200 



THE HUMAN BODY 



auditory nerve are distributed along a line across the ampulla of 

 each semicircular canal. They lose their medullary sheath close 

 to the basement membrane, a, which the axons pierce. The 



axons branch among the epithelium 

 cells, which at 'this place are sev- 

 eral rows thick, but have not yet 

 been traced into direct continuity 

 with any of them. The cells of the 

 epithelium are of two varieties. 

 The columnar cells or hair-cells, c, 

 do not reach the basejnent mem- 

 brane, are nucleated or slightly 

 granular: from the free end of each 

 projects a rigid hair process, d. 

 The remaining cells, rod-cells, b, 



Fm. 75.-Diagram of epithelium are in several rows: each has a 



in nervous region of ampulla of a slender inner process extending to 



semicircular canal. 



the basement membrane and an 



outer which reaches to the bases of the columnar cells and appears 

 there to end in a rigid membrane, e, which is perforated for the 

 passage of the hairs. They probably are mere supporting struc- 

 tures. 



In some parts of the utricle and saccule are regions of epithelium 

 very similar to that above described, and also supplied with nerve- 

 fibers. In connection with them are found minute calcareous 

 particles, otoliths or ear-stones. 



The Equilibrium Sense. An important group of afferent im- 

 pulses concerned with the maintenance of bodily equilibrium is 

 derived through the semicircular canals and vestibule of the ear, 

 which are supplied by the vestibular portion of the auditory nerve. 



Experiment shows that cutting a semicircular canal is followed 

 by violent movements of the head in the plane of the canal di- 

 vided; the animal staggers, also, if made to walk; and, if a pigeon 

 and thrown into the air, cannot fly. All its muscles can contract 

 as before, but they are no longer so coordinated as to enable the 

 animal to maintain or regain a position of equilibrium. It is like 

 a creature suffering from giddiness; and similar phenomena fol- 

 low, in man, electrical stimulation of the regions of the skull in 

 which the semicircular canals lie. 



