SECT. 125.] NERVOUS SYSTEM. 26 I 



already been remarked, that in part they increase very consider- 

 ably in thickness. According to Harting, the fibres of the median 

 nerve of the human embryo of four months, measure, on an 

 average, 3'4 mm , in a newly born infant, io # 4" im , in the adult, i6 - 6 mm . 

 From the fourth month onwards, the increase in thickness of the 

 nerves themselves appears, according to Harting, to depend simply 

 and solely upon the enlargement of the already existing elements, 

 since the foetus and newly-born infant possess the same number 

 of primitive fibres as the adult. 



Extremely few pathological changes of the nerve-elements are known. In 

 the nerve-cells of the brain, especially in old age, excessive deposits of pig- 

 ment aud deposits of fat are very frequently met with. Valentin believes he 

 has seen a regeneration of ganglion-globules in the superior cervical ganglion 

 of the rabbit, and Waller in the vagus ganglion of the rabbit, while, on the 

 other hand, Schroder could find nothing of the kind. The nerve-tubes are 

 easily destroyed by extravasations of blood, by tumours, by fibrous growth, 

 etc., and by softening of the brain or cord, in which cases the white substance 

 of the tubes breaks down into larger or smaller, very variously-shaped, coagu- 

 lated or fluid masses, whilst the axial fibre seems to disappear. 



In atrophied nerves, the nerve-tubes are thinner, readily break down, and, 

 instead of white substance, are partly, often entirely, filled with small fat mole- 

 cules. Nerves, when cut through, readily re-unite, and even pieces 8'" to 12'" 

 long cut out of peripheral nerves, are replaced by true nervous tissue. Till 

 quite recently, it was assumed, that, in these cases, the nerve-fibres simply 

 coalesced, or were united by the new fibres formed between the two cut 

 extremities ; but Waller has lately asserted, that the nerve-fibres in the 

 part of the nerve below or beyond the place of section always perish, and 

 that the regeneration is effected by means of new fibres, which are developed 

 between the necrosed nerve-fibres. According to Waller, the old nerve- 

 fibres degenerate as far as their ultimate terminations, their white substance 

 first coagulating and then breaking down into larger and smaller irregular 

 masses, which at last entirely disappear. 



How the new fibres, which are very pale, transparent and narrow, arise, is 

 not stated by Waller, and it appears to me as still by no means proved, that 

 such a formation takes place, seeing that I at least found no trace of such a 

 formation in regenerated nerves, but observed numerous collapsed, non- 

 medullated old tubes which, perhaps, subsequently receive medulla again. 

 In fact, Bruch also (Zeitsehriftf. Wits. Zool. vi.) believes he has seen a simple 

 coalescence of the separated nerve-fibres. Schiff and Lent also found a short 

 time since, that the fibres, which Waller regarded as new-formed, are nothing 

 but the old tubes which have become non-medullated. When the ends of 

 the nerves coalesce, these fibres receive anew their medulla, and consequently, 

 Waller's hypothesis cannot be maintained. When a cut nerve does not heal, 

 the peripheral end, contemporaneously with the extinction of the nervous 

 activity, gradually alters in a particular manner. The nerve-tubes, as a whole, 

 become yellowish, soft, friable, and lose their transversely-banded appear- 

 ance, together with their lustre. The individual nerve-tubes no longer 

 present a trace of double contours ; their medulla is completely coagulated, 



