STRUCTURE OF THE LABYRINTH. 



bone (Fig. 325, III, R). Another small canal, the aqueduct of the cochlea, is a 

 narrow passage that begins in the scala tympani, just in front of the round 

 window and emerges near the jugular fossa. It forms a communication be- 

 tween the perilymph of the cochlea and the subarachnoid space. 



Semicircular Canals and Saccules. The membranous semicircular canals 

 do not fill the corresponding osseous cavities completely, but are separated from 

 the walls by a rather wide space, which is filled with the perilymph. On the 

 concave margin alone they are more closely attached to the bone by connective 

 tissue. The ampullag, however, fill the bony cavities completely. Semicircular 

 canals and saccules consist of an outer vascular connective-tissue layer, upon 

 which lies a hyaloid membrane, bearing a single layer of squamous epithelium. 

 The vestibular branch of the auditory nerve sends a twig to each ampulla and to 

 the saccule and the utricle. In the ampullae (Fig. 325, II, A) the nerve-ending 

 (c) lies on a yellowish, equatorial ledge, which projects into the interior (crista 

 acustica). The medullated nerve-fibers (n), passing through ganglia, form a 

 plexus in the connective-tissue layer, then lose their sheaths near the basement 

 membrane and end by means of telodendrites by contact in the characteristic 

 cells, each of which is provided with an immovable, rigid bristle (o, p), 90 



FIG. 326. Organ of Corti. 



long, and which are situated on the crista; between them are indifferent cylin- 

 drical cells (hair-cells, a), which often contain yellowish pigment-granules. The 

 bristles or auditory hairs are composed of many fine fibers. An exceedingly 

 delicate membrane (membrana tectoria) covers the hairs. The nerve-ending's 

 in the maculae acusticse of the saccule and utricle are exactly the same as in the 

 ampullae, except that the free surface of the membrana tectoria is covered with 

 small chalky-white otoliths (II, T) composed of calcium carbonate. These are 

 partly amorphous, and partly in the form of arragonite, with a minute central 

 nucleus, and they lie fixed in the homogeneous membrane of the otoliths. Here 

 also the nonmedullated axis-cylinders of the saccular nerves come into contact 

 with the hair-cells, through the medium of telodendrites. 



Cochlea. Only that portion of the cochlear canal or duct (Fig. 325, I, C.c, 

 and III, Cc, and Fig. 326) that is covered by the membrane of Reissner, and 

 whose endolymph surrounds the organ of Corti, contains in the latter the end- 

 organs of the cochlear nerve. The organ of Corti lies on the fibrous lamina spiralis 

 membranacea (membrana basilaris) and consists of a supporting structure com- 

 posed of the so-called arches of Corti, each of which consists of two rods of Corti 



