74 THE ANATOMY OF VERTEBRA TED ANIMALS. 



wliethei' in these fishes, and in the Lampreys, the ej'e is de- 

 vehjped in the same way as in other Vertehrata. 



In all other Vertehrata, the eyes have the typical structure, 

 though sometimes, as in the Blind-fish {Amblyo2)sis) and the 

 Mole, they have no functional importance. In the Ichthy- 

 opsida and Sauropsida, but not in JSIammalia, the sclerotic 

 is often partially ossified, the ossification usually forming a 

 ring around its anterior moiety. It becomes enormously 

 thickened in the CeUicea. 



Except in A.mphtoxus and the Myxinoid fishes, the ej'e- 

 ball is moved by six muscles ; of these, four, proceeding from 

 the interior of the orbit to the periphery of the eyeball, and 

 surrounding tlie optic nerve, are termed superior, inferior, in- 

 terna], and external recti. The other two are connected with 

 the upper and the lov/er margins of the orbit respectively, and 

 pass thence to the outer side of the bulb. These are the supe- 

 rior and the inferior ohliqui. In many Reptiles and Mam- 

 mals a continuous funnel-shaped sheet of muscle, the muscit- 

 lus choanoides, lies within the four recti, and is attached to 

 the circumference of the posterior moiety of the ball of the eye. 

 It would appear, from the distribution of the nerves, which 

 has already been described, that the musculus choanoides, 

 tlie external rectus, and the nictitating muscle, constitute 

 a group of eye-muscles morphologically distinct from the othei 

 three recti, the ohliqui, and the levator palpebroB superioris. 

 In many Reptiles, and in the higher Vertehrata, the eyelids 

 are closed by circular muscular fibres, constituting an orbicu- 

 laris palpelrrarum, and are separated by straight fibres pro- 

 ceeding from the back of the orbit, usually to the upper eye- 

 lid only, as the levator palpjelnM superioris ; but sometimes to 

 both lids, when the lower muscle is a depressor palpehrm infe- 

 rioris. 



The Harderian and lachrymal glands are not found in 

 fishes ; but the former is met with in the Satrachia , and both 

 are of common occurrence in the Sauropsida and Mamnicdia. 



In Zidcertilia, Crocodilia, Aves, and many Fishes, a pecu- 

 liar vascular membrane, covered with pigment, like the cho- 

 roid, projects from near the entrance of the optic nerve, on the 

 outer side of the globe of the eye, info the ^dtreous humor, 

 M rid usually becomes connected with the capsule of the lens. 

 This is i\\e pecten, or marstipium. 



Tlie Ear. — The first rudiment of the internal ear is an in 

 i^ohuion of the integument into a small sac, which is situated 



