402 SPECIAL HISTOLOGY. 



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 super- 

 added 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 pro- 

 bable that the medullary sheath, which is also albuminous, is 

 developed merely from a metamorphosis of the outermost part 

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

 development 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 

 disposed to give the preference. In addition I would remark, 

 that the pale nerve-fibres are also met with 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 ramifications 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 appa- 

 rently 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, corresponding 

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

 nou-medullated nerve-tubes, therefore, although genetically 



