84 



GENERAL PRINCIPLES OF ZOOLOGY 



a 



telegraph system. These are to be 

 considered as the specific element 

 of the nervous system. 



In the vertebrates the ganglion 

 cells vary greatly in size; besides 

 small elements there are large cells, 

 only exceeded by the eggs in size, 

 which correspondingly have large 

 nuclei recalling the germinal vesi- 

 cles. Unipolar, bipolar, and multi- 

 polar ganglion cells are recognized, 

 the differences depending upon the 

 number of processes (nerve-fibres) 

 which arise. In multipolar cells 

 the number is large (fig. 52) and 

 they are of two kinds, dendrites and 

 axons or neurites. Dendrites are 

 so called because they branch again 

 and again, not far from their origin 

 from the cell. The axons (of which 

 there is usually but one to a gang- 

 lion cell) can extend a long dis- 

 tance without giving off branches, 



except here and there side twigs (collaterals) which arise at right angles 



to the main fibre; they often 



continue into peripheral 



nerves. They branch at their 



tips (telodendra) so the mor- 



phological distinction from 



dendrites lies in the greater 



distance of the region of 



branching from the body of 



the ganglion cell. In bipolar 



ganglion cells both processes 



appear as neurites, but if one 



is to be regarded as a dendrite 



with its branching at some 



distance from the cell, the 



definite physiological distinc- 



, , , 

 tion has to be invoked that 



the neurite carries the im- 



FIG. 52. Multipolar ganglion cell of man 

 (after Gegenbaur). a, axon. 



^- Motor ganglion cell from the thoracic 

 the spinal cord of a dog (after Bethe). 



n, nucleus. 



FIG. 

 region 



