INTEGRATION OF ACTION WITHIN THE REFLEX ARC 943 



these limbs forward preparatory to supporting the body at the next 

 step. The reflex figure is seen from this to be an integral mechanism 

 out of which is built the functional act of stepping away from a stim- 

 ulus which endangers the hind foot. 



The entire reflex figure may be considered to be an unified reflex 

 act depending on the central connections of a single afferent path, for 

 much the same reasons which lead us to conclude that excitation and 

 inhibition of the antagonistic muscles at a single joint are parts of a 

 single reflex. In all parts of the reflex figure reciprocal inhibition 

 may be seen to occur, as may be proved by studying that component 

 known as the crossed-extension reflex. As we have pointed out above, 

 stimulation of an afferent nerve from one foot produces a contraction 

 of the contralateral extensors and an inhibition of any contraction 

 which may exist in the flexor muscles (Pig. 226). Certain collaterals 

 of the afferent paths of the reflex must be assumed to cross the cord 

 and exert an inhibitory effect upon motor neurons of the flexors of 

 the crossed hind limb somewhat in the manner which Fig. 240 indicates. 

 In a similar way the paths which lead to the fore limbs must give 

 off collaterals which inhibit the contraction of those muscles which 

 would oppose the assumption of the reflex figure. A single afferent 

 path may in this way, not only produce contraction in a large group 

 of muscles, but by the inhibition of the activity of their antagonists 

 it can preoccupy a large part ' of the reflex mechanism of the spinal 

 cord to the exclusion of other reflexes. Because of the compensatory 

 nature of the component parts of the reflex figure, (including the occur- 

 rence of reciprocal inhibition in each part), which is brought about by 

 the connections of the various collaterals of the afferent path, the spinal 

 cord is enabled, quite independently of the higher centers in the brain, 

 to effect a high degree of coordination in reflex response. 



The various parts of the reflex figure do not respond to the same 

 threshold value of stimulation. Weak stimulation brings into activity 

 only those muscles which affect the part to which excitation is applied. 

 If the strength of stimulation is gradually increased, more and more 

 parts of the total reflex figure appear. In stimulating the hind foot 

 with increasing intensity, first the ankle alone is flexed, then the knee, 

 and finally the hip. The response then spreads to include the extension 

 of the crossed hind leg and finally involves the fore legs also. The 

 mechanism which determined the course of this march of the reflex 

 figure is the graded resistance of the synapses described on page 842. 

 It may be reemphasized, however, that as the reflex spreads to each 

 new joint the inhibition of the antagonists occurs coincidently with the 



