CENTRAL NERVOUS SYSTEM. 613 



type begins its development as a bipolar cell, a neuron growing from each pole 

 (Fig. 147). In the adult spinal ganglion of the higher mammals, however, 

 no such bipolar cells are to be found, but only cells having a single neuron 

 which soon divides into two branches. 



Figure 148 beautifully illustrates the phases of this change as seen in a 

 single section. At first one neuron arises from each pole of the ovoid cell- 

 body. Later the cell-body occupies a position at the side of the two neurons, 

 which appear to run into one another. Finally the cell-body is separated from 

 the two neurons by an intervening stem. The stem has the characters of a 

 nerve-fibre and from the end of it the original two neurons pass off as branches. 



From this mode of development it is plain that the single stem must be 

 looked upon as containing a double pathway, although it appears to be in all 

 ways a single fibre, for on the one hand it contains the path for the incoming 

 and on the other for the outgoing im- 

 pulses. Recent investigations have 

 shown in a striking way that cells 

 modified in this manner are by no 

 means limited to the spinal ganglia, 

 but occur in the cortex of the cerebel- 

 lum and elsewhere. The study of this 



modification brings With it the follow- FlG - 148 -~ Dmeuric changing into mononeuric 



-,-., . , . cells : from the Gasserian ganglion of a develop- 



ing Suggestion: If the Single Stem in ing guinea-pig (van Gehuchten). 



the modified spinal ganglion-cells must 



by virtue of its development contain a double pathway, it is fair to inquire 

 whether the same may not be true of the other forms of the nerve-cell in which 

 the neuron also appears to be single. Among the cortical cells the arrange- 

 ment of the branches is such that, for aught that is known, the stem of the 

 neuron may functionate in the manner suggested, and contain more than one 

 pathway. 



Classifying the nerve-cells, therefore, in the light of these facts, we find 

 (1) The pyramidal type, in which the dendrons and neuron are both well 

 developed, and in which the greater part of the impulses most probably enter 

 the cell by way of the dendrons and leave by way of the neuron ; (2) The 

 spinal ganglion type, in which originally the impulse passes in at one pole of 

 the cell and out at the other, but in the course of development the two neurons 

 become attached to the cell-body by a single stem, and by inference there must 

 be in this stem a double pathway. In this special case there are no dendrons. 



Growth of Branches. After the cells have taken on their type form, the 

 branches still continue to grow, not only in length, but in diameter. In man, 

 for example, the diameter of the nerve-fibres (neurons) taken from the periph- 

 eral nerves at birth is 1.2-2 p for the smallest, up to 7-8 p. for the largest, with 

 an average of 3-4 /jt, while at maturity it is 10-15 /j. for the larger fibres. 1 



In the second spinal nerve of the frog, Birge found the fibres 2 to increase 



1 Westphal : Neurologische Centralblatt, 1894, No. 2. 



2 Birge : Archiv fur Anatomic und Physiologic, 1882. 



