220 Mr. F. Nansen on the Ilistological Structure 



found it impossible to trace the nerve-processes in the poste- 

 rior horn (therefore in the middle part of the grey substance) — 

 just as the fibrillffi running to the dorsal nerve-rods can only 

 be traced for longer or shorter distances, and only very rarely 

 quite to the grey substance. This appears also to show an 

 agreement with Golgi's description of the human spinal cord, 

 and I must assume that the nerve-processes in this middle 

 part of the grey substance (the posterior horn) at any rate 

 for the most part divide up into small branches and lose them- 

 selves in the fibrillar web, from which again most of the 

 fibrillar in the dorsal nerve-rods originate ; the correctness 

 of this view, however^ remains to be proved by more certain 

 methods of investigation. 



Besides the above-mentioned cells, a second kind of cell 

 occurs in the grey substance. Similar cells have also been 

 described by Ahlborn in Petromyzon. They appear, as that 

 author also admits, not to be of nervous nature, but rather to 

 belong to the connective substances ; they are chiefly situated 

 around the central canal, and perfectly agree in appearance 

 and form with the epithelial cells in the epithelium surrounding 

 the central canal. Each of these epithelial cells has a very 

 long process, which is very easy to trace by suitable methods 

 of treatment j I have even frequently been able to trace them 

 quite to the periphery of the spinal cord j probably, indeed, 

 this is the case with all these processes, so that they form 

 connective fibrils radiating to the periphery. This appears 

 indeed to be the general condition in the Vertebrata. Prof. 

 Golgi long since found a similar condition in the fowl's 

 embryo; 1 have myself seen his preparations, in which it 

 was to be observed remarkably distinctly (he has, however, 

 as yet published nothing on the subject). I have myself also 

 recognized it in the spinal cord of the tench {Tinea vulgaris) 

 which had been treated in accordance with Golgi's method. 

 The cells situated outside of the central canal in Myxine have 

 similar processes, and it is probable that these comport them- 

 selves in the same way. As previously stated, the connective 

 tissue or supporting substance existing between the central 

 canal and the ventral longitudinal furrow seems to consist of 

 similar intercrossing fibrilla3. As will be stated further on, 

 there occur everywhere in the white substance similar fibrillaj 

 radiating from the grey substance to the periphery j it is also 

 possible that all these come from similar cells situated in the 

 grey substance, and that the whole primarily in the embryo 

 originate from the epithelial cells surrounding the central canal. 

 The White Substance. — The general divisions of the white 



