NERVE-FIBRES 253 



processes of neighbouring neurones. The cell-body is 

 sometimes known as the nerve-cell. It should be remem- 

 bered, however, that the whole neurone, processes and all, 

 is one cell. The cell-body contains a well-marked nucleus, 

 within which is a nucleolus. In a perfectly fresh cell the 

 protoplasm surrounding the nucleus contains fine granules 

 uniformly distributed. Shortly after death these granules 

 clump together and form the Nissl bodies, which stain 

 readily with methylene blue. But although entirely a 

 post-mortem phenomenon, the formation of Nissl bodies 

 fails to occur {ehroinatolysis) unless the cell was previously 

 in a healthy state. It fails when the cell has undergone 

 prolonged disuse or excessive fatigue. 



Among the granules, and traversing the cell-body from 

 dendrites to axon are fine fibrils which join together to 

 form a plexus. 



According to the number of processes arising from the 

 cell-body the nerve-cell is known as unipolar, bipolar, or 

 multipolar. However many processes the cell may possess, 

 only one is termed an axon. Collections of cell-bodies 

 outside the central nervous system are known as gangha, 

 and inside are often termed nuclei. 



Nerve-fibres 



These are the processes of the nerve-cells. They are 

 of two kinds : white or medullated, and grey or non- 

 medullated. Medullated fibres consist typically of three 

 layers. The innermost layer — the axis cylinder — is com- 

 posed of fine longitudinal fibrils continuous with those of 

 the cell-body. Surrounding the axis cylinder is the 

 medullary sheath, composed of a hpoid substance known 

 as myelin. It is non-nucleated and probably structureless. 

 It is interrupted at intervals — the nodes of Ranvier. The 

 medullary sheath probably serves to protect, nourish and 

 insulate the axis cylinder. 



Surrounding the medullary sheath is the neurolemma or 

 sheath of Schwann. This forms a thin nucleated and un- 



