THE LABYRINTHINE SENSATIONS 675 



in which auditory sensations *are wanting the cochlea is absent, 

 and in fishes is represented merely by a small diverticulum known 

 as the lagena. With the development of air-breathing vertebrates 

 we see the first signs of a special organ of hearing. Thus a primitive 

 cochlea is present in the amphibia, and especially in the anura, and 

 in some of the reptiles as well as in birds it acquires a bend and shows 

 the beginning of a spiral arrangement. Only in the mammals does 

 it attain a degree of development at all comparable with that found 

 in man, and characterised by the formation of one and a half to four 

 spiral turns in the cochlea as well as in the canalis media. 



This development of auditory functions cannot involve any 

 abrogation of the important part played by the otolith organ through 

 out all the lower classes of the animal kingdom. In man, as in the 

 crayfish, it is the otolith organ which determines his behaviour 

 in relation to the force of gravity, and is therefore responsible not 

 only for the maintenance of equilibrium but also for the sensations 

 which enable him consciously to orientate himself and to know 

 the position in which he happens to be at any given moment. 

 With the increasing importance of visual sensations in deter- 

 mining the behaviour of the animal, close connections are established 

 between the central connections of the nerves running from the 

 otolith organ and the parts of the brain concerned with the innerva- 

 tion of the eye muscles. By this means the position of the eyes 

 is constantly adapted to the position of the head. 



The auditory part of the internal ear has already been described. 

 That part of the labyrinth which represents the primitive otolith 

 organ consists of a bony framework containing perilymph, in which 

 is contained the membranous labyrinth with the endings of the 

 vestibular division of the eighth nerve. The osseous labyrinth 

 consists of a cavity, the vestibule, into which open behind the three 

 bony semicircular canals. In the vestibule are contained two little 

 membranous sacs, the utricle and saccule, the cavities of which are 

 connected by means of the saccus endolymphaticus. Into the utricle 

 open the three semicircular canals, the three canals having five 

 openings. These semicircular canals are arranged in three planes, 

 each of which is at right angles to the other two, so that in the organ 

 are represented the three planes of space. .We may distinguish on 

 each side an external or horizontal canal, an anterior or superior 

 vertical canal, and a posterior vertical canal. The two outer canals 

 lie always exactly in the same plane, which is practically horizontal 

 in the normal position of the head. Each posterior vertical canal lies 

 in a plane which is parallel to that of the superior vertical canal of the 

 opposite side. We thus see that these semicircular canals form 

 together three planes one horizontal and two vertical, the two latter 



