NERVE. 



[CH. VII. 



wish to dwell upon in this chapter. Under the microscope 

 nervous tissue is found to consist essentially of 

 two elements, nerve-cells and nerve-fibres. 

 A The nerve-cells are contained in the brain and 



spinal cord, and in smaller collections of cells on 

 the course of the nerves called ganglia. The part 

 of the nerve-centres containing cells is called grey 

 matter. 



The nerve-fibres are contained in the nerves, 

 and in the white matter of brain and spinal cord. 

 The nerve-fibres are long branches from the nerve- 

 cells, which become sheathed in a manner to be 

 immediately described. 



Nerve-cells differ in size, shape, and arrange- 

 ment, and we shall discuss 

 these fully when we get to 

 the nerve-centres. For the 

 present it will be convenient 

 to confine ourselves to the 

 nerve-fibres as they are found 

 in a nerve. 



Nerve-fibres are of two his- 

 tological kinds, medullated and 

 non-medullated. Medullated 

 nerve-fibres are found in the 

 white matter of the nerve- 

 centres and in the nerves origi- 

 nating from the brain and 

 spinal cord. Non-medullated 

 nerve-fibres occur in the sym- 

 pathetic nerves. 



The medullated or white 

 -B fibres are characterised by a 

 sheath of white colour, fatty 

 in nature, and stained black 

 by osmic acid ; it is called 

 the medullary sheath or ivhite 

 substance of Schwann ; this 

 sheathes the essential part of 

 the fibre which is a process 

 Fig. 1 1 8. Nerve- from a nerve-cell, and is called 

 with osmic the axis cylinder. Outside the medullary sheath is 

 B? d ' "nucleus'. a tnul homogeneous membrane of elastic nature 

 (.Key and Ret- called the primitive sheath or neurilemma. 



Fig. 1 19. A nodeof Ran- 

 vier in x medullated 

 nerve - fibre, viewed 

 from above. The me- 

 dullary sheath is inter- 

 rupted, and the primi- 

 tive sheath thickened. 

 Copied from Axel Key 

 and Retzius. x 750. 

 (Klein & Noble Smith.) 



