THE REFLEX CIRCUITS 59 



discharge into a single center, which, in turn, by means of a final 

 common path (FCP) excites a single effector (E). If the two re- 

 ceptors upon stimulation normally call forth the same response, 

 they will reinforce each other if simultaneously stimulated, 

 the response will be strengthened, and we have another type of 

 allied reflex. But there are cases in which the stimulation of 

 Rl and R# (Fig. 18, D) would naturally call forth antagonistic 

 reflexes. Here, if they are simultaneously stimulated, a phys- 

 iological dilemma will again arise which can be resolved only by 

 one or the other afferent system getting control of the final com- 

 mon path. 



Figure 18, E illustrates still another form of combination of 

 reflexes. Here there are connecting tracts (A, A) between the 

 two centers so arranged that stimulation of either of the two 

 receptors (Rl and R2] may call forth a response in either one of 

 two effectors (El and E2). These responses may be allied or 

 antagonistic, and much more complicated reflexes are here pos- 

 sible than in any of the preceding cases. 



A few illustrations of the practical operation of these types 

 of reflex circuits will be given here and many other cases are 

 cited throughout the following discussions. A case of a simple 

 reflex has already been mentioned in the sudden twitch of the 

 hand in response to a painful stimulation of the skin. The 

 simplest possible mechanism of this reaction involving only two 

 neurons is shown in Fig. 1 (p. 25). In actual practice, however, 

 the arrangement figured is one element only of a more complex 

 reaction (see p. 61). Figure 19 illustrates a more usual form 

 of this type of reaction, where a series of three or more neurons 

 is involved and at least two cerebral centers. An auditory im- 

 pulse coming to the brain from the ear through the VIII cranial 

 nerve terminates in a primary acoustic center in the superior 

 olive (a deep nucleus of the medulla oblongata, see p. 201), 

 where it is taken up by an intercalary neuron of the second order 

 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. So far as this reaction alone is concerned, it is a simple 

 reflex, but in practice the external rectus muscle of one eye is 

 never contracted apart from the other five muscles of that eye 



