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 t*he 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. Palpebrae. 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 Meibomlan 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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