356 . SPECIAL HISTOLOGY. 



sheath, I considered myself fully justified in asserting that the dark- 

 bordered nerve- fibres were in direct connection on the one side through the 

 axis-cylinder, with the pale processes of the nerve- cells and the contents 

 of those cells, and on the other, that they passed into the pale terminal 

 nerves in the situations above mentioned. But this, by itself as I believe, 

 affords no ground for the identification of the pale fibres in question, or 

 the contents of the nerve-cells, with the axis-cylinders. This could 

 only be established, if we knew with certainty that the medullary sheath 

 of the dark-bordered nerve-fibres is superadded from without to the 

 contents of the pale embryonic fibres during the development of the 

 nerves, and is an entirely new formation between those contents and 

 the membranous sheath. This is not the case, however, it being on the 

 contrary more probable that the medullary sheath, which is also albu- 

 minous, is developed merely from a metamorphosis of the outermost 

 part of the embryonic nerve-contents, that is to say from the develop- 

 ment of fat in it, and that the axis-cylinder is the unaltered innermost 

 part of those contents. In this case all the structures, the nature of 

 which we are now discussing, would represent, not bare axis-cylinders, 

 but an entire embryonic nerve-tube, the contents of which were still 

 homogeneous, or had not undergone differentiation, and would also be 

 in continuous connection with all the parts of the dark-bordered fibres, 

 a mode of explaining them to which, at all events at present, I am dis- 

 posed to give the preference. In addition I would remark, that the pale 

 nerve-fibres are also met ivith in different stages of development. The 

 nucleated fibres in the olfactory membrane remain altogether in the 

 stage of embryonic fibres, as also, to all appearance, do the pale rami- 

 fications in the electric organ, and the contents of both these kinds of 

 nerve-tubes would appear to have little agreement, in their consistence, 

 with an axis-fibre ; in the Pacinian bodies the contents of the pale fibres 

 in all respects, represent an axis-fibre, for it is probable that a sheath also 

 exists in this situation ; in the cornea, the contents of the transparent 

 terminal nerve-tubules are apparently more fluid ; and, lastly, with 

 respect to the processes of the nerve-cells, they consist, whether they 

 have a delicate sheath or not, of a substance often exactly resembling 

 an axis-cylinder, but which is also frequently of softer consistence, cor- 

 responding with the contents of the nerve-cell. The contents of the 

 pale, non-medullated nerve-tubes, therefore, although genetically com- 

 prehending more than an axis-fibre, still in all probability are capable 

 pretty nearly of assuming its nature. 



111. The nerve-cells (accessory corpuscles (Belegungskorper), nerve- 

 corpuscles, Valentin), (Fig. 140), are nucleated cells, occurring in great 

 numbers in the gray or colored substance of the central organs, in the 

 ganglia, and occasionally also in the trunks, and peripheral expansions 

 of the nerves (retina, cochlea, vestibule). The nerve-cells are covered 



