AUDITORY ORGANS. 



'85 



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. 



FIG. 185. FIG. 186. 



FIG. !&$.'- 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; u, utriculus; v, vestibular nerve. 



FIG. 186. Section through one of the coils of cochlea of guinea pig, after Schneider. 

 Bone lined; Is, spiral ligament; r, Reissner's membrane; sg, spiral ganglion; sm, st, sv, 

 scalae 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 epithelium 

 divides into separate areas (fig. 184), some of which (maculae 

 acusticse) have sensory cells with short hairs or bristles, while others 

 (cristae acusticae), characteristic of the ampullae, 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 



