CH. XVII.] AXIPETAL CONDUCTION 205 



lost time is 0'004 sec. If we assume that in the latter case, two 

 extra synapses have to be jumped, the delay at each is 0'002 sec. 



2. In the case of the eye and ear in man the total length of the 

 pathway to the brain is approximately the same, and so the reaction 

 times might be expected to be equal ; but this is not the case ; the 

 reaction time in response to a sudden sound is 0150 sec., in response 

 to a sudden flash of light 0195 sec. The greater delay in response 

 to a visual stimulus directly corresponds to the greater number of 

 synapses through which it has' to travel (see later, in the structure 

 of the visual and auditory mechanisms). 



The valved condition of nervous paths also explains another 

 difficulty. We have seen in p. 173 that under certain circumstances 

 a nervous impulse will travel in both directions along a nerve. Yet 

 when we stimulate the motor fibres in an anterior spinal root, the 

 only effect is a contraction of muscles ; there is no effect propagated 

 backwards in the spinal cord. No doubt a nervous impulse does 

 travel backwards to the anterior horn cells, but it is there extin- 

 guished, it cannot jump the synapses backwards, and there is no 

 negative variation to be detected in a galvanometer connected to the 

 pyramidal tracts in the cord. 



The law of axipetal conduction is no doubt true for the majority 

 of neurons. But there is at any rate one very striking exception, 

 namely, in the typical afferent root cells ; here the impulse passes 

 to the body of the cell by one axon from the periphery, and away 

 from it to the spinal cord by the other. To say, as some do, that the 

 peripheral process is really a dendron because it conducts impulses 

 centrifugally is simply arguing in a circle. 



