232 Reciprocal Innervation and Symmetrical Muscles. 



absolute values rise and fall pari passu with the stimulus intensity. What 

 has just been said has to be remembered in relation to that statement. 

 With decrease of stimulus intensity, as just said, a value of stimulus is 

 ultimately reached, at which in the decerebrate preparation contralateral 

 effect still remains excitatory, but ipsilateral becomes excitatory instead of 

 inhibitory. The biological meaning of this may be that with these weak 

 stimuli the reflex produced is that of standing, i.e. a local reflex contributory 

 to the great compound reflex of standing, whereas, with stronger stimuli, the 

 reflex produced is the nociceptive flexion reflex, or the flexion phase of a 

 locomotor step-reflex. 



With the flexor muscles of the hip, psoas, tensor fasciae femoris, and 

 sartorius, strong stimulation of one peroneal nerve sometimes excites 

 contraction in these muscles in the contralateral, as well as in the ipsilateral 

 limb. The contraction of the contralateral muscles is less strong than that 

 of the ipsilateral. It tends to be followed on withdrawal of the stimulus by 

 marked rebound contraction. The contralateral contraction of the hip- 

 flexors recalls the contralateral contraction of the ankle flexor, tibialis 

 anticus, noted by T. Graham Brown,* as sometimes occurring in both 

 decerebrate and spinal preparations. 



VII. Summary of Conclusions. 



1. The occurrence and distribution of reciprocal innervation extends to 

 cases of muscular co-ordination beyond those • involving simple mechanical 

 antagonism. Thus it is exemplified also in reflexes actuating symmetrical 

 muscles, for instance, muscles symmetrically placed in the right and left limbs. 



2. These muscles present the problem that, in reflexes, though often 

 worked reciprocally, they are also often worked identically. 



3. Experiments cited show certain ways in which the stimulations can be 

 experimentally arranged to give either reciprocal or identical innervation of 

 symmetrical muscles of right and left limb. 



4. It is shown that algebraic summation of excitation and inhibition can 

 explain this result. 



5. It is further 'shown that there is evidence that other factors besides 

 simple algebraic summation of the individual component reflexes have a 

 share in changing the reciprocal innervation into an identical. A reversal of 

 the weaker element of one of the components appears to occur. Thus, the 

 excitatory effect on the contralateral extensor tends occasionally to be 

 reversed to inhibitory effect, and the inhibitory effect on the contralateral 

 flexor tends to be reversed to excitatory effect. 



* ' Journ. Physiol.,' 1912, vol. 44, p. 125. 



