CENTRAL NERVOUS SYSTEM. 611 



tioii). 1 But with so much liberty of movement and with directing influences 

 that are so complicated, the chances for deviation from a fixed arrangement are 

 much enhanced. 



Polarization of Neuroblasts. Moreover, very early in the history of the 

 neuroblast the point on the cell-body from which the neuron will grow appears 

 in many cases to be fixed, and the cell is thus physiologically polarized. 2 This 

 polarity being established, the direction in which the neuron first grows is 

 determined, and where the cells are misplaced this polarization can lead to 

 the confusion of arrangement found in the brains of some congenital idiots. 3 



The volume of either the germinal cell or of the first form of the neuro- 

 blast was found by His * to be 697 cubic jut in a human fetus (embryo R-length 

 5.5 millimeters, aged 3 to 3.5 weeks). It has previously been shown that the 

 volume of a spinal-cord nerve-cell is, taken altogether, 78,500,000 cubic //, 

 and that of this the neuron occupies 78,450,000 cubic ^/, and the cell-body 

 50,000 cubic //. If we take half of this total volume, it gives under the con- 

 ditions chosen an increase in volume between the neuroblast and the mature 

 cell of 57,456-fold. 



Maturing- of the Nerve-cell. The maturing of the nerve-cell involves 

 several changes. First, the outgrowth of the neuron or neurons ; next, the 

 formation of the dendrons ; and finally, in some cases, the medullation of the 

 neuron, while simultaneously and with greater or less rapidity the absolute 

 amount of substance in both cell-body and neuron is being increased, together 

 with a chemical differentiation of the contents of the cytoplasm and the 

 nucleus. The time in the life-history of the individual at which these several 

 events occur is variable, and may be delayed beyond puberty at least, while 

 the rate at which they occur is different in different cases. Furthermore, many 

 nerve-cells never develop beyond the first stages of immaturity (Fig. 146). 



Form of the Neuron as a Means of Classification. Of the various 

 devices used to classify nerve-cells, the form of the neuron is the most useful. 



Physiologically, the nerve-cell is significant as a pathway for the nerve- 

 impulse. The current conception of the change called the nerve-impulse is 

 that it begins at one point of the cell and travels from there to the other parts ; 

 one of the other parts is the neuron, and along this the impulse can be shown 

 to pass. Although it cannot be directly demonstrated, there is reason to think 

 that primitively all the branches of a cell had similar physiological powers. 

 Indeed, the nerve-cell body stimulated at any point may be responsive just as 

 an amoeba is responsive at any portion of its surface. When, however, the 

 branches are formed they become the channels through which the impulses 

 pass, and hence assume a special significance without indicating any funda- 

 mental change in the structure of the cell. Where the cell has well-developed 



1 Davenport : Bulletin of the Museum of Comparative Zoology, Harvard College, Nov., 1895 ; 

 Herbst : Biologische Centralblatt, 1894, Bd. xiv. 



2 Mall : Journal of Morphology, 1893, vol. viii. 



3 Koster : Neurologische Centralblatt, 1889, Bd. viii. 



4 Archivfur Anatomic und Physiologic, 1889. 



