NEUKO-MUSCULAK MECHANISM 



93 



they may follow one another as in the scratch reflex, which 

 may be elicited when the skin over the shoulder of the dog 

 is stimulated. The hind leg then performs rhythmic scratching 

 movements, involving alternate contraction and relaxation of 

 the flexor muscles. The mechanism involved in the ordinary 

 flexion action of scratching is indicated in fig. 41. 



That specific channels for excitation and inhibition do not 

 exist is shown by the fact 

 that under the influence of 

 strychnine, the inhibitory 

 effects may be abolished and 

 converted to excitor effects. 

 As a result of this, havoc is 

 played with the co-ordina- 

 tion of the reflexes. 



A simple uncomplicated 

 reflex action probably does 

 not exist, for every ingoing 

 impression sets up a series 

 of changes which in turn act 

 upon the reflex arc. Thus 

 an ingoing impulse from the 

 skin sets up reflex contrac- 

 tion of muscles, and this 

 contraction stimulates per- 

 ipheral structures in the 

 muscles, tendons, and joints, 

 from which impulses pass in- 

 wards to act upon the arc. 

 At the same time changes 

 in the viscera may be pro- 

 duced which lead to excita- 

 tion of ingoing fibres, and 

 these again react upon the 



FIG. 41. To show play of different Reflex 

 Arcs upon a motor neuron FC to the 

 vasto-crureus muscle of dog, e. Stimu- 

 lation of the ear, tail, fore foot, and 

 pressure on the pad of the hind foot of 

 the same side, all cause excitation, as 

 also do stimulation of the shoulder of 

 the opposite side and nocuous stimuli of 

 the opposite hind foot. On the other 

 hand, it is inhibited by stimulation 

 of the shoulder of the same side, as 

 in the scratch reflex and by nocuous 

 stimuli of the hind foot of the same 

 side. (SHERRIXGTON.) 



central synapses (fig. 42). 



The activity of every spinal arc is carried on alongside that 

 of many others, and each of these may materially modify its 

 character. Two reflexes induced by similar stimulation from 

 adjacent areas may reinforce one another and increase the 

 general reaction. On the other hand, two stimuli of different 



