182 



HISTOLOGY 



because it is through this that a nerve impulse is conducted. While 

 it might be conceived that the impulse passed through the entire mass 

 as through a homogeneous medium (so far as 

 the impulse was concerned), and very much as 

 an electric current passes through a copper wire, 

 yet the visible presence of a fibrillar structure 

 in parts of the cytoplasm of nerve cells drew 

 attention to the fact that here were nerve fibrils 

 that perhaps were specific cell-organs of conduc- 

 tion. 



For a long time, while it was recognized that 

 these fibrils might form such paths, it could not 

 be proved that any one or more of them ran con- 

 tinuously through the cytoplasm for any distance 

 that would warrant regarding them as such or- 

 gans. Apathy and other investigators, however, 

 learned how to stain these cells in such a man- 

 ner that particular fibrils were differentiated out 

 of the mass of fibrillar tissue and shown to run 

 continuously over courses that were more than 

 probably the same as those taken by the nerve- 

 impulse. Figure 159 shows a representation of 

 a nerve cell prepared by Apathy. His method 

 differentiated out several kinds of fibrils. At present it is known that 

 the several kinds of fibrils thus discovered do form possible paths of 

 conduction. It is not fully demonstrated that these paths picked out 

 by the stain are the only ones. It is probable that the method has 

 picked out some fibrils physiologically or functionally different from the 

 others just as Golgi's method selects certain neurons to the exclusion of 

 others. Figure 159 shows a cell in which Apathy's method has de- 

 monstrated two different sets of fibrils, one of which is supposed by 

 Apathy to bring the impulse into the cell and the other to take it out 

 again. 



Seen in most preparations, the cytoplasm of a nerve cell is ordinarily 

 found to show, besides the fibrils, a number of easily seen masses of a 

 granular substance and a certain proportion of undifferentiated cyto- 

 plasm, or neuroplasm, as it should be called in this case. Besides these 

 usual features it may contain pigment bodies and other rarer structures, 

 as a centrosphere, centrosome, cell-caps, etc. The granular substance 

 stains very deeply in the ordinary staining reagents. Most nerve cells 

 show it throughout the greater part of their cytoplasm as masses of a 

 material that has taken the stain fully as deeply as the chromatin of the 

 nucleus, while all other cytoplasm around it is very slightly stained. 



FIG. 159. A small nerve 

 cell from the medicinal 

 leech, stained to show 

 the two sets of fibrils. 

 (From SCHNEIDER after 

 APATHY.) 



