LACRYMAL ORGANS, or lacrymal pas sages, organlacrymatia S. vi& lacrymales; Fr. ies organs lacrymawr ou voles lacrymales; Ital. Gli organi spettanti die lagrime ; Germ. Die Thriinenorgane.
Under this head it is proposed to describe not only the lacrymal organs properly so called, but also the eyelids and conjunctiva. This article therefore comprehends all the accessory or protecting parts of the eye (tutamina oculi of Haller) except the orbit and muscles of the eye ball, for which see the articles Fe CE and ORBIT. Those parts of the orbit directly connected with the lacrymal organs are however noticed here.
I. The eyelids.—Palpebrvr.* Fr. Les pau pires ; Ital. Le palpebre ; Germ. Die Augcn-J lieder.
The eyeball is invested in front by a mucous membrane called conjunctiva. Towards the margin of the orbit, this membrane leaves the eyeball and forms together with the skin, with which it is continuous, two horizontal folds, an upper and a lower, intended occasionally to cover and so to protect the delicate and trans parent front of the eyeball. The folds thus formed by the application against each other of a layer of mucous membrane and a layer op skin are eyelids.
Such is the simplest idea of eyelids, and such are they found in the salamander and axolotl among reptiles, and so far as in certain in stances they exist among fishes; such even is their state in man and the higher animals at the commencement of development. But, as in the perfect condition of the organ of vision, it is essential that the eyelids should admit of being readily drawn over the front of the eye ball, and as readily retracted in order again to permit the access of light, so something more than a mere tegumentary fold was required to constitute a perfect eyelid. There was, in fact, required something to impart firmness, espe cially to the margins of the folds,—a structure which, whilst it served as an advantageous point on which the muscles necessary for the movements of the eyelids might exert their action, should cause no undue pressure on the. eyeball, but rather give it an equable support and shield it from that irregular compression which might otherwise have been produced. All these desiderata we find supplied by a thin fibro-cartilaginous lamina, called tarsal carti contained in either eyelid, within the fold formed by the skin and conjunctiva.
The tarsal cartilages do not occupy the whole of the folds, but only a part at their free mat gins. Between the upper edge of the carti lage of the upper eyelid and the lower edge of that of the lower, respectively, and the cor responding edges of the orbit, there intervenes a cellulo-membraneous expansion. This (and
in the upper eyelid, the expansion of the le vator palpebrw superioris,) together with the conjunctiva on its inside and the skin on its outside, serves as it were the office of a loose hinge for the firm part of the eyelid. But the pivots on which the motions of the eyelids, especially of the upper, more immediately take place, are the angles of the eye. The upper eyelid, indeed, moves somewhat in the manner of the visor of a helmet, its firm part, when the eye is open, being drawn up and retracted within the margin of the orbit whilst its loose part is thrown into folds.
Erternal conformation of the eyelids.--The extent and form of the eyelids are best seen when they are closed in sleep. The convexity of their external surface then bespeaks a cor responding concavity of the internal adapted to the prominent front of the eyeball. The opening between the two eyelids is called the palpebral fissure, rime palpebrarura. In the closed state of the eye, this fissure represents a mere curved line with the convexity down wards; but on account of the way in which the eyelids are moved, it is, in the open state, a wide elliptical aperture.
It is chiefly by the motions of the upper eyelid that the open or closed state of the eye is commonly produced. The upper eyelid is larger than the lower, and in the closed state of the eye from relaxation simply, as during sleep or in death, it covers much more of the front of the eyeball than the lower. But in forced closure of the eye by the action of the orbicularis palpebrarum muscle, the lower eyelid is drawn up, being impressed at the same time with a horizontal movement to wards the inner angle, and meets the upper half-way, so that the latter cannot descend so far as it does during sleep. Hence, in active closure of the eye the skin of the upper eyelid is thrown into folds, whereas, in passive clo sure, it is smoothly extended in a convex form over the eyeball. The lower, eyelid is capable of pretty extensive motion. It can of itself alone cover almost entirely the whole front of the eyeball, either when the upper eyelid is held immoveably retracted under the edge of the orbit, or in that morbid shortening or re traction of the upper eyelid known by the name of lagophthalmos. But as the covering of the eye by the lower eyelid is always the effect of a muscular exertion, the eye in lagoph thalmos will not be covered during sleep, hence the lower can never serve as a substitute for the upper eyelid.