198 NERVE-CENTRES. [CH. xvn. 



fibres correspond to the wires that carry the messages to the 

 central offices, and the efferent nerve-fibres are represented by 

 the wires that convey messages from the central offices to more 

 or less distant parts of the country. This illustration will serve 

 us very well for our present purpose, provided that it is always 

 remembered that a nervous impulse is not electricity. Suppose, 

 now, one wishes to send a message from the metropolis, which 

 will represent the brain, to a distant house, say in the Highlands 

 of Scotland. There is no wire straight from London to that 

 house, but the message ultimately reaches the house ; one wire 

 takes the message to Edinburgh ; another wire carries it on to 



Fig. 204. Reflex action. 



the telegraph station in the town nearest to the house in question ; 

 and the last part of the journey is accomplished by a messenger 

 on foot or horseback. There are at least two relays on the journey. 

 It is just the same with the nervous system. Suppos'e one 

 wishes to move the arm; the impulse starts in the nerve-cells 

 of the brain, but there are no fibres that go straight from the 

 brain to the muscles of the arm. The impulse travels down 

 the spinal cord, by what are called pyramidal fibres, to the nerve- 

 cells of the spinal cord, and from these cells, fresh nerve-fibres 

 pass to the arm-muscles, and continue the impulse. Here again 

 the connection between the nerve-units is by contiguity, not by 

 continuity. This is shown in the accompanying diagram (fig. 205). 

 The cell of the cerebral grey matter is represented by C. C., the 

 pyramidal nerve-fibre arborises around the cell of the spinal cord 



