304 PHYSIOLOGY 



latent period in the case of a reflex action, i.e. the lost time in the conversion 

 of an afferent into an efferent impulse in the central nervous system, would be 

 appreciable and would increase with the complexity of the response that is, 

 with the number of neurons involved in the reaction. Such is indeed the case. 

 , In determining the actual ' lost time ' in the central nervous system 

 for any given reflex, it is necessary to subtract from the total delay, inter- 

 posed between the application of the stimulus and the resultant movement, 

 the time taken by the impulse in travelling to and from the central nervous 

 system, as well as the latent ^period of the muscles themselves. The re- 

 mainder is known as the ' reduced reflex time.' Wundt found in the frog, 

 when a reflex contraction of the gastrocnemius was excited by a stimulation 

 of a posterior root of the same side, that the reduced reflex time was -008 sec. 

 For a crossed reflex the delay was increased by -004 sec. If we assume that 

 one additional neuron is involved in the crossed reflex, the lost time at a 

 synapse would be -004 sec. ; if two cells are intercalated, the synapse delay 

 would be only -002 sec. Since the uncrossed reflex has a delay of -008 sec., 

 at least two, and possibly four, synapses are involved in the path of this 

 simple reflex. 



The blinking excited by stimulation of the eyelid has a reduced reflex 

 time of -047 sec. 



(3) SUMMATION. When contractile tissues, such as striated or un- 

 striated muscle, are excited by single shocks, a certain minimal strength 

 of stimulus is necessary in order to produce a contraction. Weaker stimuli 

 are spoken of as sub-minimal, an( i when applied singly have apparently 

 no effect on the muscle. In dealing with the properties of involuntary 

 muscle we saw that a sub-minimal stimulus is not necessarily devoid of 

 effect because it fails to evoke a contraction, since, if repeated at sufficiently 

 frequent intervals, a summation of stimulus occurs, so that at the fifth or 

 sixth application a stimulus, which was previously ineffective, becomes 

 effective and a contraction results. The muscle will now continue to respond 

 to each stimulus, but, if the excitations be discontinued for a time, reapplica- 

 tion of a stimulus of the same strength becomes once more ineffective. This 

 summation of stimulus is a prominent feature in all reflex actions, so much 

 so that it may be often impossible to evoke a reaction to a very strong 

 single .induction shock, whereas the application of a tetanising current too 

 weak-4oLbe felt on the tongue may produce a marked reaction. We shall 

 have occasion laterjpn to deql wif.li ^pom^l examples of this summation of 

 stimulus. 



(4) FATIGUE. In the muscle-nerve preparation the weakest point 

 and that which soonest suffers from fatigue is the end-plate, or rather the 

 field of conjunction of nerve fibre and muscle fibre. In the central nervous 

 system the synapses of the different neurons are equally susceptible, and 

 since several of such synapses are involved in every reflex action, we should 

 expect to find that the central. nervous system would show signs of fatigue 

 before the peripheral structures. If a given reaction be repeatedly elicited 

 by applying a stimulus to a certain area of the surface, the reaction becomes 



