The progress of the development of the eye lids is sometimes arrested, so that mere folds of skin have been found occupying their places, or, development having proceeded a little further, the eyelids have been found presenting their regular conformation indeed, but too short to cover the eyeball and incapable of motion.
Having met, the edges of the eyelids ad here by the extension of the epidermis from the one to the miter. In the human embryo the adhesion between the eyelids by the exten sion of the skin ceases towards the latter months, but the edges continue sticking together by the Meibomian secretion until the period of birth. In the young of several of the mammi fera, as the carnivora and rodents, the eyelids continue closed for some time after .birth— from one to two weeks. In birds, even in the embryo state, the eyelids never unite.
Sometimes adhesion between the eyelids in the human subject is found at birth, constitut ing what is called congenital anehyloblepAaran ; and this may be either immediate or by the intervention of a membranous structure.
The closure of the pupil by the pupillary membrane in the fmtus corresponds to the ad hesion of the eyelids to each other at that period.
According to Meckel the pupillary membrane continues entire in animals born blind, as it is expressed, as long as the eyelids remain closed.
The tarsal cartilages first appear distinctly in the fifth month ; and at birth, that of the up per is perfectly developed. The eyelashes first appear free about the sixth month.
The lacrymal gland is already evident in the last half of the fourth month.
The inner canthus of the eye is at first more elongated than it is afterwards.
On the •first appearance of the eyelids, Burdach tells us, the lacrymal caruncle presents itself; and at the inner angle a di verticulurn of the conjunctiva sinks down to the oro-nasal cavity as the commencement of the lacrymal sac and nasal duct. The lacrymal points project very much in the fifth month, and in the seventh are somewhat more retracted. The lacrymal apparatus in general, as also the Meibomian follicles, are proportionably much developed at an early period.