560 THE NERVOUS SYSTEM. 



pulses reach the membranous labyrinth, not directly from the stapes, 

 but through the intermediate vibration of the perilymph. 



The main point of interest in regard to the membranous labyrinth 

 relates to the distribution and termination of the auditory nerve. 



The auditory nerve sends to the vestibule two branches ; one dis- 

 tributed to the sacculus, the other to the utricle and ampullae. The 

 mode of termination of the nerve fibres in both divisions is essentially 

 the same. They are not distributed generally over the membrane, but , 

 terminate in well-defined spots, characterized by a thickening of the 

 membranous wall, and by a peculiar form of epithelium provided with 

 stiff, pointed cilia the so-called auditory hairs. 



In the sacculus and in the utricle, the terminal nerve spot, or " mac- 

 ula auditiva," is an oval plate, 3 millimetres by 1.5 in the sacculus, 

 and 3 millimetres by 2 in the utricle. In the ampullae, it forms a trans- 

 verse fold of the membranous wall, projecting inward like the valvulse 

 conniventes of the small intestine, but occupying only about one-third 

 of their circumference. Elsewhere, the sacs are lined, according to 

 Kolliker, by a single layer of pavement epithelium cells. But at the 

 spots in question the epithelium is twice or three times as thick as in 

 the remaining portions, and consists of elongated cylindrical and fusi- 

 form cells. It also presents, standing upright upon its surface, the cilia, 

 or auditory hairs, which in man are about 25 mmm. in length. The 

 terminal fibres of the auditory nerve, which pass toward these thick- 

 ened spots, may be traced, according to all recent observers, into the 

 epithelial layer ; and certain appearances give rise to the supposition 

 that the axis-cylinder of each fibre is prolonged through a fusiform 

 epithelium cell, and projects, in the form of an auditory hair, from its 

 free extremity. This is inferred mainly from the similarity in appear- 

 ance between the axis-cylinder of the nerve fibres and the slender 

 downward prolongation of the fusiform cells ; and from the fact that 

 both structures are stained blackish or brown by osmic acid (Rudinger). 

 However this may be, there is no doubt that the projecting cilia, either 

 mechanically or by nervous sensibility, receive and transmit the sonor- 

 ous vibrations of the surrounding fluid. 



A remarkable feature connected with the auditory spots of the sac- 

 culus and utricle is the so-called otoconia, or ear sand. This consists 

 of calcareous grains, embedded in a gelatinous material, and forming 

 a white, chalky-looking layer immediately over the auditory spot. The 

 grains are rounded, elongated, or prismatic and crystalline in form ; 

 the largest measuring, according to Kolliker, about 10 mmm. in length. 

 Their exact office is unknown, but it is evident, from their constant 

 existence in this situation in different animals, that they have some 

 important relation to the sense of hearing. In man, mammalians, 

 and birds they are pulverulent. In reptiles and fish they are some- 

 times of friable concretions, sometimes rounded masses, hard and dense 

 as porcelain. According to Wagner, they are completely absent only in the 

 cyclostomi, or fishes of the lowest order, including the lamprey and hag. 



