186 INTRODUCTION TO NEUROLOGY 



lus (f.l.m.\ by which fibers descend to the spinal cord (chiefly 

 for turning movements of the head by the neck muscles) and 

 ascend to the midbrain. The last-mentioned fibers connect 

 chiefly with the nuclei of the motor nerves for the eye muscles 

 (III, IV, and VI pairs of cranial nerves), thus providing for 

 the conjugate movements of the eyes which accompany head 

 movements (in this way, for instance, enabling one to keep the 

 gaze fixed upon a stationary object while the head is moving, 

 cf. p. 211). 



It will be noticed that there is no important pathway from 

 the vestibular nucleus to the thalamus and cerebral cortex, for 

 the equilibratory reactions excited from the semicircular canals 

 are normally unconsciously performed. This is in marked 

 contrast with the connections of the cochlear nerve, for the audi- 

 tory reactions are often consciously directed (p. 202). There 

 is, however, an important connection with the cerebellum, 

 partly directly by root fibers of the vestibular nerve and partly 

 by secondary fibers from the superior and lateral vestibular 

 nuclei (Fig. 86). The cerebellum is, accordingly, an important 

 center of adjustment for the proprioceptive reflexes, and to this 

 our attention will next be directed. 



The Cerebellum. This important organ is an overlord which 

 dominates the proprioceptive functions of the body in some- 

 what the same way that the cerebral cortex directs and controls 

 the exteroceptive reactions. Both of these organs are second- 

 arily added to the more primitive segmental structures of the 

 brain stem, that is, they are suprasegmental (p. 113). 



The correlation centers of the brain stem, and particularly 

 those of the cerebral cortex, analyze the afferent impulses enter- 

 ing the brain and determine what particular reactions are ap- 

 propriate in each situation. After the character of the move- 

 ment has been determined in this way, the proprioceptive sys- 

 tems cooperate in its execution, and the cerebellum is the cen- 

 tral coordination station for the proprioceptive reactions. 

 None of its activities come into consciousness. 



The cerebellum, therefore, is intimately connected with all 

 sensory centers which are concerned in the adjustment of the 

 body in space and motor control in general. The maintenance 

 of bodily equilibrium is the most important of these functions, 



