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 each other (Fig. 167). 

 The horizontal 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 

 J 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 witnin 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 cylin- 

 drical 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 //. 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. 



Flourens's Experiments upon the Semicircular Canals. 

 Modern experiments and theories concerning the functions of the 

 semicircular canals date from the classical researches of Flourens* 



* Flourens, "Recherches expe"rimentales sur les propri6tes et les fonc- 

 tions du systeme nerveux," second edition, 1842. 



372 



Fig. 167. Diagrammatic hori- 

 zontal section through the head to il- 

 lustrate the planes occupied by the 

 semicircular canals (after Waller) : S, 

 Superi9r canal; P, posterior canal; 

 11, horizontal canal. 



