THE EAR. 81 



ball, and surrounding the optic nerve, are termed superior, 

 infei-ior, internal, and external recti. The other two are 

 connected with the tipper and the lower margins of the 

 orbit respectively, and pass thence to the oiiter side of the 

 bulb. These are the superior and the inferior ohliqui. In 

 many Reptiles and Mammals a continuous funnel-shaped 

 sheet of muscle, the nmscidus clioanoides, 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 nmsculus clioanoides, the external rectus, and the 

 nictitating muscle, constitiite a group of eye-muscles mor- 

 phologically distinct from the other three recti, the obliqui, 

 and the levator palpebroe superioris. In many Reptiles, and 

 in the higher Vertebrata, the eyelids are closed by circular 

 muscular fibres, constituting an orbicularis palpebrarum, 

 and are separated by straight fibres proceeding from the 

 back of the orbit, usually to the upper eyelid only, as the 

 levator palpebrm superioris; but sometimes to both lids, 

 when the lower muscle is a depressor palpebr(B inferioris. 



The Harderian and lachrymal glands are not found in 

 fishes; but the former is met with in the Batrachia, and 

 both are of common occurrence in the Sauropsida and 

 Mammalia. 



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

 liar vascular membrane, covered with pigment, like the 

 choroid, projects from near the entrance of the optic nerve, 

 on the outer side of the globe of the eye, into the vitreous 

 humour, and usually becomes connected with the capsule 

 of the lens. This is the pecten, or marsupiwm. 



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

 involution of the integument into a small sac, which is 

 situated on each side of the posterior cerebral vesicle, just 

 above the end of the second visceral cleft. The mouth of 

 the involution soon closes, and a shut sac results. The sac 

 enlarges, and, by a remarkable series of changes, its upper 

 part becomes (ordinarily) converted into three semicircular 



