NEUEO-MUSCULAR MECHANISM 91 



cation of a stimulus to a frog's foot and the drawing up of 

 the leg is many times longer than the time which would be 

 taken in the passage of a nerve impulse up to the cord and 

 down again (Practical Physiology). The change takes an 

 appreciable time to pass across the synapse. The duration of 

 this time varies very greatly and is dependent on the strength 

 of the stimulus and on the condition of the synapse. 



2. While the latent period varies with the strength of 

 stimulus, there is not the same correspondence between the 

 strength of the stimulus and the extent of reaction in reflex 

 arcs as there is in simple nerve fibres, and the extent of the 

 reaction depends very largely upon the condition of the 

 synapses. Thus, in a decapitated frog poisoned with strychnine 

 the least touch produces very powerful reactions, while if ice 

 be put on the spinal cord, very strong stimulation may call 

 forth no response (Practical Physiology). 



3. When a nerve is directly stimulated, the effect' stops 

 with the stoppage of the stimulus. But with reflex arcs 

 the effect may be continued as an after-discharge for some 

 considerable time after the stimulus is stopped. The extent of 

 this after-discharge varies with the strength of the stimulus 

 and with the condition of the synapse (Practical Physiology). 



4. In a nerve the smallest effective stimulus causes the 

 passage of an impulse and repetition of the stimulus does 

 not increase its effect. But in reflex arcs a subminimal 

 stimulus if repeated again and again breaks down the resist- 

 ance to its passage across the synapse and leads to a re- 

 action ; there is a summation of stimuli (Practical Physiology). 



5. In a nerve rhythmic stimuli lead to results at the same 

 rate. But rhythmic stimulation of the receptors of a reflex 

 arc is apt to set up reflex movements of quite independent 

 rhythm. This is well seen in the scratch reflex which may be 

 elicited by stimulating the skin over the shoulder in a dog 

 with the spinal cord cut across high up. At whatever rate 

 the stimuli are applied to the skin, the scratch movement 

 of the hind leg recurs regularly four or five times per second. 



6. While a nerve conducts impulses in both directions, a 

 reflex arc allows its passage across the synapse in one direction 

 only from the receiving to the reacting neuron. There is, as 

 it were, a valve action. 



