84 YETERINAPtY PHYSIOLOGY 



reflexes, such as the knee-jerk (p. 89), have very .much 

 shorter latent periods than more complex reflexes. 



2. As a result of the impulse having to traverse these 

 various synapses, there is not the same strict correspondence 

 between the strength of the stimulus and the resulting action 

 as is found in stimulating: single sets of neurons in a 

 nerve (p. 68). The result does vary with the strength of 

 the stimulus — a weak stimulus producing a feeble contraction 

 of only a few muscles, and a stronger stimulus a more 

 vigorous response in a larger number of muscles. But it 

 also varies with the condition of the spinal cord, after 

 the administration of strychnine the least touch produces 

 a powerful generalised contraction, while if the cord is cooled 

 a much feebler response is obtained. 



3. When a nerve is stimulated, the excitation stops as 

 soon as the stimulus ceases. But in reflex action there 

 is apt to be a continuance, an after-discharge, for some time 

 after the stimulus is removed. This is well seen when the 

 toes of the spinal frog are pinched ; the leg, when drawn up, 

 remains in position for a considerable time. The duration of 

 this after-discharge varies with the strength of the stimulus, 

 and with the condition of the synapses in the cord (Practical 

 Physiology). 



4. The rhythmic repetition of a subminimal stimulus is 

 more capable of liberating a reflex action than it is of 

 causing a stimulation of a neuron. Apparently the resist- 

 ance to the passage of the impulse over the synapses 

 is decreased by repetition ; just as while one knock at a 

 door may not secure its opening, a series of knocks may 

 do so. , 



5. 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. This was demonstrated 

 by using the electric variation which accompanies the passage 

 of a nerve impulse (p. 60). If the ingoing neuron A 

 (fio-. 35) and the outgoing neuron B are connected with 

 sralvanometers, when A is stiuuilated, an electric variation 

 occurs in A and in B. But if B is stimulated while an 



