NERVE-FIBRES AND NERVE-CELLS. 105 



gradually become thinner. At last only separate fibres 

 are to be seen, these being, however, still in appearance 

 exactly like those constituting the main stem. Such 

 fibres as up to this point have had medullary sheaths 

 now frequently lose them, and therefore become exactly 

 like grey fibres. The axis-cylinder itself then some- 

 times separates into smaller parts ; so that a nerve-fibre, 

 thin as it is, embraces a very large surface. The ends of 

 the nerve-fibres are connected sometimes with muscles, 

 sometimes with glands, and sometimes, again, with 

 peculiar terminal organs. 



In the central organs of the nervous system many 

 nerve-fibres are found which are in appearance in- 

 distinguishable from those of the peripheric system. 

 There are fibres with axis-cylinder, medullary sheath, 

 and neurilemma, others without medullary sheath, and, 

 finally, others in which no neurilemma can be detected, 

 and which may therefore be described as naked axis- 

 cylinders. But, besides these, very delicate fibres, far 

 finer than the axis-cylinders, occur. The central organs 

 of the nervous system are how^ever especially marked 

 by the abundant occurrence of a second element, which, 

 though it is not altogether unrepresented in peripheric 

 nerves, yet is only found in the latter distributed in a 

 few places, whilst in the central organs it constitutes 

 an important portion of the whole mass. This consists 

 of certain cell-like structures called nerve-cells, or gan- 

 glioii-cells. In each ganglion-cell it is possible to dis- 

 tinguish the cell body, and a large kernel (nucleus) 

 within this ; wdthin the kernel, a smaller kernel {nu- 

 cleolus) may also frequently be distinguished. Some 

 ganglion-cells are also surrounded by a membrane 

 which occasionally passes into the neurilemma of 

 6 



