THE NERVOUS SYSTEM 425 



nodes. The portion of nerve included between two nodes has a 

 nucleus somewhere in it lying beneath the neurilemma. A 

 bundle of such fibres enclosed in a sheath — and they may number 

 thousands to the bundle — constitutes a nerve. A non-medullated 

 nerve resembles the above in every respect, excepting the 

 medullary sheath. Of this it has none. It consists of an axis 

 cylinder covered by a neurilemma. The fibre is freely nucle- 

 ated, and a bundle of such fibres in a sheath constitutes a non- 

 medullated nerve. This class of nerve belongs solely to one 

 branch of the nervous system — viz., the sympathetic — whereas 

 the medullated fibres are confined to the larger system of cerebro- 

 spinal nerves. 



The essential feature in the nerve is the axis cylinder ; it is 

 the true impulse-conducting substance. The function of the 

 medullary sheath is not definitely known, and it has been assumed 

 to play the part of a non-conductor and insulate one fibre from 

 another ; but evidently this does not exhaust its uses, for, as just 

 mentioned, the fibres of the sympathetic system are without a 

 medullary sheath. Even medullated nerves lose their sheath 

 before terminating in the tissues. Medullated nerves are more 

 sensitive to stimuli than non-medullated, and the longer the 

 nerve the greater the thickness of the sheath, but at present 

 these facts cannot be connected with its function. 



The axis cylinder, as previously stated, is the important 

 part of the nerve ; it is the conducting substance, and its nature 

 and origin are of the utmost significance. We shall see presently 

 that the entire nervous system consists of nerve-cells and fibres. 

 The nerve-cell is the essential feature, and no matter whether the 

 brain, spinal cord, or ganglia be examined, nerve-cells character- 

 istic of the tissue are present ; the nerve-cells of the cerebellum 

 are absolutely distinct from those of the cerebrum ; the cells in 

 the ganglia are different from either, or from those found in the 

 spinal cord. It does not matter how greatly the nerve-cells 

 differ in type, they all conform to one law, and that is they each 

 furnish from one pole of the cell a process which becomes a nerve- 

 fibre. No cell furnishes more than one such process, and that 

 process constitutes the axis cylinder of the nerve. When the 

 neurone doctrine is dealt with, more will be said on this subject, 

 but one piece of information must be anticipated, and that is a 

 nerve-fibre is really an elongated process of a nerve-cell, and may 

 run for a considerable distance without a break ; even when it 

 is broken the thread is again taken up, so that the axis cylinder 

 runs from its origin to its termination. There is no union of 

 nerve-fibres ; of the thousands in a large bundle each and every 

 one is complete in itself, and it is this which enables the same 

 nerve-trunk to convey impulses of many and opposite kinds, of 



