SENSE-ORGANS AND INTEGUMENT. 
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 sepa- 
rated 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, con- 
necting 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. Palpebree.—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 
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