92 VETERINARY PHYSIOLOGY 



7. Nerve fibres do not manifest fatigue, but reflex arcs 

 readily do so, apparently through a change in the synapses, 

 and these synapses are also much more susceptible to the 

 influence of poisons e.g. deficiency of oxygen or the action of 

 such drugs as chloroform than are nerve fibres. 



8. A nerve may be stimulated again and again at very short 

 intervals of time. If it loses its excitability after stimulation 

 the refractory period is very brief. Reflex arcs manifest much 

 more prolonged refractory periods, during which it is impossible 

 to elicit another response. This is of great importance in 

 preventing confusion of movements. For, since a reflex act 

 takes an appreciable time to be performed, it is of importance 

 that it should be completed before another is started. 

 Sherringtou has studied this in " a spinal dog," a dog with the 

 spinal cord cut high up. One of the best examples is to be 

 seen in the result which follows pressing the finger between 

 the toes of the hind leg; the leg is forcibly extended as in 

 forward progression, an act in which the other limbs must 

 take their part before the first leg is again extended. Here a 

 refractory period of very considerable duration supervenes on 

 stimulation to allow of the other legs acting in proper sequence. 



The spinal reflexes are definite and purposive in character. 

 This may be shown by placing on the thigh of a decerebrated 

 frog a little piece of blotting paper dipped in acetic acid. 

 Definite and purposive movements for the removal of the 

 paper are made by the leg, movements involving the co- 

 ordinated and orderly consecutive action of certain muscles 

 and the relaxation of other muscles (Practical Physiology). 

 This implies the co-ordinated action of a number of outgoing 

 neurons in response to a particular stimulation of a few in- 

 going neurons, a co-ordinated action involving excitation of 

 certain muscles and inhibition of others. This co-ordination 

 in action must owe its origin to a process of evolution by which 

 appropriate lines of conduction have been established in the 

 spinal cord. 



The reciprocal excitation and inhibition of muscles is a feature 

 of very great importance. It has been very fully studied by 

 Sherrington in the "spinal dog." Excitation and inhibition 

 may occur (a) at the same time as when the flexors contract 

 and the extensors are inhibited in drawing up the leg, or (&) 



