AUDITORY ORGANS. 185 
creasing powers of hearing the lagena becomes greatly elongate, until 
in the mammals it acquires a peculiar development and is known as the 
scala media, the structure and relations of which are described below. 
In the cyclostomes utriculus and sacculus are not differentiated. In the 
myxinoids there is but a single semicircular canal, with, however, an ampulla at 
either end. In the lampreys there are two canals, both in the vertical plane, and 
each with an ampulla at its lower end. 
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Fic. 185. Fic. 186, 
Fic. 185.—Labyrinth of human embryo, 30 mm. long, after Streeter. a, ampulla; ac, 
anterior canal; c, cochlea; cr, crus; de, endolymph canal; nc, cochlear nerve; s, sacculus; se, 
endolymph sac; 4, utriculus; v, vestibular nerve. 
Fic. 186.—Section through one of the coils of cochlea of guinea pig, after Schneider. 
Bone lined; /s, spiral ligament; 7, Reissner’s membrane; sg, spiral ganglion; sm, st, sv, 
scale media (ductus cochlearis), tympani and vestibuli. 
These parts of the internal ear form the membranous labyrinth. 
With the formation of canals, lagena, etc., the sensory epithclium 
divides into separate areas (fig. 184), some of which (macule 
acusticz) have sensory cells with short hairs or bristles, while others 
(cristee acustice), characteristic of the ampulle, have cells with longer 
hairs. The membranous labyrinth is filled with a fluid, the endo- 
lymph, in which are solid particles, the otoliths. These are usually 
microscopic crystals of calcium carbonate which give the endolymph 
