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 
Fic. 263.—Diagram showing the ear and related parts 
in a young cat. 
P., Pinna; Sg., squamosal: £.A.M., external auditory meatus; 7., 
tympanum; 47., malleus; /., incus; S7., stapes abutting on foramen 
ovale; Z., bulla of tympanic bone; Se., a septum in the bulla; £.7., 
eustachian tube leading from the tympanic cavity to the back of the 
mouth; B.O., basi-occipital: C., cochlea; S., sacculus; U., utriculus ; 
D.£., ductus endolymphaticus; .V., auditory nerve; S.C., semi- 
circular canal; PZ., periotic bone. 
change in the direction or velocity of movement. Tow far the ears of 
Invertebrates (¢.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, 
