THE CENTRAL NERVOUS SYSTEM 



661 



as consisting of nerve-fibres and nerve-cells, but the antithesis of a 

 time-honoured distinction must not lead us to forget that the essential 

 part of a nerve-fibre, the axis-cylinder, is a process of a nerve-cell, 

 and the medullary sheath probably a product of the axis-cylinder.* 

 In strictness, the term ' nerve- cell ' ought to include not only the cell- 

 body, but all its processes, out to their last ramifications. But the 

 habit of speaking of the position of the cell-body! as that of the nerve- 

 cell is so ingrained that it seems better to continue the use of the 

 latter term in its old signification, and to speak of the cell and 

 branches together as a neuron. 



The Neurons. A typical nerve-cell (Plate V., 2 ; Figs. 227-229) 

 is a knot of granular protoplasm, containing a large nucleus, in- 

 side of which lies a highly refractive nucleolus. A centrosome 

 and attraction sphere (p. 18) have also been found in some nerve- 



FlG. 227. MULTIPOLAR NERVE-CELL (BARKER, AFTER KOLLIKER). 



(Golgi preparation.) n, axon ; c, collaterals. 



cells, though not as yet demonstrated in all. Pigment may also 

 be present, especially in old age. By certain methods of stain- 

 ing it may be shown that fibrils run through the cell, while in 



* Although each internode of the medullary sheath of a peripheral 

 nerve-fibre has been supposed to be formed from a cell that, in the course 

 of development, comes in contact with the axis-cylinder and ultimately 

 encircles it, a similar origin can hardly be admitted for the medulla of the 

 fibres of the spinal cord and brain, where, indeed, a segmental genesis 

 seems excluded by the absence of regularly placed internodal nuclei and 

 nodes of Ranvier. In any case the nuclei of the peripheral fibres belong 

 to the neurilemma and not to the medullary sheath, and while the medul- 

 lary sheath, like the axis-cylinder, is as regards its nutrition under the 

 control of the nerve-cell, and must therefore be looked upon as an integral 

 portion of the neuron, the neurilemma in respect both of its nutrition 

 and its development appears to be an independent structure. 



f Foster and Sherrington call the cell-body the perikaryon. 



