THE NERVOUS TISSUES. 



157 



the cell-bodies or the dendrites of other neurones, they are, without 

 losing their identity, linked into chains, so that a physiologic conti- 

 nuity exists between them. In such neurone chains the dendrites 

 are regarded as cellulipetal, transmitting the stimulus to the cell ; the 

 neuraxes as cellulifugal, transmitting the impulse imparted by the 

 cell to the motor nerve-endings or central organs (Kolliker, 93), 

 The entire nervous system may therefore be said to be made 

 up of such neurone chains, the complexity of which varies 

 greatly according to the number of neurones which enter into 

 their construction. This subject will be considered more fully 

 in a chapter on the nervous system. 



Fibrils of axial 

 cord. 



Neurilemma. 



Segment of 

 Lantermann. 



THE NERVE-FIBERS. 



The neuraxes of the cells of type I, and the dendrites of the 

 peripheral sensory neurones (spinal ganglia and homologous cranial 

 ganglia), form the chief elements in all the 

 nerve-fibers. In the nerve-fibers they pos- 

 sess a distinctly fibrillar structure. The 

 fibrils composing them, the axis-fibrils, are 

 imbedded in a semifluid substance, the 

 neuroplasm (Kupffer, 83, II) the whole 

 being surrounded by a very delicate 

 membrane, the axolemma. In the nerve- 

 fibers, the axis-fibrils and the neuroplasm 

 form axial cords w r hich are surrounded 

 by a special membrane or membranes, 

 the presence or absence of which serves 

 as a basis for a classification of nerve- 

 fibers. Two kinds are distinguished, 

 medullated and nonmedullated nerve - 

 fibers. 



In medullated nerve-fibers, the axial 

 cords (neuraxes of cells of type I, and 

 dendrites of spinal ganglion cells) are sur- 

 rounded by a highly refractive substance 

 very similar to fat, which is blackened 

 in osmic acid, the so-called medullary or myelin sheath. In a fresh 

 condition this sheath is homogeneous, but soon changes and presents 

 segments separated from each other by clear fissures. These seg- 

 ments vary in size and are known as " Schmidt-Lantermann-Kuhnt's 

 segments." On boiling in ether or alcohol the entire medullary 

 sheath of a nerve-fiber does not dissolve, but a portion is left in the 

 shape of a fine network which is not affected by exposure to the 

 action of trypsin. From the latter circumstance it has been thought 

 that this network consists of a substance very similar to horn, and 

 is therefore known as neurokeratin (horn-sheath, Ewald and Kuhne). 

 On burning isolated neurokeratin, an odor exactly like that of burn- 



Fig. 1 1 8. Longitudinal 

 section through a nerve- fiber 

 from the sciatic nerve of a 

 frog; X 8 3- 



