CHAPTER XII 



THE VESTIBULAR APPARATUS AND CEREBELLUM 



THE general somatic sensory systems considered in the last 

 chapter include some of the most primitive reflex mechanisms. 

 These fall into two groups the exteroceptive systems and the 

 proprioceptive systems (pp. 77-89) and each of these groups 

 comprises, in addition to its primitive generalized members, 

 certain so-called organs of special or higher sense. The special 

 exteroceptive sense organs are the organ of hearing (p. 195) 

 and the organ of vision (p. 204). The special proprioceptive 

 sense organs are the semicircular canals of the internal ear; and 

 those will next be described, together with their central mechan- 

 isms in the medulla oblongata and cerebellum. 



The Vestibular Apparatus. The internal ear contains two 

 quite distinct groups of sense organs, the organ of hearing in the 

 cochlea and the vestibular organs (utricle, saccule, and semicircu- 

 lar canals), both of which are supplied by the VIII cranial nerve, 

 which accordingly has two parts, the cochlear and the vestibu- 

 lar nerves. The semicircular canals are the most highly spe- 

 cialized end-organs of the proprioceptive series and are- con- 

 cerned chiefly with the maintenance of bodily equilibrium. The 

 general structure of the internal ear is described on p. 195; here 

 we need merely mention that the three semicircular canals 

 (ductus semicirculares) of each ear lie approximately at right 

 angles to each other, as shown diagrammatically in Fig. 85, and 

 each canal is dilated at one end to form the ampulla, within 

 which is a patch of sensory epithelium from which hairs project 

 into the contained fluid (see Figs. 32 and 91). A movement of 

 the head in any direction will cause a flow of the fluid in one or 

 more of these canals in each ear, which in turn will excite a 

 nervous impulse in the hair-cells of the corresponding ampullae. 

 These nervous impulses will be transmitted to the vestibular 

 centers of the brain, where they will be so analyzed as to call forth 



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