PROTECTIVE MECHANISMS OF THE EYEBALL 95 



All terrestrial mammals, with the exception of man and 

 monkeys, possess three eyelids for the protection of the 

 surface of the eye. Man and monkeys have two, the third 

 eyelid being reduced to a vestigial structure, the semilunar 

 fold. 



In birds the third eyelid, or nictitating membrane, is met 

 with in the most highly developed condition; it has in them 

 a muscle directly attached to it which draws it forward in 

 front of the eyeball. In the Mammifera the movements 

 of the third eyelid are most extensive in the Ungulata, and 

 are produced indirectly by the contraction of a muscle 

 which draws backward the eyeball and pushes forward the 

 orbital fat. The third eyelid in mammals is composed of 

 a plate of cartilage covered on both surfaces by mucous 

 membrane. Anteriorly it has a free even margin fitting 

 accurately the surface of the cornea, over which it glides. 

 Posteriorly it terminates in a considerably thickened edge 

 which penetrates into the depths of the orbit on the inner 

 surface of the globe, where it is continuous with a mass of 

 fat. The muscle which draws back the eyeball —the retractor 

 bulbi— arises from the bones at the apex of the orbit, and 

 is inserted into the sclerotic inside the attachments of the 

 recti muscles, between them and the optic nerve. It encircles 

 the latter in a cave-like fashion, hence it is often spoken of 

 as "the choanoid muscle" (Fig. 22). When the eyeball is 

 drawn back in the orbit by this muscle the fat of the orbit, 

 with the base of the third eyelid attached to it, is pressed 

 forward, so that the anterior margin of the eyelid becomes 

 swept across the front of the eye. 



In mammals which possess this choanoid muscle the bony 

 outer wall of the orbit is incomplete, the space between the 

 bones being filled up by a fibrous membrane termed the 

 "cornet," which contains muscular tissue, "the orbital 



