THE MACHINERY OF THE BRAIN 251 



destination, which we shall suppose in this case to be 

 the skin of the sole of the foot. Thus the muscles of 

 the sole as well as its skin are linked directly to the 

 machinery of the spinal cord by means of nerve wires. 

 Many uncertainties were cleared away from the nerve 

 system by the discoveries of Professor His. We became 

 assured that every nerve fibre is simply an outgrowth or 

 branch of a nerve unit or cell. Besides sending out a 

 main process or fibre from one end, a cell-unit in the 

 cord or in the brain always threw out from its opposite 

 end a series of shorter branches or antennae — dendrites 

 they are called, — which we shall see are of the utmost 

 consequence in the mechanism of the brain. The 

 meaning of Dr Waller's discovery became also quite 

 clear. Nerve fibres die when they are divided because 

 they are merely prolongations from mother units ; they 

 die when separated from their source, just as a finger 

 dies when it is amputated. Further, the machinery of 

 the whole nerve system is constructed on a unit system. 

 The units differ in size and shape ; the antennas vary 

 with the particular duty which units have to carry out ; 

 the fibre processes vary in thickness, length, and in 

 manner of insulation. In one region we may find the 

 units grouped in great masses, while in another they are 

 scattered to form small stations. The fibres may form 

 great trunk cables or mere meshworks. But everywhere 

 throughout the whole nerve system we can be certain 

 that we are dealing with a system composed of units 

 formed on a common ground plan. In the next chapter 

 we are to see how far we can apply the conception of a 

 telephone system to the central nerve system. There is 

 a great temptation to regard the cell units as microscopic 

 batteries and the fibres connected with them as wires. 

 But for reasons which will appear as we proceed, we 

 prefer still to speak of the units as " operatives," meaning 

 thereby that they also share in carrying on the work 

 which operators do in telephone exchanges. 



