THE FUNCTIONS OF THE CEREBELLUM 



397 



back part there are formed two sacs the saccule and utricle and the three 

 semicircular canals. The saccule and the utricle, which receive each a large 

 branch of the vestibular nerve, represent the otolith organ, which is found 

 in almost all classes of animals. The crayfish, for instance, at the base 

 of its antennae presents a small sac lined with 'hairs and richly supplied 

 with nerves. In this sac a small calcareous particle rests on the hairs. 

 It is evident that the incidence of the pressure of the small stone or otolith 

 on the hairs will vary according to the position of the animal (Fig. 200), 

 so that any change in the position of the head will be attended by altera- 



A 



abc 



FIG. 200. Diagram of an otolith organ, to show how alterations 

 in its position will cause the weight of the otolith (ot.) to press on 

 different sense cells, and therefore to affect different nerve fibres. 



abc 



tion in the nerve fibres which have been stimulated by the pressure of 

 the otolith, and therefore in the nature of the impulses flowing to the central 

 nervous system. The importance of these impulses in regulating the loco- 

 motion and the maintenance of the equilibrium of the anima] is well shown 

 if the otolith be replaced by a small fragment of iron. Under normal 

 circumstances the iron particle will act quite as well as an otolith. If 

 however a powerful magnet be brought in the neighbourhood of the animal, 

 the pressure of the particle will not be determined simply by gravity and 

 therefore by the position of the animal, so that there will be a discordance 

 between the impulses arriving from the otolith organ and those arising from 

 the sense-organs of the body, and marked disorders of equilibrium are the 

 result. 



In the saccule and utricle the vestibular nerve ends in similar otolith 

 organs known as the maculae acousticae. These are small elevations 

 covered with long hairs and supplied with nerves. One or two calcareous 

 secretions or otoliths are embedded in the hairs, so that any change in position 

 will cause a corresponding change in the nerve fibres which are being excited 

 by the weight of the otoliths. The semicircular canals, which lie in the three 

 planes of space, are also provided with end organs, somewhat similar in 

 structure to the maculae acousticae, but devoid of otoliths. The end organs 

 are excited by mass movements of the fluid endolymph, which are set 

 up by rotation of the head. 



Since the nervous apparatus of the labyrinth is excited not by changes 

 in the environment, from which it is carefully shielded, but by changes in the 



