656 THE SENSES. 



they are lodged. Each membranous semicircular canal presents at one 

 of its extremities, at the expanded part of its bony canal, a similar 

 rounded dilatation, known as the " ampulla." 



The membranous sacs and semicircular canals are considerably 

 smaller than the osseous cavities which contain them, and occupy 

 nearly everywhere an eccentric position ; being, at certain points, in 

 contact with and adherent to the internal periosteum, while at others 

 they are surrounded by the perilymph. The sacculus and utricle 

 together occupy about two-thirds of the cavity of the vestibule; and, 

 according to Riidinger, are so placed that neither of them touches the 

 base of the stapes at the fenestra ovalis, but are separated from it by 

 an appreciable layer of fluid. Thus the sonorous impulses which reach 

 the membranous labyrinth come to it, not directly from the stapes, but 

 through the intermediate vibration of the perilymph. 



The sacculus and the utricle are adherent to the internal periosteum 

 of the vestibule at the points of entrance of their corresponding branches 

 of the auditory nerve. The ampullae of the semicircular canals fill 

 almost completely the bony cavities in which they rest, their outer 

 surface lying for the most part in contact with the periosteum. On 

 the other hand, the membranous semicircular canals are very much 

 smaller in calibre than the osseous excavations which contain them, 

 and lie in contact with the periosteum only along the inner or smaller 

 curvature of the bony canals ; so that they are surrounded externally 

 by a comparatively large quantity of perilymph. They are, however, 

 attached and held in place, as shown by Riidinger, by slender, scattered, 

 fibrous bands and partitions, which traverse at various points the peri- 

 lymphic space. 



The main point of interest in regard to the membranous labyrinth 

 relates to the mode of distribution and termination of the auditory 

 nerve. 



The auditory nerve sends to the vestibule two branches; one of 

 which is distributed to the sacculus, the other to the utricle and 

 ampulla?. The mode of termination of the nerve fibres in both these 

 divisions is essentially the same. They are not distributed generally 

 over the membrane, but terminate only in particular well-defined spots, 

 characterized by a thickening or prominence of the membranous wall, 

 and by the presence of 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 " macula 

 auditiva," is in the form of an oval plate, or lamina, 3 millimetres by 

 1.5 in the sacculus, and 3 millimetres by 2 in the utricle. In the am- 

 pullae, it forms a transverse ridge or fold of the membranous wall, pro- 

 jecting inward after the manner of the valvnlae conniventes of the small 

 intestine, but occupying only about one-third of the circumference of 

 the ampulla. Elsewhere, the membranous sacs of the labyrinth 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 



