258 



NERVOUS SYSTEM. 



[sect. 125. 



consist only of such fibres and of rudiments of the neurilemma, 



and are grey or dull- 

 white, like sympa- 

 thetic fibres; at a 

 later period, in the 

 human embryo of 

 four or five months, 

 they become pro- 

 gressively whiter, 

 and the proper white 



or 



1. Two nerve-fibres from the ncrcus ischiadic™ of an embryo 16 

 weeks old. 2. Nerve-tube from a newly-born rabbit, a. Envelope 

 of the same. b. Nucleus, c. Medullary sheath. 3. Nerve-fibre 

 from the tail of the tadpole, a be. As above, d. The fibre still 

 presents the embryonic character ; the dark-bordered fibre shows 

 a division. 



With reference to the pro- 

 duction of the white sub- 

 stance, direct observation 

 shows only this much, that 

 the contents of the pale 

 embryonic tubes gradually 

 acquire darker and darker 

 contours, and they finally 

 appear as genuine dark- 

 bordered fibres. Now, since 

 it can be demonstrated, that 

 the fibres, while becoming 

 so altered, do not change 

 their diameter, we may, 

 perhaps, be warranted in 

 assuming, that the white 

 substance is formed by 

 a chemical metamorphosis 

 of a part of the contents 

 of the primitive nerve- 

 tubes, whilst the remain- 

 der becomes the axis- 

 cylinder. 



The development of the 

 nerve - terminations, which 

 appears to differ in some 

 respects from that of the 

 nerve-trunks, may, as I have 

 shown (Annal. d. Sc. Nat., 



medullary sub- 

 stance becomes more 

 and more developed 

 in their fibres. 



Fig. 108. 



Nerves from the tail of the tadpole, magnified 350 

 times. 1. Embryonic nerve-fibre, in which two or 

 more dark-bordered nerve-tubes have been developed. 

 2. Embryonic fibres, containing only one dark-bordered 

 tube, which in the one fibre ceases at b. 3. Embryonic 

 ] ale fibres. 4. Fusiform cells connected with each 

 other, and with a fully developed nerve-fibre. 



