In man the upper eyelid is the more de veloped and the mare moveable. Descending from him the lower is found gradually to assume the superiority in this respect. In the cetacea the upper and lower eyelids are tumid folds of skin enclosing fat, but no tarsus, and the Meibomian glands are entirely wanting. There is no orbicularis palpebrarum muscle, but muscular fibres proceed from the anterior and posterior end of the orbital process of the frontal bone to the eyelids in the region of the outer and inner canthi. Instead of the levator palpebrre superioris there is a hollow conical muscle which arises from the circumference of the foramen opticum and terminates in the eye lids. A similar in four divisions occurs in the seal.* In the echidna there is a circular eye lid. In birds the lower eyelid is in general very much larger and more moveable than the upper. In the ostrich and some parrots both eyelids are equally moveable.
In birds the eyelids are closed in death. In the gallinaceous birds which I have examined this appears to be owing to an expansion of elastic membrane attached to the margin of the orbit and interlaced round the eyelids. During life the lower eyelid is opened by muscular action by a proper depressor muscle. The upper eyelid retains its levator. There is a tarsal cartilage in the lower eyelid; in the upper eyelid there is a less dense fibrous struc ture. The skin of the lower eyelid is naked and finer than that of the upper in accordance with its greater mobility.
In chelonian reptiles the lower eyelid is, as in birds, the larger and more moveable ; and as the more moveable the eyelid the finer the integument, so in them the lower eyelid is naked as in birds.
In lizards the eyelids form a sort of sheet stretched before the eyeball with a horizontal fissure closed by a circular muscle and opened by a levator and depressor. In the chameleon the palpebral fissure is represented by a small hole opposite the pupil. Tracing the eyelids in a general way from batrachian reptiles to fishes, we find a gradual depreciation of struc ture ; thus salamanders have two folds of skin, upper and under, for eyelids, but not sufficient to cover the eyeball, whilst the pipa has none. In fishes generally, the skin before passing over the front of the eye becomes finer and forms a slight circular fold, often only well-marked above. Sometimes there is an anterior and a posterior semilunar fold, as in the herring, salmon, but especially in several of the shark tribe. It is the Orthagoriscus mola which has circular folds for eyelids, and an orbicular mus cle for closing them. The eyelids are again opened by five radiating muscles.
Among the invertebrata some of the cepha lopoda have large palpebral folds. In the octopus " the eyes are small in proportion, and the skin is drawn over them so as to cover them entirely at the will of the animal." There are
no eyelids in the sepia officinalis, but a con tinuation of the integuments passes over the eye.
Eyebrows and eyelashes occur only in a few mammalia. Eyelashes exist in the pachyder mata, ruminants, &c. but are wanting especi. ally in small mammals. Meibomian glands are commonly found.
In birds the eyelids sometimes present cilia. This is the case only in some birds of prey, in some parrots, in the ostrich, &c. but seldom iu other orders. Very small Meibomian follicles are said to exist inothe eyelids of birds. In the eyelids of a common fowl at present before me, there are small transverse fissures on the mar gin of the lids filled with a sebaceous matter. They have the appearance of very small Mei bomian glands, not closed as in the mammifera, but open along their whole length.
In a preceding part of this article I remarked on some points of resemblance between the iris and eyelids, as regards functions, and sym pathy in the performance of those functions. In some of the lower animals there are certain points in which they even approximate in form. In connexion, therefore, with the subject or the eyelids it will not be out of place to allude to that flocculent growth of the uvea hanging from the upper margin of the iris over the trans versely elongated pupil in the horse, &c. and which appears to serve the purpose, if it may be so expressed, of an internal eyelid. An analogous but more curiously and highly deve loped structure—a blending as it were of the iris with some remains of the structure of the eyelids—exists in the eyes of several fishes ; among others in the skate.
The structure alluded to is a digitated exten sion of the whole substance of the upper part of the iris, hanging over the pupil, which it is large enough entirely to cover. There being no intrinsic power of motion in the iris of fishes, the mechanism by which this digitated veil is drawn up from over the pupil is this :—Where the upper part of the ciliary margin of the iris is connected with the sclerotica, the latter is very flexible and is externally intimately con nected with a fold or rudimentary eyelid which the integument forms before passing as con junctiva over the eye. Muscular fibres are inserted into this fold at the point of connexion, and draw it upwards ; the flexible part of the sclerotica of course follows this movement and the upper part of the iris, the sclerotica, so that the digitated veil is drawn up from over the pupil. In a young skate which I removed from the egg and preserved alive for some weeks, I observed that the digitated veil was kept down during the day, but was drawn up toward evening, and the large black pupil exposed.