492 SPECIAL HISTOLOGY. 



are quite distinct, nuclei are still occasionally to be seen in the 

 now wider fibres, but in some they have disappeared, although 

 the fibres are without dark contours; which are not developed 

 before the middle period of foetal life (in the foetal Calf, when 

 more than 12" long, according to Valentin), and, indeed, first 

 in the spinal cord. 



As regards the subsequent changes in the nerve -fibres, it 

 has already been remarked, that they occasionally increase very 

 considerably in thickness. According to Harting (1. c, p. 75), 

 the fibres of the median-nerve which have not yet acquired 

 dark contours, measure in a four months' human embryo, on the 

 average 3-4 n, - m - J in a new-born child 104 mm , in the adult 16-6 mn \ 

 The increased thickness of the nerves themselves appears, 

 according to Harting, from the fourth month onwards, to 

 depend solely upon the enlargement of the already existing 

 elements, the foetus and new-born child already possessing the 

 same number of primitive fibres as the adult. 



[It remains to be observed, that extremely few pathological 

 changes of the nervous elements are known. In the nerve- 

 cells of the brain, the deposition of pigmentary matter becomes 

 excessive, particularly in old age; and fatty deposition also 

 takes place (Virchow, 'Archiv/ I, 1). Valentin thinks that 

 he has observed a regeneration of nerve-cells in the superior 

 cervical ganglion of the Rabbit. Nerve-fibres are readily 

 destroyed, as in consequence of extravasation of blood, tumours, 

 softening, fibroid growths, &c, in which cases the medullary 

 substance breaks up into larger or smaller, coagulated or fluid 

 masses, of very various configuration, whilst the axis-fibres 

 seem to disappear. In atrophied nerves, the fibres are ob- 

 served to be thinner, easily broken up, and, instead of the 

 medullary matter, are frequently, in parts, entirely occupied 

 by minute fatty molecules, as was seen on one occasion by 

 Virchow, in a human optic nerve, and by myself in the nerves 

 of a Frog. Nerves that have been cut across, readily unite ; 

 portions of peripheral nerves from 8 — 12'" in length, even, are 

 restored by true nervous tissue (Bidder, 1. c, p. 65; Valentin 

 'defunct, new.; p. 159, §323; and ( Phys./ 2 Aufl. I, 2, 716). 

 Should the union of a divided nerve not take place, the peri- 

 pheral end undergoes a gradual change in a particular way, 



