THE EYE. 387 



In Fishes there is rarely any special path by which 

 impressions of sound travel from the outer world to the ear. 

 In frogs and higher Vertebrates, however, the ear has sunk 

 further into the recesses of the skull, and a special path for 

 the sound is necessary. The hyo-mandibular or spiracular 

 gill-cleft, near which the ear originally lies, is replaced by an 

 outgrowth from the pharynx called the Eustachian tube. 

 The outer part of this outgrowth expands into a tympanic 

 cavity, across which there stretches a drum or tympanum. 

 In the frog this tympanum lies on the surface of the head, 

 and is connected with the capsule of the ear by a rod known 

 as the columella. The inner end of this closes a small 

 aperture — the fenestra ovalis — in the wall of the ear-capsule. 

 In higher Vertebrates there is an external chamber or meatus 

 outside the tympanum, and the columella — which persists 

 in Sauropsida — is replaced in Mammals by three ossicles, an 

 outermost malleus, a median incus, an internal stapes. It is 

 only in Mammals, and not in all of them, that we find a 

 pinna or external projection around the opening of the 

 external chamber of the ear. From the tympanic cavity, 

 the Eustachian tube is continued down to the back of the 

 mouth. 



The homologies of the little ossicles which connect the 

 drum with the ear-capsule are not quite certain. I adhere in 

 the meantime to the following interpretation : — 



Columella of Birds, Reptiles, and Amphibians = Stapes 

 of Mammals = Hyomandibular or upper part of 

 the hyoid arch. 

 Incus of Mammals = Quadrate of Sauropsida. 

 Malleus of Mammals = Articular element of Meckel's 

 cartilage. 



(d) The Eye. — There is no eye in Amphioxus, it is rarely 

 more than larval in Tunicates, it is degenerate in Myxine, 

 and in the young lamprey. In higher forms the eye is always 

 present, though occasionally degenerate, e.g., in fishes from 

 caves or from the deep-sea. It is hidden under the skin in 

 Proteus, an aimphibian cave-dweller, and in the subterranean 

 amphibians like Ccecilia, very small in a few snakes and 

 lizards, and abortive as to its nerves in the mole. 



The adult eye is more or less globular, and its walls consist 

 of several distinct layers. The innermost layer bounding 



