DEVELOPMENT OF NERVE-FIBKES. 433 



the periphery, and finally form a very fine terminal plexus, with 

 anastomoses and free ends. (Fig. 278.) Next, the fibres gradually 

 enlarge to from two to four times their original diameter, and dark 

 bordered fine fibres are gradually developed in them in the peri- 

 pheral direction. Sometimes two to four dark-bordered tubules 

 are developed within the same embryonic fibre, in the tadpole ; a 

 fact not yet established in regard to the higher animals. 



3. The development of the nerve-fibres in the central organs has 

 not been thoroughly, investigated. Tubules begin to form, how- 

 ever, in the white substance of the foetal brain at the end of the 

 2d month, by the coalescence of cells. In the 4th month, nuclei 

 are still occasionally to be seen in the now wider fibres, though the 

 latter do not become dark-colored before the middle period of foetal 

 life; and in the white portion of the cord sooner than in that of the 

 brain. 



The fibres in the ganglia are developed subsequently to those of 

 the nerve-trunks, and apparently in the manner just described. 



The growth of the nerve-fibres is apparently secured, after the 

 4th month of embryonic life, solely by the enlargement of the 

 already existing elements. No new fibres are found after this pe- 

 riod. According to Harting, the size of the fibres in the median 

 nerve in the human foetus at 4 months, the new-born child, and the 

 adult, is as 3.4, 10.4, and 166. 



When nerve-fibres are cut across, they readily unite ; and por- 

 tions, 8 to 12 lines long, may be entirely regenerated, when removed 

 from the peripheral nerves. Incised wounds of the spinal cord 

 unite also. (Brown Sequard.) 



Pathological States and New Formations of the Nerve-Fibres. 



1. The nerve-fibres are readily destroyed by extravasation of 

 blood, tumors, softening, fibroid growths, &c. ; in which case the 

 medulla breaks up into larger or smaller masses, while the axis- 

 fibre disappears. Continued pressure sometimes leads to a com- 

 plete interruption of continuity of the fibres at the point com- 

 pressed. 



2. In atrophied nerves the fibres are thinner, easily broken up, 

 and sometimes the medulla is converted into minute fatty mole- 

 cules. 



3. It is not known that hypertrophy of the nerve-fibres ever 

 occurs. 



4. In case a divided nerve does not unite (as necessarily after 

 amputation), the fibres usually become yellowish, soft, lacerable, and 



28 



