2.-- The conjunctiva.—Semilurtar fold, mem brana nictitans or third eyelid.—Lacrynial caruncle and glandalc o f harder.
The existence of eyelids supposes a con junctiva,—that is, the integument modified into a mucous membrane lining the posterior surface of the eyelids, covering the front of the eyeball, and thus connecting these two parts together. In those animals in which there is no palpebral covering either in the usual form of eyelids, or in that anomalous form in which there is no palpebral opening, there is, properly speakingom conjunctiva : at the most, the com mon integument may be softer, thinner, and more transparent where it covers the eye.
The points deserving particular notice in the comparative anatomy of the conjunctiva are, 1. The oculo-palpebral space of the con junctiva.
2. The membrana nictitans and third eyelid, with the glandule of Harder.
In most animals the oculo-palpebral space of the conjunctiva is as it has been described in man. Serpents have been generally regarded as being without eyelids and vire lacrymales. The eye, indeed, has been commonly described as covered by a transparent lamella of epider mis, which is cast with the rest of the epider mis; and this has been often adduced as an ar gument in favour of the extension of the con junctiva over the cornea in other animals. Ser pents have a palpebral covering, a conjunctiva and vim lacrymales ; but the conformation of these parts is quite peculiar. For the first true exposition of the point we are indebted to Jules Cloquet' In the article HEARING, ORGAN OF, at the end of the section on the parallel between the ear and the eye, it is said, " A part in the composition of the appendages of the eye analogous to the membrana tympani is only to be conceived by supposing the exist ence of a mediate anchylo-blepharon, that is, an irregular membrane stretched between the edges of the eyelids, uniting them together, and closing in the space lined by the conjunctiva, which space would now communicate with the exterior only by the lacrymal canalicules and nasal duct, in the same way that the tympanic cavity communicates with the exterior only by the Eustachian tube." The Mediate anchylo-ble pharon here supposed is the actual and regular structure in serpents, and is the sole cause of the apparent anomaly in the conformation of their eye appendages. The structure is this:—
Around the margin of the orbit the skin ap pears as if it formed a palpebral fold, covered with scales, and representing a sort of frame, in which is set, as it were, a transparent continua tion of the skin before the fro»t of the eye. But this is not the conjunctiva; it is a natural me diate anchylo-blepharon, or a palpebral covering without palpebral fissure. It is, however, transparent, and was therefore formerly con founded with the cornea. A conjunctiva lines the inner surface of this palpebral covering without palpebral fissure, and invests the greater part of the orbital. cavity, from which it is re flected on the sclerotica, and is from thence continued over the cornea, closely adhering to it.
The conjunctiva thus forms a sac (called by, Jules Cloquet oculo-palpebral sac of the con-, junctiva), confining a space (which may in likl manner be called oculo-palpebral) into which, open the excretory ducts of the lacrymal gland. From it the tears are drawn off by a canal be-.
tweet) the jaw and palate bone into the mout of which more will be said farther on.
From the above description it will be readily understood how it is from the palpebral exten sion of the skin that the epidermis falls when the skin is said to be cast, and not from the con junctiva.
Muller' has found the above structure in all true serpents, even in the amphisbcenm, whose eyes are covered with thick skin. I do not know if it has been observed in ccecilim also. He has found a similar structure among the saurian reptiles in the geckoes, even in the genus phyllurus. He has even found it in a mammal, spalax typhlus,-1- the eyes of which appear to be covered by the thick hairy skin, underneath which, however, there is a small sac of conjunctiva. In the chameleons, which follow the family of the geckoes, in Cuvier's arrangement, there is a near approach to the same structure, the palpebral covering present ing only a very small palpebral opening oppo site the pupil.