the upper and lower tarsus. Sometimes two glands are united towards their orifice ; sometimes, on the other hand, at their end. Frequently the tail of the gland bends laterally and describes an arch. The structure of the Meibomian glands consists essentially in a central canal running from one extremity to the other, like the duct of the pancreas, and around that canal glandular loculi or cryptx opening into it directly, or through the medium of each other. The duct suddenly contracts before opening on the margin of the eyelid. In a transverse section of the Meibomian glands this canal is seen, according to Zeiss, as a small hole around which are placed from five to six glandular grains.