948 CENTRAL NERVOUS SYSTEM 



stimuli. Their purpose is to protect the body from immediate danger 

 and consequently they tend to displace other kinds of reflexes from occu- 

 pancy of the final common path. The reflex responses to all stimuli which 

 are capable in the conscious animal of giving rise to strongly affective 

 sensations tend to prevail over all others and consequently the sexual 

 reflexes share with the responses to painful stimuli a position of domi- 

 nance in the competition of reflex activity. At the opposite extreme are 

 the postural reflexes which arise from proprioceptive stimuli and con- 

 cern chiefly the extensor muscles which must support the weight of the 

 body. These give way before competing reflexes of other types with 

 facility. It is important that they should do so, since postural tone 

 must always adapt itself to the position into which the body has been 

 forced in response to the stimuli of the environment. 



In the case of antagonistic reflexes of equal potency the result of 

 competition for the common path depends on the relative strength of 

 the exciting stimuli. Thus a flexion reflex of the hind leg will usually 

 displace a simultaneous scratch reflex, but if the stimulus eliciting the 

 scratch is strong, and that tending to produce flexion is weak, the 

 scratch reflex may persist and the flexion fail to assert itself. 



When a reflex has occupied the final common path for some time 

 it will become fatigued, and may then be displaced by an antagonistic 

 reflex which could not previously compete against it successfully. Thus, 

 ordinarily the scratch reflex is much less readily elicited than the flexion 

 reflex, and if both are excited at the same time the latter will prevail; 

 but if the flexion reflex is kept up until it shows signs of fatigue, then 

 by simultaneous excitation of both reflexes the scratch reflex will obtain 

 the mastery. The development of successive induction, which is de- 

 scribed below, also assists the competing reflex in gaining control as its 

 antagonist becomes fatigued. 



The susceptibility of reflex arcs to fatigue is probably of importance 

 in assuring the development of variety in reflex responses, since it 

 prevents prepotent responses from occupying the final common path for 

 too long. Many characteristics differentiate reflex fatigue from the 

 fatigue of an isolated nerve-muscle preparation. The most impor- 

 tant of these distinguishing features are as follows: (1) The fatigue 

 comes on intermittently; thus, when the flexion reflex is persistently 

 elicited, the first sign of fatigue is an irregular decline in the flexion 

 movement followed by its entire disappearance for a short time. These 

 lapses become more and more frequent, until at last complete fatigue 

 sets in and no flexion occurs. (2) Reflex fatigue soon passes off. (3) 

 It appears earlier for weak than for strong stimuli. (4) The movement 

 produced by the reflex action may also change in character during re- 

 flex fatigue ; thus, the beat of the scratch reflex may become slower 



