SENSE OF HEARING. 657 



times as thick as in the remaining portions, and consists of elongated 

 cells of two different forms, namely, cylindrical and fusiform. It also 

 presents, standing upright upon its free surface, the pointed cilia above 

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

 length. The terminal nerve fibres of the auditory nerve, which pass 

 up toward these thickened spots of epithelium, may be traced, accord- 

 ing to the testimony of all recent observers, into the epithelial layer 

 itself; and certain appearances give rise to the supposition that the 

 ultimate axis-cylinder of each nerve fibre is prolonged through the sub- 

 stance of a fusiform epithelium cell, and finally becomes the cilium or 

 auditory hair projecting from its free extremity. These appearances 

 are, 1st, the similarity in size and aspect between the axis-cylinder of 

 the nerve fibres and the slender downward prolongations of the fusiform 

 cells; and, 2d, the fact that both these structures become stained more 

 or less deeply of a blackish or brown color by the action of osmic acid 

 (Riidinger). Whatever the precise relations of the terminal nerve 

 fibres to the other elements of the epithelial layer may be, there is no 

 doubt that the projecting cilia act either mechanically, or by virtue of 

 a real nervous sensibility belonging to them, and are the immediate 

 recipients of the sonorous vibrations communicated by the surrounding 

 fluid. 



A remarkable secondary feature connected with the auditory spots of 

 the sacculus and utricle is the existence, at each, of a deposit of minute 

 solid calcareous grains, the so-called otoconia, or ear sand. These grains 

 are embedded in a homogenous gelatinous material, and form a white 

 chalky-looking \ayer immediately over the auditory spot, by which the 

 situation of this spot is easily recognized. The grains are composed 

 almost exclusively of lime carbonate. They are rounded, elongated, 

 or distinctly prismatic and crystalline in form ; the largest measuring, 

 according to Kolliker, about 10 mmm. in length. The exact office per- 

 formed by these calcareous deposits is unknown, but it is evident from 

 their constant existence in the same situation in different animals, that 

 they have some important relation to the sense of hearing. In mam- 

 malians and birds they are pulverulent, as in man. In reptiles and fish 

 they assume the form, sometimes, of friable chalk} 7 concretions, some- 

 times of rounded masses of considerable size, hard and dense as por- 

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

 cyclostomi, or lowest order of true fishes, including the lamprey and 

 the hag. 



Physiological Action of the Membranous Labyrinth. The sacculus 

 and utricle, contained in the cavity of the vestibule, are membranous 

 formations, to which the fibres of the auditory nerve are distributed, and 

 in which they terminate. These membranous expansions are supported 

 by the contact of fluid on each side, and are held in place by the partial 

 fibrous attachments which connect them with the wall of the vestibule. 

 They are the structures upon which the impressions of sound are finally 

 received, and correspond, in this respect, to the retina in the organ of 



