-466 



NEUROLOGY, 



^ NERVOUS TISSUE. 



Two substances, distinguishable by their colour, enter into the 

 formation of nervous tissue — viz., the white and the grey matter. 

 Botb are soft, fragile, and easily injured, in consequence of which 

 the principal nervous centres are always well protected by the 

 skeleton. When examined under the microscope, two distinct 

 structures, fibres and cells, are found; the fibres being of two 

 kinds, the white or meduUated, and grey or non-medullated, the 

 former being more abundant. 



The white fibres, which form the white matter of the brain, 

 spinal cord, and the nerves, and are also 

 sparingly met with in the ganglia and 

 nerves of the sympathetic system, have 

 three distinct components — Is^, An exter- 

 nal envelope, the neurilemma^ which is 

 fine, transparent, and nearly homogeneous; 

 2nd, A layer of white, fatty matter, the 

 medullary sheath, or ivhite suhstaTice of 

 Schwann ; and 3rcZ, the cylinder axis, or 

 ^primitive hand of Remak, in the centre 

 of all. The diameter of these fibres varies • 

 between xTtnrth and -jTrWth of an inch, 

 though some are very much less. . 



The grey or non-m^dullated fibres 

 constitute the chief nerves of the gang- 

 lionic system, and are also present in the 

 cerebro-spinal nerves. They are smaller than the white fibres, . 

 resembling their cylinder axis, measuring from ToWth to 

 a 0^0 th of an inch in diameter, and are apparently almost 

 homogeneous in their structure. 



The nerve cells, or ganglionic corpuscles, are found in the 

 nerve centres, and in the peripheral end organs of some of the 

 nen^es. In shape they are spheroidal, caudate, or stellate. 

 Each cell is composed of a transparent membrane, filled with a 

 homogeneous, colourless matrix, in which is a number of diffused 

 minute granular bodies, and an, internal vesicular nucleus, 

 presenting one or more minute, but distinct nucleoli. The 

 processes of the cells become continuous with the cylinder axes 

 of nerve fibres, vrhile the medullary sheath appears to be 

 prolonged over the cell ; hence these cells have been described 



Fio. 174. 



• Nerre tissue — *bi-polar cells 

 and fibres. 



