THE FUNCTIONS OF THE CEREBELLUM 



449 



segmental reactions of the intercostal muscles are entirely subordinated 

 to the grey matter in the medulla around the entry of the vagus fibres 

 from the lungs. 



This subordination of the hinder to the anterior sense-organs is 

 paralleled in the case of the proprioceptive system. Entering the 

 hind-brain at the upper border of the medulla is the eighth nerve, 

 composed of two parts which differ widely in functions, viz. the 

 cochlear division and the vestibular division. The former is entirely 

 concerned with the reception of sound waves, and is therefore the 

 auditory nerve. The vestibular nerve, which is distributed to the 

 rest of the membranous labyrinth, must be assigned to the proprio- 

 ceptive system. The labyrinth is practically a double organ. The 

 primitive auditory sac arises as a simple involution of the surface. 

 In the course of development the front part is modified to form 



sbc 



a be 



o u c. 



FIG. 201. 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 

 nfirvfi fibres. 



cbc 



the canal of the cochlea, which is set apart entirely for the reception 

 of sound. From the 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, may be regarded as representing 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 which is 

 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. 201), so that any change 

 in the position of the head will be at once attended by alteration 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 locomotion and the maintenance of the equilibrium 



29 



