652 



PHYSIOLOGY 



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 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 import- 

 ance of visual sensations in 

 determining the behaviour 

 of the animal, close connec- 

 tions are established be- 

 tween the central connec- 

 tions of the nerves running 

 from the otolith organ and 

 the parts of the brain con- 

 cerned with the innervation 

 of the eye muscles. By 

 this means the position of 

 the eyes is constantly adap- 

 ted 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 contain ing 

 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 

 AS hid) open Ix'hind the three bony semicircular canals. In the vestibule arc iron, 

 t aincd two little membranous sacs, the utricle and saccule, the cavities of 

 \\hieh arc connected by means of the aaccus endolymphaticits. Into the utricle 

 open the three semicircular canals, the three canals having five openings. These semi- 

 circular 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 dist inrush 

 on < ich 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 plain 1 . 

 \\hich is practically horizontal in the normal position on the head. Each posterior 

 vertical canal lies-in a plane which is parallel to that of the superior vertical .anal of 

 the opposite sidr. \\V thus see that tin se semicircular canals form together three planes 

 one horizontal and two vertical, the two latter being at right angles to one another 

 (Fig. &>:t). The membranous canal lies \\ithin the osseous canal, a considerate spa< -c 

 intervening between the two canals. At one end the osseous canal is dilated and the 



FIG. 323. Figure from Ewald showing the situ- 

 ation of the three semicircular canals in the 

 skull of the pigeon. 



