358 



THE NERVOUS SYSTEM 



FIG. 93. 



certain, according to the observations of Key and Retzius,* in the 

 cerebro-spinal ganglia of the rabbit, where the cell-process follows for 

 a time a winding course and becomes covered with a layer of myeline, 

 which may be rendered perfectly distinct by staining with perosmic 



acid (Fig. 93). It thus forms a complete 

 nerve fibre, often exhibiting its charac- 

 teristic annular constrictions and inci- 

 sions, and sometimes dividing into two 

 secondary fibres. 



Some cell-processes, on the other hand, 

 without acquiring a medullary layer, 

 join nervous bundles in the neighbor- 

 hood, and become to all appearance non- 

 medullated nerve fibres. In all these 

 instances the tubular prolongation, from 

 the capsule of the nerve cell, is after a 

 time closely applied to the exterior of 

 the nerve fibre, becoming continuous 

 with the sheath of Schwann. 



A transition of the cell-process into a 

 medullated nerve fibre has been also 

 found in the spinal ganglia of the frog 

 and toad, the layer of myeline reaching 

 nearly to its junction with the nerve 

 cell. But it is most distinctly marked, 

 and has been most frequently seen in 

 the ganglia and trunks of the trigeminus 

 and vagus nerves of fishes, particularly 

 the pike and lamprey. In these situa- 

 tions there are scattered nerve cells of 

 peculiar form ; namely, elongated or 

 ovoidal, with a nerve process at each 

 They are thence called " bi- 



stained with perosmic acid. From Gas- polar " cells. In the pike, the medul- 



^JJ n * llonoftherabbit - < Ke y and kry layer surrounds the nerve fibre 



quite to its origin from the cell ; and it 



sometimes extends over the cell itself, which, as well as the nerve fibre, 

 is thus invested with a layer of myeline. These bipolar cells, as well 

 as similar ones observed in the auditory nerve-trunk in fishes, some- 

 times appear hardly more than nucleated enlargements of the axis 

 cylinder ; and they are generally situated about midway between two 

 annular constrictions. In the lamprey, the nerve fibres are non-medul- 

 lated; but they are connected with bipolar cells in the ganglion of the 

 trigeminus and in the trunk of the auditory nerve, in the same manner 

 as above described. 



NERVE CELL, with axis cylinder process , ., 



and medullated nerve fibre attached; e 



* Anatomie des Nervensysterns und des Bindegewebes. Stockholm, 1876. Zweite 

 Halfte, p. 39. 



