466 



SPECIAL HISTOLOGY. 



Fig. 158. 



nerve-tubes, the medullary sheath, and the axis-cvlinder. In 

 older nerve-cells, or by the operation of re-agents (arsenious 

 acid, chromic acid, iodine), the contents of the cell become 

 detached from the membrane, and the axis-cylinder appears 

 as a direct continuation of the former (fig. 158), as was first 



shown by Harting (vid. also Stannius 

 in 'Gott. Anzeig./ 1850, and Leydig, 

 I 1. c. Tab. 1, fig. 9), which is the best 

 proof that the contents of the nerve- 

 cells cannot be understood as con- 

 tained in a dilated nerve-tube. The 

 nerve-tubes or ganglion-fibres thus 

 originating, which frequently arch 

 round or embrace the cells with several 

 circular turns, are at first fine, measur- 

 " ing 0-0015 — 0-0025'", but (not con- 

 tinuing so as I formerly supposed, when 

 I was acquainted only with their 

 origin), they all very soon increase in 

 size, as may be very readily observed 

 in many fibres, whilst still within the 

 ganglion, up to 0-003'" and 0004.'",, 

 many even to as much as 0-005 and 

 0-006'" ; becoming, consequently, 

 •" medium-sized, and thick nerve-Jibres. 



The processes of the cells and the nerve-fibres springing from 

 them are also furnished with nucleated sheaths like the cells 

 themselves, the vaginal processes, as they are termed, which 

 they lose, however, at the point where they join the emergent 

 trunk, obtaining instead of it, as a coat, the common neuri- 

 lemma of the nerves. 



The description I have above given of the conditions ob- 

 servable in the spinal -ganglia in Man and the Mammalia, 

 differs very considerably from what was found by Bidder, 

 Reichert, R. Wagner, and Robin, to be the case in Fishes. 



d...9 



Fig. 158. Nerve-cell of the Pike (bipolar, as they are termed), which is continued 

 at each end into dark-bordered nerve-tubes, treated with arsenious acid, x 350 diam.: 

 a, sheath of the nerve-cell ; b, sheath of nerve ; c, nerve-medulla ; d, axis-cylinder 

 continuous with the contents of the nerve-cell ; e, which have shrunk away from the 

 sheath. 



