THE EYE-MUSCLES IN SAUROPSIDA. 261 



In tlie Ophidia and some I/acertiUa (the AmpMsboenoidea^ 

 some Scincoidea^ all Ascalohota)^the integument is continued 

 over the eye, and becomes transparent. These reptiles are 

 commonly said to possess no eyelids ; but it must be remarked 

 that this is not true of them in the sense in which it is true of 

 most osseous fishes, as the transparent covering of the eye 

 really represents the two eyelids of the higher 'Vertehrata^ and 

 is separated from the eyeball by a chamber lined by conjunc- 

 tiva, 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 Lacertilla it contains a cartilage or an ossification. 



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

 a nictitating membrane moved by special muscles, which pre- 

 sent three different arrangements. 



In the lizards a short thick muscle {hursalis) 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 pre- 

 sphenoidal region of the inner wall of the orbit, passes back- 

 ward through the sheath, and then forward 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 

 pyramidalls muscle) arise from the inner side of the eyeball, 

 and, arching over it and the optic nerve, are inserted partly 

 into the outer edge of the nictitating membrane, partly into 

 the lower eyelid. The Crocodilia have ?i pyramidalis muscle 

 taking the same origin and course ; but it sends no fibres to 

 the lower eyelid, its tendon being inserted altogether into the 

 nictitatino- membrane. 



The third arrangement, which in a manner combines to- 

 gether the first and the second, is that seen in birds. A/'y- 

 ramidalis 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 nictitating 

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

 muscle, which however arises, not, as in lizards, from the wall 

 of the orbit, but from the upper surface of the sclerotic itself, 

 whence it passes backward and ends in a fibrous sheath which 

 encloses the tendon of the pyrmnidalis. The contraction of 



