CHAPTER XXI. 



THE FUNCTIONS OF THE SEMICIRCULAR CANALS AND 

 THE VESTIBULE, 



Position and Structure. — ^The membranoui? semicircular canals 

 lie within the bony semicircular canals, the wpace 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. 179). 

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



angles to the mesial or sagittal 

 plane of the body, and the verti- 

 cal 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 op- 

 posite side, as represented in the 

 figure. At one end of each canal, 

 near its junction with the utricu- 

 lus, 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, 

 therefore, 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 28m. 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. 



402 



Fig. 179. — Diagram to show the posi- 

 tion of tlie 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). 



