748 



A MANUAL OF PHYSIOLOGY 



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-bodyf 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 (also spelled neurone) . 



The Neurons. A typical ^nerve-cell (Figs. 300, 302, 304) is a knot 



of granular protoplasm, con- 

 taining a large nucleus, in- 

 side of which lies a highly 

 refractive nucleolus. A cen- 

 trosome and attraction 

 sphere (p. 5) have also been 

 found in some nerve-cells, 

 $/ c though not as yet demon- 



strated in all. Pigment may 

 also be present, especially 



FIG. 300. ANTERIOR HORN CELL FROM 

 MAN SHOWING FIBRILS (BETHE). 



FIG. 301. MEDULLATED NERVE- 

 FIBRE SHOWING FIBRILS OK 

 AXIS-CYLINDER (BETHE). 



The fibrils are seen passing, 

 without interruption, across a 

 node of Ranvier. 



in old age. By certain methods of staining it may be shown that 

 fibrils (neuro-fibrils) run through the protoplasm of the cell, forming 

 a felt- work in it, and entering the dendrites on the one hand and the 



* The nuclei of the peripheral fibres belong to the neurilemma and not 

 to the medullary sheath, and while the medullary 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 pevikavyon. 



