NERVE-FIBRES. 



In further pursuing the subject, we shall first examine the fibres and cells 

 by themselves, and afterwards consider the structure of the parts which they 

 contribute to form, viz. , the cerebro-spinal organs, the ganglia, and the nerves. 



The fibres are of two kinds : 1, the ivhite, tubular, medullated, or dark 

 bordered, and 2, the grey, pale, non-medullated, or gelatinous. The former 

 are by far the most abundant ; the latter are found principally in the 

 sympathetic nerve, but are known to exist also in many of the cerebro- 

 spinal nerves. 



The White or Tubular Fibres (fig. LXXIV). These form the white part of 

 the brain, spinal cord, and nerves. When collected in considerable num- 

 bers and seen with reflected light, the mass which they form is white and 



Fig. LXXIV. 



, IV< 



Fig. LXXIV. A. WHITE OB TUBULAR NERVE-FIBRES, showing the sinuous outline and 



double contours. 



B. DIAGRAM to show the parts of a tubular fibre, viz. 1, 1, membranous tube. 2, 2, 

 white substance or medullary sheath. 3, axis or primitive band. 



o. DIAGRAM intended to represent the appearances occasionally seen in the tubular 

 fibres. 1, 1, membrane of the tube seen at parts where the white substance has separated 

 from it. 2, a part where the white substance is interrupted. 3, axis projecting beyond 

 the broken end of the tube. 4, part of the contents of the tube escaped. 



opaque. Viewed singly, or few together, under the microscope, with trans- 

 mitted light, they are transparent ; and if quite fresh from a newly killed 

 animal, and unchanged by cold or exposure, they appear as if entirely homo- 

 geneous in substance, like threads of glass, and are bounded on each side by 

 a simple and usually gently sinuous outline. Their size differs considerably 

 even in the same nerve, but much more in different parts of the nervous 

 system ; some being as small as the -pgiro^k anc * otners upwards of the 

 j-^o-y-th of an inch in diameter ; moreover, the same fibre may change its 

 size in different parts of its course, and it is generally smaller at its central 

 and peripheral ends. Very speedily after death, and especially on exposure 

 to the action of water, these seemingly homogeneous fibres become altered : 



k 2 



