204 NERVE-CENTRES [CII. XVII. 



ganglia ; they are situated outside the central nervous system ; they 

 receive impulses from efferent cells in the central nervous system, 

 and distribute them to involuntary muscles and secreting glands. 



Earth-worm 



Nereis 



Vertebrate 



FIG. 215. Diagram to illustrate the primitive conditions of the afferent nerve-cell, and the manner in 

 which it becomes altered in the process of evolution. (After Retzius.) I, integument ; C, central 

 nervous system ; the arrows show the direction in which the impulse passes. 



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 



