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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



within the cochlea and known as the canalis cochlearis. The auditory nerve 

 really consists of two distinct divisions having separate origins and different 

 distributions. One of these branches passes finally to the cochlea, and the 

 other to the vestibule and the semicircular canals. The nerve approaches the 

 labyrinth by way of a canal known as the meatus auditorius internus, and 

 on reaching the angle between the vestibule and the base of the cochlea the 

 cochlear division passes to the cochlea. The remainder of the nerve consists 

 of two divisions, the superior of which is distributed to the utricle and to the 

 ampullae of the anterior and horizontal semicircular canals ; the inferior branch 

 supplies the saccule and the posterior semicircular canal. The inner wall of 

 both utricle and saccule is developed at a particular spot into a low mound, 

 the macula acustica, made up of an accumulation of the connective-tissue ele- 

 ments of the membranous wall and covered by a peculiarly modified epithe- 

 lium, the auditory epithelium (Fig. 279). All the auditory-nerve filaments that 

 enter the saccule and utricle respectively pass to these mounds and there enter 

 into relation with the auditory epithelium. 



As the auditory-nerve endings are confined to a particular area in the 



utricle and the saccule, so the nerve-fibres supplying the semicircular canals 



, are limited to a certain part of the 



ampulla of each canal. The tissue of 

 the wall of the ampulla is developed 

 into a ridge projecting into the cavity 

 in a direction across its long axis. 

 This ridge, present in each ampulla, is 

 called the crista acustica ; it is capped 

 by a thick layer of columnar epithelial 

 cells, the auditory epithelium, which 

 thins away at the border of the crista 

 into the sheet of flattened cells by 

 which the rest of the ampulla is lined. 

 The auditory cells(Fig. 279) are said to 

 be of two kinds one, cylindrical in 

 shape and reaching only part way to 

 the basement membrane, the hair-cells; 

 the other, narrow and elongated, the 

 supporting or sustentacular cells. The former are peculiar in the fact that 

 from their free ends there project long, stiff, hair-like processes. The fila- 

 ments of the ampullary-nerve branches pass through the cristse and encircle 

 the bodies of the hair-cells. The cells covering the maculce acusticce have 

 essentially the same structure as those just described, though in the macula 

 the auditory hairs are shorter than in the cristse. Seated on the free surface 

 of the macular epithelium is a fibrous mass which is said to be a normal 

 structure, and not, like a somewhat similar mass found covering the crista? in 

 post-mortem sections, a coagulum due to the method of preparation. Im- 

 bedded in the membrane over the macula? of both saccule and utricle are 



FIG. 279. Diagram showing the epithelial cells of 

 a macula or a crista (after Foster) : 1, cylinder or 

 hair-cell ; 2, the same, enveloped in a nest of nerve- 

 flbrils ; 3, 4, 5, forms of rod- or spindle-cells. 



