424 



ANATOMY FOR NURSES 



[Chap. XX 



of these bones are mesial (frontal, ethmoid, and sphenoid) there 

 are only eleven bones forming both orbits. 



The orbit is shaped like a four-sided pyramid ; the apex, directed 

 backward and inward, is pierced by a large opening — the 

 optic foramen — through which the optic nerve and the ophthalmic 

 artery pass from the cranial cavity to the eye. A larger opening 

 to the outer side of the optic foramen — the sphenoidal fissure — 

 provides a passage for the ophthalmic vein and the nerves which 



SUPERIOR RECTUS 



CHOROrO 



OPTIC NERVE 



iNFERIOBRfCTUS 



Fig. 198. — Diagrammatic Section of the Eye. (Flint.) 



carry impulses to and from the muscles, i.e. the motor oculi, 

 the pathetic, the abducens, and the ophthalmic. The base of the 

 orbit, directed outward and forward, forms a strong, bony edge 

 for protecting the eyeball from injury. 



Each orbit averages about 2 inches (50 mm.) in depth, is lined 

 with fibrous tissue, and contains a pad of fat, which serves as a 

 support for the eyeball. A condition of emaciation is usually 

 accompanied by sunken eyes, which results from the absorption 

 of this fat, and the consequent sinking of the eyeballs in the orbits. 

 Between the pad of fat and the eyeball is a serous sac — =- the 

 capsule of Tenon — which envelops the eyeball from the optic 

 nerve to the ciliary region and forms a socket in which the eyeball 

 rotates. This sac secretes a lubricating fluid, the function of 

 which is to prevent friction when the eyeball moves. 



