224 C. JUDSON HERRICK 



ing that system into relation with several others, so that the 

 nervous discharge which emanates from it may be the efferent 

 link in a very complex reflex arc. This efferent discharge may 

 descend to the appropriate motor center, or it may ascend to 

 enter a still higher association center, all of whose afferent tracts 

 come from similar lower centers and therefore carry nervous 

 impulses which represent a sort of physiological resultant of the 

 functional factors there interacting. 



A few selected illustrations of the various types of correla- 

 tion centers may clarify these relations. Fig. i illustrates the 

 simplest reflex arc. An auditory impulse coming to the brain 

 terminates in a primary acoustic center in the superior olive, 



21 nerve 



Figure 1. — 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 neurone of the second 

 order to the nucleus of the VI nerve. The fibres of this nerve end on the ex- 

 ternal rectus muscle of the eyeball. 



where it is taken up by an intercalary neurone and transmitted 

 to the nucleus of the VI nerve. The result is a contraction of 

 the external rectus muscle of the eyeball, turning the eye toward 

 the side from which the auditory stimulus was received. (There 

 is another path leading to the nucleus of the III nerve for the 

 innervation of the internal rectus muscle of the other eye, thus 

 providing for conjugate movements of the two eyes — but that 

 is another story.) This reflex arc operates per se in a purely 

 mechanical fashion to produce a determinate invariable type of 

 response. 



Other fibers arising in the primary acoustic center may ascend 

 to the roof of the midbrain (corpora quadrigemina), where they 

 are brought into relation with fibers belonging to other func- 

 tional systems. In the lower amphibian brains (e.g., the com- 



