THE MECHANISM OF CO-ORDINATED MOVEMENTS 387 



excitor for the flexor muscles of the same side and for the extensor 

 muscles of the opposite side, and inhibitor for the extensor muscles of 

 the same side and for the flexor muscles of the opposite side. 



The ascending branches of the nerve fibre in the same way will 

 have endings which, while inhibitor for the greater number of other 

 possible reflex changes, will be excitor in a slight degree for certain 

 efferent neurons whose action is allied to that of the primary reflex. 

 The diagram shows also that the contraction of the flexor muscle, 

 set up as the result of stimulating a , itself initiates a secondary reflex 

 process from muscle up the nerve fibre ' and back again to the muscle 

 by the efferent neuron. This muscular afferent nerve also has central 

 terminations of two signs excitor to itself and inhibitor to the 

 antagonistic muscles. For the sake of clearness the diagram omits 

 a number of other channels coming from other regions of the cord, or 

 from other afferent nerves, the sign of which would be negative, i.e. 

 which would tend to inhibit the activity of the whole reflex arc. 



We see therefore that from every sensitive point on the surface of 

 the body impulses can be initiated which will set into action whole 

 chains of neurons, and will have a widespread influence throughout 

 the central nervous system. It is important to note that the efferent 

 path innervating, say, the flexor muscles of one side is common to 

 many reflexes. It is used, for instance, by mutually antagonistic 

 reflexes such as the scratch reflex and the flexor or pain reflex. We 

 must assume therefore that the mutual inhibition of different reflexes 

 occurs, not in the ' final common path ' i.e. in the motor neurons, 

 which must always remain open but further back in the arc, probably 

 near its afferent side. 



We have reason to believe that the propagation of impulses 

 through the central nervous system involves expenditure of energy, 

 and that the seat of this expenditure may be located in all probability 

 at the synapses. It follows that the result of any particular sensory 

 stimulation will not be absolutely invariable, but that the spread of 

 the nerve process in the nervous system, and the degree of block pre- 

 sented by the various synapses and determining the potency of any 

 given reaction, will depend on the condition of the various synapses 

 at the time of the stimulation. 



This condition may be altered in various ways. Repeated excita- 

 tion causes in the synapses, just as in the nerve-endings of the skeletal 

 muscle, a condition of fatigue. Stimulation confined to a single point 

 in the ' scratch area ' of- the spinal dog excites a scratch reflex which 

 rapidly dies away. On shifting the exciting electrodes a little to one 

 side the reflex act begins again, often with greater force than at first, 

 and a very prolonged reaction can be induced by gradually moving the 

 electrodes along the surface of the skin. A reflex arc therefore rapidly 



