THE REGULATING MECHANISM 171 



flattened nucleated cells. The analogy of 

 a nerve-fibre to a copper wire surrounded 

 by an insulating sheath is striking, the wire for 

 conduction representing the central rod or 

 axis, while the insulating sheath is the white 

 substance. Still it is only an analogy. Nerve 

 fibres vary much in diameter. Many have no 

 white substance ; primitive fibres are desti- 

 tute of it, and it makes its appearance late 

 in development. Nerves consisting of bundles 

 of fibres divide and subdivide into more 

 and more delicate fibres, until, as already 

 pointed out, they are so minute as to be 

 invisible to the naked eye. If we trace the 

 axis of a fibre to its beginning we find that it 

 always originates from or in a nerve cell. 



95. Suppose a nerve were laid bare and it 

 were stimulated, say by gentle shocks of 

 electricity, so feeble as barely to be felt by the 

 tip of the tongue, one or more results might 

 follow : (1) a muscle might contract, and then 

 we call the nerve motor because it produces 

 motion of a muscle ; (2) a gland might begin 

 to secrete, showing the action of a secretory 

 nerve ; (3) blood vessels might diminish in 



