OIL XVI.] LAW OF AXIPETAL CONDUCTION 191 



as that already described in the nerves, except that, as there is no 

 primitive sheath, there can be no multiplication of its nuclei; there 

 is instead an overgrowth of neuroglia. Degenerated tracts conse- 

 quently stain differently from healthy white matter, and can by this 

 means be easily detected. 



Another method of research which leads to the same results as 

 the degeneration method is called the embryological method. The 

 nerve-fibres which grow from different groups of nerve-cells become 

 fully developed at different dates, and so, by examining brains and 

 cords of embryos of different ages, one is able to make out indivi- 

 dual tracts before they have blended in the general mass of white 

 matter. 



We shall, however, return to this subject when later on we are 

 studying the physiology of the central nervous system in detail. 



The Law of Axipetal Conduction. 



A general law has been laid down by van Gehuchten and Cajal, 

 that all nerve impulses are axipetal, that is, they pass towards the 

 attachment of the axon, by which they leave the body of the cell. 

 In other words, the direction of an impulse is towards the body of 

 the cell in the dendrons, and away from it in the axon. When we 

 further consider that every nervous pathway is formed of a chain of 

 cells, and that the impulse always takes the " forward direction," we 

 see that there is what we may compare to a valved action which 

 permits the passage of impulses in one direction only. The synapses 

 are the situations of these so-called valves. 



On the onward propagation of a nerve impulse through a chain 

 of neurons, its passage is delayed at each synapse, hence there is 

 additional " lost time " at each of these blocks. The relative number 

 of the blocks furnishes a key to the differences found in reaction 

 time for different reflexes and psychical processes. This we may 

 illustrate by two examples, one taken from the frog, the other from 

 man. 



1. If a frog's posterior root is stimulated, the time lost in the 

 spinal cord when the gastrocnemius of the same side contracts is 

 0*008 sec. ; if the opposite gastrocnemius contracts, the additional 

 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 0*150 sec., in response 

 to a sudden flash of light 0*195 sec. The greater delay in response 

 to a visual stimulus directly corresponds to the greater number of 



