74 THE ANATOMY OF YERTEBRATED ANIMALS. 



whether in these fishes, and in the Lampreys, the eye is de 

 veloped in the same way as in other Vertebrata. 



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

 though sometimes, as in the Blind-fish (Ai^ibl^/opsis) and the 

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

 opsida and S>auropsida^ but not in 3Iain)nalia^ the sclerotic 

 is often partially ossified, the ossification usually forming- a 

 ring around its anterior moiety. It becomes enormously 

 thickened in the Cetacea. 



Except in Amphioxus and the Myxinoid fishes, the eye- 

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

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

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

 ternal, 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 siipe^ 

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

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

 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 rtmseidus cJioanoides^ 

 the external rectus^ and the nictitating muscle, constitute 

 a group of eye-muscles morphologically distinct from the othei 

 three o^ecti^ the obliqui^ and the levator palpebrce superioris. 

 In many Reptiles, and in the higher Vertebrata^ the eyelids 

 are closed by circular muscular fibres, constituting an orbicvr 

 laris palpebrarum^ and are separated by straight fibres pro- 

 ceeding from the back of the orbit, usually to the upper ej'C- 

 lid only, as the levator palpebrm superioris / but sometimes to 

 both lids, when the lower muscle is a depressor palpebrm mfe- 

 rioris. 



The Harderian and lachrymal glands are not found in 

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

 are of common occurrence in the Sauropsida and 3Iammalia. 



In Lacertilia, GrocodiHa, 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, into the vitreous humor, 

 and usually becomes connected with the capsule of the lens. 

 This is the p)ecten^ or marsupimn. 



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

 volution of the integument into a small sac, wliich is situated 



