REFLEX ACTION 



491 



the final common path, and, indeed, on the whole of the motor neurones of which 

 it consists. This is shown by the fact that two weak stimuli may be made to 

 alternate with each other, each stimulus of the one coming between those of the 

 other. If they acted on different neurones, signs of two rhythms should appear in 

 the muscles of the leg. There is not even a break or interference in the rhythm 

 of the first reflex when the second is added, although the amplitude may be 

 increased. The result must be due to the 

 refractory state induced by each of the first 

 series of stimuli, and in the same neurones 

 (see Fig. 152). 



This immediate induction only occurs 

 between allied reflexes, which will be re- 

 ferred to again later. 



Successive Induction, on the other hand, 

 is of a different nature. Suppose that we 

 excite an extension reflex in one leg by 

 appropriate stimulation of the opposite leg 

 and with such a strength of stimulus that 

 the reflex is a small one. This is done at 

 regular intervals and the reflex is found to 

 be very constant. In one of the intervals, 

 a strong and prolonged flexion reflex is 

 excited from the limb itself. After this, 

 the extension reflexes are increased both in 

 height and in duration, the effect gradually 

 passing off (Fig. 153). During the flexion 

 reflex, the extension arcs were inhibited 

 and the phenomenon is the same as the 

 "rebound," referred to under the head of 

 inhibition above (page 422). This exalta- 

 tion, after a period of inhibition, is not 

 shown by merely removing the exciting 

 stimulus, so that it is not due to rest. 



It will be clear that the increased excit- 

 ability produced by one reflex for its antago- 

 nist reflex plays a part in the mechanism 

 of such alternating movements as those of 

 stepping or locomotion. 



Irreversibility of Direction. It is a well- 

 established fact that nerve fibres conduct in 

 both directions.. The experiments of Bell 

 and Majendie showed, however, that stimu- 

 lation of the spinal end of a motor root gives 

 no sign of reflex nor of sensation. Similar 

 results of Gotch and Horsley have been 

 mentioned above (page 475). 



The experiments of Veszi (1909) showed 

 that fatigue of the reflex mechanism could 

 not be obtained by stimulation of the axones 

 of the motor neurones, and those of Fro'hlich 

 (1909, 2) showed that the same thing applies 



to the simple reflex arc of the stellar ganglion of the Cephalopod. While fatigue 

 of this ganglion can readily be produced by stimulation of the nerves proceeding 

 to it from the cerebral mass, it cannot be fatigued by stimulation of the motor 

 nerves which it gives off to the muscles of the mantle. 



We have seen when discussing the properties of the true nerve network of 

 Medusae that excitation passes in all directions. It appears from fixed preparations 

 that the " neuro-fibrils " are, in this case, continuous from one cell to another, 

 and, although these are probably artefacts, the fact of their continuity suggests 



FIG. 152. ABSENCE OF CHANGE OF 

 RHYTHM WHEN PLACE AND KATE OF 

 STIMULATION AKE CHANGED. 



Scratch reflex elicited from two skin points, A 

 and B. The point A gave the "low" form 

 of reflex ; B the " high " form. 



The interval between the stimuli at B much 

 shorter than at A. The slow rate at A is 

 marked between the signal lines. 



Time in fifths of seconds above. 



Note the absence of any sign of interference of 

 rhythm when the second series of stimuli 

 comes in and when the first is cut off. 

 Hence all the neurones of the final common 

 path must have been active in both cases. 



(Sherrington, 1909, 1, p. 12.) 



