CHAPTER XXI. 



THE FUNCTIONS OF THE SEMICIRCULAR CANALS AND 

 THE VESTIBULE. 



Position and Structure. The membranous semicircular canals 

 lie within the bony semicircular canals, the space between being 

 filled with perilymph which communicates freely with that in the 

 rest of the labyrinth. Within the membranous canals is the endo- 

 lymph, which communicates through the five openings with the 

 endolymph in the utriculus. The canals lie in three planes that are, 

 approximately at least, at right angles to one another (Fig. 170). 

 The horizontal or external canals lie in a horizontal plane at right 



angles to the mesial or sagittal 

 plane of the body, the vertical 

 canals on each side make an angle 

 of about 45 degrees with this 

 mesial plane. The plane of each 

 of the anterior canals is parallel to 

 that of the posterior or inferior 

 vertical canal of the opposite side, 

 as represented in the figure. At 

 one end of each canal near its 

 junction with the utriculus is the' 

 swelling known as the ampulla 

 and within the ampulla lies the 

 crista acustica containing the hair 

 cells with which the nerve fibers 

 communicate, and which, there- 

 fore, are considered as the sense 

 cells of the organ. The hair 

 cells are cylindrical and each 

 gives off a long hair, consisting 

 perhaps of a bundle of finer 

 hairs, which projects into the 

 interior of the canal for a distance 

 of at least 28 /i. The nerve fibers 

 distributed to these hair cells are given off by the vestibular branch 

 of the eighth nerve, or more properly the vestibular nerve, one 

 branch of which (ramus utriculo-ampullaris) supplies the utriculus 

 and the ampulla of the superior and horizontal canals, while the 

 other (ramus sacculo-ampullaris) furnishes fibers to the sacculus. 

 and the posterior ampulla. 



382 



FIG. 170. Diagram to show the posi- 

 tion of the semicircular canals in the head 

 of the bird. On each side it will be seen 

 that the three canals lie in planes at right 

 angles to one another. The external or 

 horizontal canals (E) of the two sides lie 

 in the same plane. The anterior canal of 

 one side (A) lies in a plane parallel to that 

 of the posterior canal (P) of the other side 

 (Ewald). 



