THE REFLEX CIRCUITS 61 



ent impulses of the muscle sense, tendon sense, etc., exerts a 

 constant influence upon the center which receives the initial 

 stimulus, so that this center is constantly under the combined 

 influence of the external stimulus which sets the reflex in motion 

 and the internal stimuli arising from the muscles themselves 

 (proprioceptors, see p. 86) which control its course. In this 

 case there is a true physiological circuit rather than an arc or 

 segment of a circuit, as is commonly implied in the expression 

 "reflex arc." This case is typical of the complex reflexes of the 

 body in general, and for this and other considerations we follow 

 the usage of Dewey (1893) and term the mechanism of a com- 

 plete reflex a "reflex circuit" rather than an arc (see C. J. 

 Herrick, 1913, and p. 308). 



It has been suggested by Loeb also that many instincts are 

 -^^xply complex chain reflexes. Even in animals whose behavior 

 ^X^is so complex as .birds, a careful analysis of the cycle of nest 

 building and rearing of young reveals many clear illustrations of 

 this principle (see the works of F. H. Herrick, cited at the end of 

 this chapter). Each step in the cycle is a necessary antecedent 

 to the next, and if the series is interrupted it is often necessary 

 for the birds to go back to the beginning of the cycle. They 

 cannot make an intelligent adjustment midway of the series. 



The complex circuit illustrated by Fig. 18, C presents two 

 possible types of reaction, either allied or antagonistic reflexes. 

 The former case is illustrated again by the sudden movement of 

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

 is brought about, as we saw in considering the simple reflex, by a 

 contraction of the arm muscles. But the muscles which move 

 the elbow-joint are not, when the arm is at rest, entirely flaccid. 

 Both flexors and extensors are always contracted to a certain 

 degree, one balanced against the other. Now at the same time 

 that the sensory stimulus from R (see Fig. 18, C) causes the con- 

 traction of the flexor muscle, El , it also causes the relaxation of 

 the antagonistic extensor, E2 the two efferent impulses coopera- 

 ting to effect the avoiding reaction as rapidly as possible. In 

 the antagonistic reflexes of our third type the physiological reso- 

 lution involved in the selection of one or the other possible 

 reaction always involves a delay in the response until one motor 

 pathway dominates the system to the exclusion of the other. 



