In operations on the eyeball when the eyelids are held apart unskilfully, the folds are thrust out between the eyelids by the action of the orbicularis muscle, so that they almost bury the front of the eyeball and consequently im pede the operator.
By long-continued catarrhal ophthalmia and the abuse of blue stone and similar escharotics, the conjunctiva is apt to become contracted and thickened, and to acquire at the same time a callous articular surface. In such cases the contraction tells very much upon the looseness of the folds of the conjunctiva at the upper and lower palpebral sinuses, which may indeed be said to be obliterated. The consequence of this is great restriction in all the movements of the eyeball.
Foreign bodies which may have entered the oculo-palpebral space sometimes get lodged in the palpebral sinuses of the conjunctiva, especially the upper, and may be retained there for a length of time without causing much or any irritation, the conjunctiva being there so loose and the adjacent cellular and adipose tissue of the orbit so soft that the body is not much pressed upon by the opposing surfaces. The contrary is the case when the foreign body lies between the eyeball and the firm part of the eyelid, for here its irritation excites the orbicularis muscle to stronger action which serves but to aggravate the distress.
Disposition of the conjunctiva at the inner canthus.—Under this head falls to be consi dered the semilunar fold, the notice of which it will be advantageous to premise by a descrip tion of the lacrymal caruncle. In consequence of the prolongation of the palpebral fissure at the inner canthus into a secondary one, the lacrymal caruncle and semilunar fold are so exposed that their external conformation can be readily and indeed best studied in the living eye.
Lacrymal caruncle, cartincula lacrymalis. Fr. La caroacule lacrymale. Ital. La carun oila lagrimale. Germ. Die Thriinenkarunkel. Tills is a small reddish yellow eminence having a slightly tuberculated surface, beset with very delicate scarcely visible hairs. It is situated, as has been said, within the secondary fissure of the inner canthus, and inclosed between the two slips of the tondo palpebrarum. To see
the lacrymal caruncle in its whole extent, it is necessary to evert slightly the lower eyelid, when it is observed running into a point down wards and outwards. The lacrymal caruncle consists of a mass of loose fibro-cartilaginous tissue, similar to that of the tarsal cartilages, in which are imbedded follicles, secreting a fluid of the same nature as that of the Meibomian glands, and pouring it out by twelve or fifteen excretory orifices on its surface, which is in vested by the conjunctiva. Anciently the la crymal caruncle was thought to be the secreting organ of the tears, and the lacrymal points the excretory orifices.
Semilunar fold, plica semilunaris. Fr. Le repli semilunaire. Ital. La piega semilunare. Germ. Die halbmondfirmigen Falte. In passing from the caruncle to the eyeball, the conjunctiva forms a vertical semilunar fold which encloses at its free edge a minute cartilage of a nature similar to the tarsal cartilages. This part of the conjunctiva is distinguished from the ocular portion by its reddish colour and greater thick ness, indeed it resembles more the palpebral conjunctiva than the ocular. The concavity of the crescent, which is also the free edge of the fold, is towards the cornea. The rolling of the eyeball outwards has a tendency to undo the fold, which on the contrary is rendered more distinct when the cornea is turned towards the nose. In quadrupeds the semilunar fold is much more developed, and contains within it a more distinct cartilaginous plate. It consti tutes what in them is called membrana nictitans. The third eyelid in birds is the same structure carried to its highest pitch of development. In man, in whom it is very small, its component structures are readily developed to a consider able size by inflammation. According to Soem rnerring the semilunar fold is larger in the negroes. We shall have occasion to recur to the membrana nictitans of quadrupeds and the third eyelid of birds.