THE NERVOUS SYSTEM. 431 



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 (Yirchow, " 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 de- 

 stroyed, as in consequence of extravasation of blood, tumors, softening, 

 fibroid growths, &c., in which cases the medullary substance breaks up 

 into larger or smaller, coagulated or fluid masses, of very various con- 

 figuration, whilst the axis-fibres seem to disappear. In atrophied nerves, 

 the fibres are observed 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 lines even are restored by true nervous tissue (Bidder, 1. c., 

 p. 65; Valentin " de fund, nerv.," p. 159, 323; and " Phys." 2 

 Aufl. I. 2, 716). Should the union of a divided nerve not take place, 

 the peripheral end undergoes a gradual change in a particular way, 

 with a simultaneous extinction of the nervous activity. The nerve- 

 fibres generally become yellowish, soft, lacerable, and lose their trans- 

 versely banded and glistening aspect. They no longer present any 

 trace of a double contour, their medullary substance is wholly coagu- 

 lated, and their breadth frequently very various (Stannius in Mull. 

 " Arch.," 1847, p. 452). Whether the axis-fibres undergo change, we 

 are, unfortunately, not informed. According to Brown-Sequard, incised 

 wounds, even of the spinal cord, in the Rabbit, united. Hypertrophies 

 of the nerve-substance itself are unknown, although probably such a 

 condition occurs in the neurilemma. Virchow noticed a new formation 

 of fine nerves in pleuritic and peritoneal adhesions, and, according to 

 the same observer, it would appear that gray nerve-substance may be 

 formed on the walls of the cerebral ventricles. 



127. With respect to the functions of the nervous system, the follow- 

 ing remarks, which are immediatelypertinent to the anatomical facts, may 

 suffice. As regards the two elementary portions of the nervous system, 

 anatomical investigation shows, that all its divisions, which preside over 

 the higher functions, contain gray substance in greater or less quantity, 

 as in the sympathetic, the ganglia of the spinal and cerebral nerves, 

 and in the spinal cord and brain ; whilst the nerves which act only as a 

 conducting apparatus, contain nothing but nerve-fibres. This being 

 admitted to be the attribute of the gray substance, it may further be 

 inquired whether it presents differences in its structure, as it does in its 

 functions. With respect to this I would remark as follows : the largest 

 nerve-cells are met with in situations from which motory effects proceed, 



