THE FUNCTIONS OF THE CEREBELLUM 399 



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 larg : 

 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 alteration in the nerve fibres which have been stimulated by the 



abc 



a br 



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

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

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



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 locomotion and the maintenance of the equilibrium of the animal 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 dissonance 

 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 he in the three 

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

 structure to the maculae acousticae, but devoid of otoliths. They are excited 

 by mass movements of the fluid endolymph, filling the canals, 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 



