X NERVE-FIBRES AND CELLS 167 



composed, like striped muscle, of cylindrical fibres, bound 

 together by connective tissue. The latter is much more 

 abundant than* in muscle, and in particular forms a thick 

 sheath round the nerve which must be torn off before the 

 nerve-fibres are reached. 



Each fibre (Fig. 54, A) is a cylindrical cord in which 

 three parts can be distinguished. Running along the axis 

 of the fibre is a delicate protoplasmic strand, the neuraxis 

 or axisfibre {?ix). Around this is a sheath formed of a 

 fatty substance and known as the viedidlary sheath {m.s) ; ^ 

 and finally, investing the whole fibre is a delicate, structure- 

 less membrane, the neurikm?na {ne). At intervals the 

 medullary sheath is absent, and a node is produced, where 

 the fibre consists simply of the neuraxis covered by the neuri- 

 lemma. Directly beneath the neurilemma nuclei are found 

 at intervals. 



In the ganglia are found, not only nerve-fibres, but 

 nerve-cells (Fig. 54) : these are cells of a relatively large 

 size, each with a large nucleus and nucleolus. In the 

 spinal ganglia (B) the cell-body is produced into two pro- 

 cesses, which may be united at their base. One of these 

 processes is continuous with the neuraxis of a nerve-fibre ; 

 the other is also a protoplasmic process which passes into 

 the spinal cord and sends off branches, each branch finally 

 ending in a complicated branch-work or arborisation, which 

 is interlaced, but not actually continuous with, a similar 

 arborisation arising from a nerve-cell in the spinal cord or 

 brain (Fig. 55). 



The white matter of the brain and spinal cord consists of 

 nerve-fibres, those in the cord having a longitudinal direction ; 

 the grey matter contains numerous much-branched (" multi- 



' The medullary sheath may be absent in certain nerve-fibres [e.^., in 

 the sympathetic and olfactory nerves. ) 



