494 STRUCTURE OF VERTEBRATA. 



The parts which we have so far considered constitute together the 

 membranous labyrinth of the ear. Round about them the mesoblast 

 (mesenchyme) forms a two-layered envelope. Its inner layer disin- 

 tegrates to produce a fluid, the perilymph, which bathes the whole 

 outer surface of the membranous labyrinth. Its outer layer forms a 

 firm case, the cartilaginous or bony labyrinth, surrounding the internal 

 ear. The membranous labyrinth itself contains another fluid, the 

 endolymph. 



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 



-PE 



FIG. 263. Diagram showing the ear and related parts 

 in a young cat. 



/., Pinna; Sq., squamosal: E.A.M., external auditory meatus; T. t 

 tympanum; M., malleus; /., incus; St., stapes abutting on foramen 

 ovale ; B. t bulla of tympanic bone ; St., a septum in the bulla ; E. T. t 

 eustachian tube leading from the tympanic cavity to the back of the 

 mouth; B.O., basi-occipital ; C 1 ., cochlea; S., sacculus; /., utriculus; 

 D*E. t ductus endolymphaticus ; IV., auditory nerve; S.C., semi- 

 circular canal ; PE., periotic bone. 



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 Elasrnobranchs, 



