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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



diminished, and thus the structure of this system in the smaller mammals 

 becomes numerically simplified. 



Size and Function. Histology shows us the nerve-cell prolonging itself 

 into branches often much subdivided, the dendrous and the neuron. Such a 

 cell contains a mass of living substances capable of being broken down and 

 built up chemically, and there is nothing against the inference that the larger 

 the cell the greater is the quantity of these living substances, and hence the 

 larger the amount of stored energy represented by it. The larger cells are 

 therefore those capable of setting free the greater amount of energy. The 

 energy-producing changes are in the greatest measure to be associated with the 

 cell-body, rather than with any of the branches. On the other hand, the 

 nerve-cells with large cell-bodies, sending out as they do branches which are 

 more voluminous than those nerve-cells that are small, furnish a greater 

 amount of material to form the ultimate twigs into which these branches 

 finally split. From this it follows that in general the large nerve-cells have 



more points of connection with the structures 

 about them, as well as the capacity for the lib- 

 eration of a greater amount of energy. 



Growth of Nerve-cells. During growth 

 and development the nerve-cells may present 

 many changes in appearance (Fig. 145). 



The nerve-elements are derived from ger- 

 minal cells found in the epiblast of the embryo. 

 Amid the columnar epiblastic elements forming 

 the medullary tube these spherical cells appear 

 in man about the third to the fourth week of 

 fetal life. 1 They divide rapidly and in such 

 a way that one daughter-cell continues as the 

 germinal cell, while the other moves away 

 from the primitive surface of the body and 

 becomes without further division a young 

 nerve-cell or neuroblast. The formation of 

 neuroblasts in man ceases or becomes very 

 slow and unimportant by the end of the third 

 month of fetal life. 



Two characters of the neuroblast are 

 worthy of careful consideration. First, there 

 is good indirect evidence that, in early life at least, and before their branches 

 have been formed, they are migratory, moving in an amoeboid manner. 

 This being so, the perfection with which they arrange themselves in the adult 

 system depends on the accuracy with which they respond to those condi- 

 tions that determine their migration as well as upon the normal character of 

 these directing influences (mechanical strain; 2 chemotaxis or nutritive attrac- 



1 His: Archiv fur Anatomif, und Physiologic, Anat. Abthlg., 1889. 



2 His : Unsere Korperform, 1874. 



FIG. 145. Portion of developing medul- 

 lary tube (human) seen in frontal section 

 X 1100 diameters (His) : G, germinal cell ; 

 N, neuroblasts. 



