152 



ESSENTIALS OF PHYSIOLOGY. 



tissue is thickened at one point to form a projection, which is covered 

 with columnar epithelium supporting a number of cells provided with 

 hairs, and which is called the crista acustica (septum transversum). 

 Similar thickenings occur in the utricle and saccule, these being called 

 maculae acusticce ; they have the same structure as the cristee, with the 

 addition of small concretions of lime, called otoliths, scattered among 



the processes of the hair 

 cells. The fibres of the 

 vestibular nerve are dis- 

 tributed to the cristae 

 and maculae acusticee, 

 and end in fibrils round 

 the hair cells. 



The bony labyrinth 

 contains perilymph, and 

 the membranous cavities 

 contain endolymph ; the 

 membranous structures 

 are attached to the bony 

 labyrinth by fibrous 

 strands. 



The functions of the 

 semicircular canals have 

 been ascertained chiefly 

 by experiments on 

 pigeons, in which they 

 are easily accessible. If 

 the horizontal canals 

 are destroyed, the head 

 oscillates from side to 

 side in a horizontal 

 plane ; after section of 

 the posterior or superior 

 vertical canals, the head 

 and body are thrown into constant movement in a vertical plane, 

 so that the animal tends to turn somersaults. 



After destruction of all the canals, the animal is in constant violent 

 movement, but can neither stand, walk, nor fly. After a time partial 

 recovery takes place, but the symptoms return when the eyes are 

 bandaged, showing that the partial recovery is due to compensatory 

 utilisation of the visual sensations for co-ordination. 



It is clear from these observations that the canals are essential 



FIG. 49. Figure from Ewald showing the situation of 

 the three semicircular canals in the skull of the 

 pigeon. (Starling's Principles of Physiology. ) 



A, plane of anterior or superior semicircular canal ; P, plane of 

 posterior ; and E, that of horizontal canal. 



Note that the anterior canal of one side and the posterior 

 canal of the other side are in the same plane. 



