904 



OF SENSATION, AND THE ORGANS OF THE SENSES. 



only contains a liquid (the endo-lympTi), but is also separated from the osseous 

 wall by another collection of'liquid, the peri-lymph; so that it is suspended, as 

 it were, in a liquid which bathes both its surfaces. In the cavity of the Vesti- 

 bule, which is subdivided by a membranous partition into two, are found small 

 masses of concretionary particles, collectively named otoconia, or ear-powder ; 

 these are obviously the rudiments of the otoliths, or ear-stones, whose presence 

 in animals with a less perfect auditory apparatus seems needful to intensify the 

 undulations. It is commonly supposed that the Semicircular Canals have for 

 their peculiar function, to receive the impressions by which we distinguish the 



Fig. 226. 



A view of the axis of the Cochlea and the Lamina Spiralis, showing the arrangement of the three Zones ; 

 the osseous zone and the membrane of the vestibule have been removed : 1, the natural size of the parts ; 

 the other figure is greatly magnified ; 2, trunk of the auditory nerve ; 3, the distribution of its filaments in 

 the zona ossea ; 4, the nervous anastomosis of the zona vesicularis ; 5, the zona membranacea ; 6, the osseous 

 tissue of the modiolus ; 7, the opening between the two scalae. 



direction of sounds ; and it is certainly a powerful argument in support of this 

 view that, in almost every instance in which these parts exist at all, they hold 

 the same relative position to each other as in Man, their three planes being 



Fig. 227. 



Cochlea of a new-born infant, opened on the side towards the apex of the petrous bone. It shows the 

 general arrangement of the two scalae, the lamina spiralis, and the distribution of the cochlear nerve. At 

 the apex is seen the modiolus expanding into the cupola, where the spiral canal terminates in a cul-de-sac. 

 The helicotrema is not visible in this view. From Arnold. 



nearly at right angles to one another. The idea, however, must be regarded as 

 a mere speculation, the value of which cannot be decided without an increased 



