I. THE ORGAN OF SIGHT. ORGANON VISUS.
1. The Orbit.—The bony framework of the orbital fossa, in which the organ of sight is situated, has already been described ([page 53]). The orbit is not protected by bone on all sides, being open caudad and ventrad, and partly laterad. The structures within the orbit are further protected and separated from other structures by a very thin, tough, transparent membranous sac, the periorbita, which surrounds them almost completely and nearly fills the orbit. The periorbita forms a conical sac with the apex of the cone directed caudoventrad and attached about the optic foramen and orbital fissure. The base of the cone is attached about the margin of the orbital fossa,—to the supraorbital margin of the frontal, to the maxillary and malar bones, and to the orbital ligament, connecting the frontal process of the malar with the zygomatic process of the frontal. The periorbita is partly separated from the walls of the orbital fossa by masses of fat. It comes in contact, aside from the bones, with the temporal and pterygoid muscles, and with nerves and blood-vessels in the orbit.
The sac thus formed encloses the eyeball with its muscles and glands ([Fig. 166]), as well as a mass of fat in which these are imbedded. The periorbita and its contents are freely movable with relation to surrounding structures.
2. The Eyelids. Palpebræ.—The eyelids are two thin folds which protect the eye. Externally they are covered with hair; internally by the thin membranous conjunctiva. The eyelids contain the Meibomian or tarsal glands, arranged in short rows passing perpendicularly inward from the edge of the lids; these may sometimes be seen with the naked eye as broad yellowish lines. They secrete a substance which prevents the adhesion of the edges of the two lids. On the edge of each eyelid, three to four millimeters from the medial angle of the eye, is one of the openings of the lachrymal canals.
At the medial angle of the eye is the large nictitating membrane (membrana nictitans), or “third eyelid,” corresponding to the plica semilunaris of man. In the cat this is large and may cover the whole surface of the eye. It is supported ([Fig. 165]) by a broad central strip of cartilage (a), passing from its edge to its inner angle, and the inner end of this cartilage is surrounded, on the medial (concave) side of the membrane, by the large lobulated Harderian glands (b).
Fig. 165.—Inner Surface of Membrana nictitans,
showing the supporting cartilage and Harderian glands. a, supporting cartilage; b, Harderian glands; c, outer edge.
The conjunctiva is the thin membrane covering the inner surface of the lids, the outer surface of the eyeball, and both surfaces of the nictitating membrane.
The muscles of the eyelids are M. orbicularis oculi, already described ([page 98]), and M. levator palpebræ superioris, [described] below.