REFLEX ACTION 501 



of one reaction excites another, and so on. Thus the reflex protrusion of the frog's 

 tongue, excited by the sight of a fly, provides the stimulus (contact with the mucous 

 membrane of the mouth) which causes closure of the mouth, swallowing of the 

 fly, and so on, in series. In such cases as the scratch reflex, where the conditions 

 can be readily controlled, each reflex increases the excitability of the reflex arc 

 for the next succeeding one. The question of compound reflexes is a large one. 

 Sherrington's book (1906, Chapters IV., V. and VI.) should be consulted. 



FATIGUE 



Sinc*e, as we have seen (page 423), the seat of reflex fatigue is not in the 

 final motor neurone itself, it is clear that the possession of this final common 

 path by a new reflex is considerably affected by the fatigue of a previous reflex. 

 The value of this fatigue of an intermediate synapse is to prevent too long 

 possession of an effector by a particular reflex. Fatigue of a certain reflex 

 enables a second one to obtain possession of the effector, although the stimulus 

 exciting the former reflex may be still going on. The final motor neurone is 

 comparatively incapable of fatigue. 



Fatigue of a steady reflex, such as the flexion reflex of the knee, is first shown 

 by its becoming rhythmic. 



Although a reflex arc is soon fatigued, it recovers again fairly rapidly ; its 

 power of responding again may be very considerable even after ten seconds 

 of rest. 



The diminution of its excitability is gradual, so that a weak stimulus ceases 

 to be effective earlier than a strong one does. 



A reflex may cease either from fatigue or from inhibition. In a rhythmic 

 reflex, such as the scratch reflex, the difference can be seen. In the former 

 case, the beats become slower, and each beat is more prolonged and sluggish ; 

 in the latter case, there is no change in rate nor in the duration of each beat. 

 They are usually abolished altogether by inhibition, without any previous change, 

 although the amplitude may sometimes be reduced. 



NOCICEPTIVE REFLEXES 



Reflexes that protect an animal from injury are usually prepotent, that is, 

 they displace others. The receptors are probably free nerve endings, since any 

 form of nocuous stimulus is capable of exciting nerve fibres, and there is no 

 need of recognition of the kind of stimulus. Great sensibility to small stimuli, 

 so important in the higher senses, would be a disadvantage in this case. The 

 scratch reflex in the spinal animal can be driven from possession of the final 

 common path by a flexion reflex produced by a pin-prick in the foot. These 

 nociceptive reflexes recover first after spinal transection. Thus, the above- 

 mentioned flexion reflex can be obtained earlier than that next described, namely, 



THE EXTENSOR THRUST 



When the spinal cord has considerably recovered from the shock of section, 

 gentle pressure between the paws will often produce an extension reflex, in which 

 the leg is straightened out. This effect is similar to that which is caused by 

 contact with the ground in walking. 



Description of the great variety of individual reflexes would be out of place in this book. 

 Those to the viscera, the heart, and the blood vessels are described in other chapters. 



AUTOTOMY 



We may devote a few words to a peculiar reflex met with in certain Crustacea. 

 If a crab be picked up by one of its ambulatory appendages, it generally, by a 

 powerful muscular contraction, breaks this leg off at a particular place and so 

 obtains freedom. This mechanism was first investigated by Fredericq and more 

 recently by Roskam (1913). The second segment of the leg in the crab consists 



