CRISTA ACUSTICA AND COCHLEA. 



809 



nective-tissue to the osseous walls. The ampullae, however, completely fill the corresponding 

 osseous dilatations. The canals and ampullae consist externally of an outer, vascular, connec- 

 tive-tissue layer, on which there rests a well-marked hyaline layer, bearing a single layer of 

 flattened epithelium. 



Crista Acustica. The vestibular branch of the auditory nerve sends a branch to each ampulla 

 and to the saccule and utricle (fig. 593). In the ampullae (fig. 591, II, A), the nerve (c) ter- 



Fig. 592. Fig. 593. 



The interior of the right labyrinth with its membranous canals and nerves. In fig. 592, the 

 outer wall of the bony labyrinth is removed to show the membranous parts within 1, 

 commencement of the spiral tube of the cochlea; 2, posterior semicircular canal, partly 

 opened; 3, horizontal; 4, superior canal; 5, utricle; 6, saccule; 7, lamina spiralis; 7', 

 scala tympani; 8, ampulla of the superior membranous canal; 9, of the horizontal; 10, of 

 the posterior canal. Fig. 593 shows the membranous labyrinth and nerves detached 1, 

 facial nerve in the internal auditory meatus ; 2, anterior division of the auditory nerve 

 giving branches to 5, 8, and 9, the utricle and the ampullae of the superior and horizontal 

 canals ; 3, posterior division of the auditory nerve, giving branches to the saccule, 6, and 

 posterior ampulla, 10, and cochlea, 4 ; 7, united part of the posterior and superior canals ; 

 11, posterior extremity of the horizontal canal. 



minates in connection with the crista acustica, which is a yellow elevation projecting into the 

 equator of the ampulla. The medullated nerve-fibres, n, form a plexus in the connective-tissue 

 layer, lose their myelin as they pass to the hyaline basement mem- 

 brane, and each ends in a cell provided with a rigid hair (0, p) 90 /x 

 in length, so that the crista is largely covered with these hair-cells, 

 but between them are supporting cells like cylindrical epithelium 

 (a), and not unfrequently containing granules of yellow pigment. 

 The hairs or "auditory hairs " (M. Schultze) are composed of many 

 fine fibres (Eetzius). An excessively fine membrane (membrana 

 tectoria) covers the hairs (Pritchard, Lang). 



Maculae Acusticse. The nerve-terminations in the maculae acus- 

 ticae of the saccule and utricle are exactly the same as in the am- 

 pullae, only the free surface of their membrana tectoria is sprinkled f&- 

 with small white chalk-like crystals or otoliths (II, T), composed 

 of calcic carbonate, which are sometimes amorphous and partly in i. 

 the form of arragonite, lying fixed in the viscid endolymph. The 

 non-medullated axis-cylinders of the saccular nerves enter directly 

 into the substance of the hair-cells. The terminations of the nerves 

 have been investigated, chiefly in fishes, in the rays. 



[Fig. 594 is a vertical section of a macula acustica of the rabbit. 

 The medullated nerves [n) lose their myelin at the external limiting 

 membrane, become non-medullated, pierce this membrane, and 

 form a basal plexus (pb) between (i) the epithelial cells, and finally 

 terminate in the sensory ciliated cells (r). The epithelium itself 

 consists of basal cells (cb), fusiform or supporting cells (/), and the 

 ciliated neuro-epithelium (r), each cell being provided with a cilium, 

 which perforates the external limiting membrane (a). There is 

 thus a remarkable likeness to the olfactory epithelium.] 



Cochlea. The terminations of the cochlear branch of the audi- 

 tory nerve lie in connection with Corti's organ, which is placed in the canalis or ductus coch- 

 learis (fig. 591, I, Cc, and III, Cc, and fig. 595), the small triangular chamber [or scala media,] 

 cut off from the scala vestibuli by the membrane of Reissner. Corti's organ is placed on the 

 lamina spiralis membranacea, and consists of a supporting apparatus composed of the so-called 

 Corti's arches, each of which consists of two Corti's rods (z, y), which lie upon each other like 



Fig. 594. 

 Vertical section of the ma- 

 cula acustica of a rabbit. 



