60 



INTRODUCTION TO NEUROLOGY 



and all six muscles of the other eye. In this way alone can con- 

 jugate movements of the two eyes be effected for the accurate 

 fixation of the gaze upon any object. The entire system of con- 

 jugate .movements is also entirely reflex and it is effected by an 

 exceedingly complicated arrangement of nerve tracts and cen- 

 ters, of which the superior olive and the nucleus of the VI nerve 

 are integral parts. 



The chain reflex (see Fig. 18, B) is a very common and a very 

 important type. Most of the ordinary acts in the routine of 

 daily life employ it in one form or another, the completion of one 

 stage of the process serving as the stimulus for the initiation 

 of the next. 



nerve 



Fig. 19. Diagram of a simple auditory reflex. Upon stimulation of the 

 endings of the VIII nerve in the ear by sound waves, a nervous impulse may 

 pass to the superior olive, whence it is carried by an intercalary neuron of 

 the second order to the nucleus of the VI nerve. The fibers of this nerve 

 end on the external rectus muscle of the eyeball. 



There are within the muscles elaborate sense organs (the mus- 

 cle spindles and their associated afferent nerves, see p. 87), 

 which are stimulated by the contraction of the muscle. These 

 afferent nerves of the muscle sense have their own centers of 

 adjustment within the central nervous system, from which in 

 turn efferent impulses go out which ultimately reach the same 

 muscles from which the sensory impulses came in. This, of 

 course, is a variety of chain reflex, and is the mechanism by 

 which refined movements of precision are executed, where differ- 

 ent sets of muscles must work against each other in constantly 

 varying relations without conscious control. In the case of a 

 sustained reflex series of this character this return flow of affer- 



