X XERVE-FIBRES AND CELLS 167 



Microscopic Structure of Nervous Tissue. Examination 

 of a piece of nerve under the microscope shows it to be 

 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) 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 axis-fibre (nx). Around this is a sheath 

 formed of a fatty substance and known as the medullary 

 sheath (m. s) ; 1 and, finally, investing the whole 

 fibre is a delicate, structureless membrane, the neitro- 

 lemma (ne) . At intervals the medullary sheath is absent, 

 and a node is produced, where the fibre consists simply 

 of the neuraxis covered by the neurolemma. 



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 come off from the cell by a common stalk. 

 One of these processes (the axon) becomes the neuraxis of 

 a peripheral nerve-fibre ; the other (dendron) 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 cell with its processes is spoken 

 of as a neurone. 



The white matter of the brain and spinal cord consists 

 of nerve-fibres, those in the cord having a longitudinal 



1 The medullary sheath may be absent in certain nerve-fibres 

 (e.g., in the sympathetic and olfactory nerves). 



