1912.] Innervation and Symmetrical Muscles. 225 



afferent is stronger -than the inhibitory effect, and the excitatory effect is 

 ipsilateral. So that with both flexors and extensors the ipsilateral effect is 



Fig. 6. — Flexor muscles, psoas and sartorius, of right and left hips. Stimulation of the 

 right and left peroneal nerves together causes contraction of both the right and 

 left muscles synchronously. The stimulation intensities right and left were approxi- 

 mately equal. Decerebrate cat. 



the stronger, but in the case of the flexors it is excitatory while with the 

 extensors it is inhibitory. 



IV. Algebraic Summation as a Factor in Producing the Change. 



In the case of these symmetrical muscles therefore the change from 

 reciprocal reflex effect to identical is clearly explicable by algebraic summa- 

 tion of excitation and inhibition.* The result may be stated numerically. 

 Eight peroneal nerve under a stimulus whose intensity may be figured as 10, 

 causes an excitation of the motoneurones of right flexor muscle of an 

 intensity expressible as 10, and a weaker inhibition of the motoneurones of 

 left flexor whose intensity may be called 6. It at the same time causes also 

 an inhibition of the motoneurones of the right extensor of intensity 10, and 

 a weaker excitation of the motoneurones of left extensor of intensity 6. 

 Similarly, the reflex effect of the left peroneal nerve under a stimulus 

 of like intensity is on the left flexor's motoneurones an excitation 

 of value 10, and on right extensor's motoneurones of value 6, while 

 left extensor's motoneurones it inhibits with value 10 and right flexor's 

 motoneurones with value 6. Denoting (fig. 7) excitation by the prefix + 

 and inhibition by the prefix — , the resultant reflex effect on the 

 motoneurones of the muscles severally is, when both nerves are stimulated 

 concurrently, + 4 for the flexor in each limb, and — 4 for the extensor 

 in each limb. So that, if we suppose all the muscles to be previously in 

 a condition of medium tonus the two symmetrical flexors then contract and 



* 'Roy. Soc. Proc.,' 1908, B, vol. 80, p. 565. 



