As to the intimate composition of the tarsal cartilages, they consist of what is called fibro cartilage, a microscopically fibrous substance, without any of the corpuscles of common carti lage.
In the human lower eyelid, the thickness of this substance is inconsiderable, and its con sistence not so great as in the upper. In the lower animals it is in the same state in both eyelids. This is what has led Zeiss-l- to say that he never found a real cartilaginous tarsus in the human lower eyelid, nor among the lower mammifera in the upper eyelid either. " In the sow only," says he, "there is a nearer ap proach to a tarsal cartilage than is to be found in any other of the lower animals." Miiller* has very well explained away all this difference of opinion, by showing that the dense cellular tissue which, according to Zeiss, occupies the place of tarsal cartilage in the human lower eyelid, and in both those of the inferior mammi fera, is the same tissue, as the more consistent fibro-cartilage of the human upper eyelid, only in a less condensed state.
The Meibomian glands are commonly de scribed as being situate between the palpebral conjunctiva and tarsal cartilage. Winslow, Haller, and Zinn describe them as lying in grooves on the posterior surface of the tarsal cartilages. The Meibomian glands are seen very distinctly from the inside of the eyelids, as if they were immediately underneath the con junctiva. But if the skin and orbicularis muscle be removed front the outside, these glands be come equally vile there. "Where, then," asks Zeiss,-I- "do they lie,—before or behind the tarsal cartilage?" Examination of sections of the cartilage shows. that the Meibomian glands lie in the substance of the tarsal cartilage itself; and in the human lower eyelid and in both those of the lower animals, in the less consistent fibrous structure which there composes the tarsus.
At the outer canthus the cellulo-membra neous expansions called tarsal ligaments are stronger, and form bands which decussate and thus tie the tarsal cartilages to each other and to the outer margin of the orbit. These bands compose what is called the external palpebral ligament.
The internal palpebral ligament is the tendon of the orbicularis palpebrarum muscle,—tendo oculi, or tendo palpebrarum. This, to adopt the description of Professor Harrison of Dub lin, " is a small horizontal tendon, nearly one quarter of an inch in length. It is inserted in ternally into the upper end of the nasal process of the superior maxillary bone ; thence it passes outwards and backwards to the internal corn missure of the eyelids, where it forks into two slips which enclose the caruncula lacrymalis, and are then inserted each into the tarsal carti lage and the lacrymal duct." I
Orbicularis palpebrarummuscle.—This is de scribed in the article FACE, VOl.ii. p. 221. Here we shall only advert to some particular points in its history. The fibres of the orbicularis pertaining to the upper eyelid arise from the internal angular process of the frontal bone, and from the upper edge of the tendo palpebrarum, and proceed, forming a curve, at first upwards and outwards, and then downwards and out wards, within the upper eyelid and along and over the upper edge of the orbit towards the temple and outer angle of the eye. Here they meet those of the lower eyelid which have come from the nasal process of the upper jaw-bone, and from the lower edge of the Fdpebral ten don, curving at first downwards and outwards, then upwards and outwards, within the lower eyelid and along the edge of the orbit, extend ing some way down over the cheek. The part of the orbicularis more immediately contained within the eyelids, sometimes called the palpc bral and ciliary portions, in contradistinction to the outermost fibres, which are without the eye lids, and encircle the base of the orbit, therefore called the orbital portion, consists ofpale thin fibres, of which those at the margin of the eye lids are collected into a considerable fasciculus, having interposed between them and the tarsal cartilage the roots of the cilia. At the outer canthus the upper and lower fibres intercross and adhere to the external palpebral ligament. That many of the fibres of the orbicularis are inserted into and exert their action in a great degree on the skin of the eyelids, may be easily ascertained in the living person, by observing, during the action of closing the eye, the traction of the skin of the eyelids towards the nasal can thus. By this traction, the skin, especially in the lower eyelid, is very much corrugated. This corrugation of the skin of the lower eyelid by the action of the orbicularis is greatest right over the lower part of the lacrymal sac,—that part which we commonly press upon when we want to evacuate any accumulation of mucus or tears,—that part where abscess of the sac gene rally bursts and leaves a fistulous opening,— that part which we open in the operation for so called fistula lacrymalis. This part of the lacrymal sac must therefore be immediately affected by the contraction of the muscle, and the pressure thus produced, together with that on the upper blind end of the sac by the supe rior fibres, will promote the transmission of the tears and conjunctival mucus into the nasal duct.