676 PHYSIOLOGY 



being at right angles to one another (Fig. 310). The membranous 

 canal lies within the osseous canal, a considerable space inter- 

 vening between the two canals. At one end the osseous canal 

 is dilated and the membranous canal undergoes a corresponding 

 dilatation so as to fill up the whole bony canal. In this dilatation, 

 which is known as the ampulla, we find the ending of a branch of 

 the vestibular nerve in a special sense epithelium, forming the 



FIG. 310. Figure from Ewald showing the situation of the three semi- 

 circular canals in the skull of the pigeon. 



crista acustica (Fig. 311). The crista is composed of hair-cells with 

 sustentacular cells between them. The fibres of the vestibular nerve 

 end in arborisations among the hair-cells, the hairs of which project 

 into the endolymph filling the ampulla. In the utricle and saccule 

 we also find special sense-organs, known as the macula acustica, 

 the structure of which is very similar to that of the crista in the 

 ampullae. Among the hairs, however, of the macula is found a 

 small concretion of carbonate of lime, the otolith. 



The first accurate experimental investigation of the functions 

 of these different parts we owe to Flourens. This observer showed 

 that, whereas extirpation of the cochlea caused deainess, extirpation 



