THE NERVE ELEMENTS. 145 



assumption that they indicate the pathways for the 

 impulses, it is interesting to note that they occasionally 

 enter the cytoplasm by one prolongation of the cell, and 

 leave it by another, without interruption and without 

 running close to the centre of the cell. When the cells 

 of a single locality are treated with staining reagents, it 

 is usually found that while some are deeply coloured 

 others are not. 



Age certainly is a factor in determining this colour 

 reaction. The youngest cells, the neuroblasts, stain diffe- 

 rently from those more developed ; these latter differ 

 according to their maturity, the condition of rest or 

 fatigue, and also according to their physiological rela- 

 tions ; those under control of the higher centres staining 

 in a way different from those which belong to the reflex 

 system. 1 All the prolongations arise primarily from the 

 cell-body. Of these outgrowths there are two sorts : 

 one, the dendrons, which branch in a tree-like manner 

 into the surrounding substance, and the other the neu- 

 rons, which have an even calibre and smooth contour, and 

 from which the branches arise at right angles. Most cells 

 have one neuron, some have two or even more, but the 

 distinctions between these groups will be made later on. 

 At its distal termination the neuron divides, in some 

 cases many times, and according to circumstances the 

 stretch of fibre between the cell-body and this terminal 

 subdivision may vary from the fraction of a millimetre 

 to half the length of a man ; moreover, in many cases 

 the neuron gives off small branches near its origin 

 (Fig. 20, A), and these branches are apparently of much 

 physiological importance. Besides the type of cell 

 which forms the basis of the above description, Fig. 19 

 shows other forms (B and C\ and the question arises as 



1 Kaiser, Die Funktionen der Ganglienzellen des Hals mar kes t 

 Haag, 1891. 



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