604 



PHYSIOLOGY 



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 ac- 

 quires 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 throughout 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 there- 

 fore 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 sensa- 

 tions in determining the be- 

 haviour of the animal, close 

 connections are established be- 

 tween the central connections 

 of the nerves running from the 

 otolith organ and the parts of 

 the brain concerned with the 

 innervation 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 inter- 

 nal 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, 

 thes 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 



FIG. 312. Figure from Ewald showing the situ- 

 ation of the three semicircular canals in the 

 skull of the pigeon. 



