SENSE ORGANS. 



495 



c.b, 



the spiracle, or first gill -cleft, is situated in the vicinity of the ear ; in 

 higher forms, according to many authors, this first gill-cleft is metamor- 

 phosed into the conducting apparatus of the ear. In development, a 

 depression beneath the closed gill-cleft unites with an outgrowth from 

 the pharynx, and thus forms the tympanic cavity, which communicates 

 with the back of the mouth by the Eustachian tube. The tympanic 

 cavity is closed externally by the drum or tympanum, which may be 

 flush with the surface, as in the frog, or may lie at the end of a narrow 

 passage, which in many Mammals is furnished externally with a projec- 

 tion or pinna. In Amphibia and Sauropsida the tympanic cavity is 

 traversed by a bony rod the columella, which extends from the drum 

 to the fenestra ovalis, a little aperture in the wall of the bony labyrinth. 

 In Mammals this is replaced by a chain of three ossicles, an outermost 

 malleus, a median incus, an internal stapes. 



The homologies of these 

 ossicles are still uncertain. 

 One interpretation has 

 been stated on p. 480 ; the 

 following is Hertwig's : 

 Malleus = Articular -f 

 angular elements of 

 Meckel's cartilage. 

 Incus = Palato-quad- 

 rate of lower Verte- 

 brates. 



Stapes of Mammals 

 has a double origin, 

 being formed from 

 the upper part of 

 hy oid arch + an ossi - 

 fication from the 

 wall of the ear cap- 

 sule = (wholly?) col- 

 umella of Birds, 

 Reptiles, and Am- 

 phibians. 



y.sp. 



FIG. 264. Diagram of the eye. 



C., Cornea ; a./t., aqueous humour ; c,b., ciliary 



The eye. There is body; /., lens; /., iris; Sc., sclerotic; CA., 



A .-! choroid ; /?., retina; v.7i., vitreous humour ; 



no eye in Amphtoxus, y . sp ^ ye f llow ' spot . .; optic ' nerve . 

 it is rarely more than 



larval in Tunicates, it is rudimentary 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 amphibian cave-dweller, and in the 

 subterranean amphibians like Ccedlia, very small in a few 

 snakes and lizards, and its nerves are abortive in the 

 mole. 



33 



