THE LABYRINTH 319 



impressions we are made aware of the position of the 

 body as a whole and of the head in particular, in relation 

 to the outside world. These impressions arise partly in 

 the labyrinth, the end-organ of the vestibular branch of 

 the eighth nerve. The labyrinth consists of a system of 

 passages within the temporal bone (osseous labyrinth). 

 Within the osseous labyrinth, and separated from it by a 

 membrane, is an inner system, the membranous labyrinth. 

 The osseous labyrinth is filled with perilymph and the 

 membranous with endolymph. The labyrinth contains, in 

 addition, the cochlea, but we are here concerned only with 

 that part of the membranous labyrinth from which the ves- 

 tibular branch of the eighth nerve arises. This part con- 

 sists of two sacs, the utricle and saccule, which are connected 

 together by a tube, the saccus endolymphaticus, and the 

 three semicircular canals. The utricle and saccule are two 

 small sacs, into which open, from a projection on its wall, 

 a number of hairs which are the terminations of some of 

 the fibres of the eighth nerve (Fig. 51, p. 301). Among the 

 hairs are a few calcareous nodules, the otoliths. 



In lower animals, notably the crayfish, the otohth organ 

 is cup-shaped, the hairs pointing inwards. In these animals 

 the otohth can be removed and a small piece of iron 

 inserted in its place. When a magnet is then brought near 

 the head the equihbrium of the animal is disturbed. This 

 experiment suggests that the otohth organ in the crayfish 

 and, by analogy, the saccule and utricle in higher animals 

 serve to give the individual information regarding the 

 position of the head in relation to gravity. For any 

 position of the head the weight of the otohth falls in a 

 particular manner on the hairs, and this is interpreted 

 centrally as a sensation of position. 



The three semicircular canals, which are continuous with 

 the cavity of the utricle, are disposed in three planes at 

 right angles to one another, one horizontal and two 

 vertical. Of the two vertical canals, the anterior canal 

 of the one side hes in the same plane as the posterior 



