136 THE CONTROL OF LIFE 



are like tlie wires, and one set of wires (called sensory 

 or afferent) carry messages to a cell-body or telephone, 

 wHle another set (called motor or efferent) carry mes- 

 sages from a cell-body or telephone. The comparison 

 is a useful one, but it must not be pressed far. Thus the 

 * Central ' Office in a telephonic system may connect one 

 telephone with another for purposes of intercommunica- 

 tion, but it is not supposed to store and combine messages 

 as our brain does. 



Besides (a) the sensory neurons which carry messages 

 inwards, and receive, store, shunt, or otherwise deal 

 with them, and (b) the motor neurons which issue and 

 carry orders, there are (c) many neurons which link the 

 other two kinds together within the central nervous 

 system. These are called connecting, communicating, 

 or internuncial cells, and they actually form the bulk 

 of the central nervous system. In some of them intelli- 

 gence is at home. 



Besides the neurons there are in the nervous system 

 supporting cells (the neuroglia) and blood-vessels, and 

 these weigh much more than the nerve-elements alone. 

 It is interesting to notice in passing a quite unique 

 thing about the nervous system, and that is the extra- 

 ordinary length of some of the nerve-fibres, which, 

 as we have seen, are outgrowths of nerve-cells. In 

 Man, for instance, some of the fibres that issue from the 

 spinal cord in the small of the back and reach to the 

 toes are far more than a yard long, and they must be 

 longer still in a giraffe. 



(C) A great part of our nervous activity consists of 



