55^ 



STRUCTURE OF VERTEBRATA 



With regard to. the function of the parts of the ear, the semicircular 

 canals are believed by many to be concerned with the appreciation of a 

 change in the direction or velocity of movement. How far the ears of 

 Invertebrates {e.g. Crustacea and Mollusca) are adapted for any function 

 except this, is still doubtful, and we can hardly see that any other 

 would be of much use to purely aquatic animals. It seems likely at 

 any rate that the primitive function of the ear was the perception of 

 vibrations, and that from this both the sense of hearing and the sense 

 of equilibration have been differentiated. 



It is in accordance with the facts mentioned above that we rarely 

 find in Fishes any special path by which impressions of sound may 



travel from the external 

 world to the ear. In 

 Amphibians and higher 

 Vertebrates, however, the 

 ear has sunk farther into 

 the recesses of the skull, 

 and a special path for the 

 sound is present. In 

 Elasmobranchs, the spir- 

 acle, or first gill-cleft, is 

 situated in the vicinity 

 of the ear ; in higher 

 forms, according to many 

 authors, this first gill-cleft 

 is metamorphosed into 

 the conducting apparatus 

 of the ear. In develop- 

 ment, 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 

 projection 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 interpreta- 

 tion has been stated on p. 793 ; the following is Hertwig's : — 



Malleus = Articular + angular elements of Meckel's cartilage. 

 Incus =Palato-quadrate of lower Vertebrates. 



Fig. 312. — Diagram of the eye. 



Cornea ; a.h., aqueous humour ; c.b., ciliary 

 body ; I., lens ; /., iris ; Sc, sclerotic ; Ch., 

 choroid ; R., retina ; v.h., vitreous humour ; 

 y.sp., yellow spot ; n., optic nerve. 



