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MEDULLATED NERVE-FIBRES. 



NERVE-FIBRES. 



Two kinds of nerve-fibres are met with in the body, differing from one another 

 both in their microscopical character and in their more obvious aspect : those of 

 the one kind have received the name of white fibres, on account of the appearance 

 which they present when collected in considerable numbers, as in the nerve trunks 

 or white matter of the nerve centres, the others being denominated grey fibres. 

 When examined with the microscope it is found that this difference of aspect 

 depends upon the presence or absence of a peculiar sheath to the fibre, formed of a 

 kind of fatty substance, this fatty or medullary substance, as it is termed, giving a 

 dark double contour to the white fibres (when seen by transmitted light), which is 



Fig. 352. WHITE OR MEDULLATED NERVE-FIBRES, SHOWING THE SINUOUS OUTLINE AND 

 DOUBLE CONTOURS (after Bidder and Volkmann). 



Fig. 353. A SMALL PART OP A MEDULLATED FIBRE HIGHLY MAGNIFIED (E. A. S.). 



The fibre looks in optical section like a tube hence the term tubular, formerly applied to these 

 fibres. Two partial breaches of continuity are seen in the medullary sheath, which at these places 

 exhibits a tendency to split into laminae. The primitive sheath is here and there apparent outside the 

 medullary sheath, and the delicate striae which are visible in the middle of the fibre probably indicate 

 the fibrillaled axis-cylinder. 



Fig. 354. VARICOSE MEDULLATED FIBRES FROM THE ROOT OF A SPINAL NERVE (from Valentin). 



altogether absent from those of the other kind. On account of this the white fibres- 

 are also known as the double-bordered or medullated fibres, the grey fibres being 

 termed in contradistinction the pale or non-medullated fibres, or from their dis- 

 coverer, foe fibres of RemaJc. 



The medullated nerve-fibres form the white part of the brain and spinal cord, 

 and by far the greater part of the cerebro-spinal nerves. Viewed singly under the 

 microscope with transmitted light they are transparent, and, as before stated, are 

 characterised by their well-defined even outline and, except the smallest, by their 

 double contour, which gives them a tubular aspect. 



Their size differs considerably even in the same nerve, but much more in 

 different parts of the nervous system ; some being less than 12 ^ 00 th inch (2 /*), 

 and others upwards of rsVoth inch (17 /x) in diameter. Speaking generally, they 



