9 o8 THE CENTRAL NERVOUS SYSTEM 



are borne by some of the columnar cells, between which lie more 

 elongated fibre-like supporting cells. The hairs project into a mucus- 

 like mass, sometimes containing otoconia, or crystals of calcium car- 

 bonate. The ampullae, like the rest of the membranous labyrinth, is 

 filled with a watery fluid called endolymph. The utricle and saccule 

 have each a somewhat similar but broader elevation, the macula 

 acustica, covered with epithelium and hair-cells of the same character, 

 and the hairs project into a similar mass in which otoconia are con- 

 stantly present. In some animals, as fishes, the calcareous matter in 

 the utricle and saccule forms masses of considerable size (otoliths). 

 Fibres of the auditory nerve end in arborizations around the bodies 

 of the hair-cells of the maculae and cristae acusticae. We have already 

 seen that it is the ventral or vestibular division of the nerve which is 

 especially related to the vestibule (p. 898). 



There is very strong evidence that the semicircular canals are con- 

 cerned, not in hearing, but in equilibration. A pigeon from which 

 the membranous canals have been removed still hears perfectly 

 well so long as the cochlea is intact, but exhibits the most profound 

 disturbance of equilibrium. If the horizontal canal is destroyed 

 or divided, the pigeon moves its head continually from side to side 

 around a vertical axis; if the superior canal is divided, the head 

 moves up and down around a horizontal axis. The power of co- 

 ordination of movements is diminished, but not to the same extent 

 in all kinds of animals. Thrown into the air, the pigeon is helpless; 

 it cannot fly; but a goose with divided semicircular canals can still 

 swim. The condition is only temporary, even when the injury 

 involves the three canals on one side ; but if the canals on both sides 

 are destroyed, recovery is tardy, and often incomplete. In mam- 

 mals the loss of co-ordination is much less than in birds ; and move- 

 ments of the eyes, the direction of which depends on the canal 

 destroyed, take to a large extent the place of movements of the 

 head. The effects of destructive lesions have their counterpart in 

 the phenomena caused by stimulation; excitation of a posterior 

 canal, for example, in the pigeon causes movements of the head 

 from side to side. 



Lee's results in fishes are, on the whole, of similar tenor. Mechan- 

 ical stimulation of the ampullae in the dogfish, by pressing on them 

 with a blunt needle, calls forth characteristic movements of the 

 eyes and fins, and electrical stimulation of the auditory nerve 

 causes movements compounded of the separate movements obtained 

 by stimulation of the ampullae one by one. Lee concludes that the 

 semicircular canals are the sense-organs for dynamical equilibrium 

 (i.e., equilibrium of an animal in motion), and the utricle and saccule 

 for statical equilibrium (i.e., equilibrium of an animal at rest). 



The evidence from all sources points strongly to the conclusion 

 that afferent impulses are actually set up in the fibres of the auditory 

 nerve, through the hair-cells, by alterations of pressure or by stream- 

 ing movements of the endolymph when the position of the head is 



