THE EYE-MtrSCLES IN SAUEOPSIDA. 305 



sense in whicli it is true of most osseous fishes, as the 

 transparent covering of the eye really rej^resents the 

 two eyelids of the higher Vertebrata, and is separated 

 from the eyeball by a chamber lined by conjunctiva, 

 which communicates with the nose by a lachrymal canal. 

 In the other Sauropsida two lids are developed, and each 

 generally possesses a special palpebral muscle, which 

 acts as an elevator of the upper, and a depressor of the 

 lower, lid. In some Scincoidea the middle of the lower lid 

 is transparent. In many Lacertilia it contains a cartilage 

 or an ossification. 



Most lizards, all Chelonia, Crocodilia, and Aves, possess 

 a nictitating membrane moved by special muscles, which 

 present three different arrangements. 



In the lizards a short thick muscle (hursalisj is attached 

 to the inner and posterior wall of the orbit, and ends in a 

 fibrous sheath. A tendon, one end of which is attached to 

 the presphenoidal region of the inner wall of the orbit, 

 passes backwards through the sheath, and then forwards to 

 be attached to the nictitating membrane. "When the muscle 

 contracts, it necessarily pulls the latter over the eye. A 

 Harderian gland is always developed, and a lachrymal gland 

 very generally, though not always. 



In the Chelonia, muscular fibres (forming the so-called 

 pyrainidalis muscle) arise from the inner side of the eye- 

 ball, and, arching over it and the optic neiwe, are inserted 

 partly into the outer edge of the nictitating membrane, 

 partly into the lower eyelid. The Crocodilia have a py- 

 ramidalis mxiscle taking the same origin and coiu'se ; but it 

 sends no fibres to the lower eyelid, its tendon being inserted 

 altogether into the nictitating membrane. 



The third arrangement, which in a manner combines 

 together the first and the second, is that seen in birds. A 

 pyramidcdis muscle, arising from the inner and under surface 

 of the eyeball, soon ends in a tendon which sweeps round 

 the upper and outer surfaces of the sclerotic to the nicti- 

 tating membrane, as in the crocodiles. But there is also a 

 bursaUs muscle, which however arises, not, as in lizards, 



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