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AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



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G. 



these brandies 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 liberation of a greater amount of energy. 



Growth of Nerve-cells. — The nerve-elements are derived from germinal 

 cells found in the epiblast of the embryo (Fig. 69). 



These 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 neurone, 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 

 f>?;~ ?f rr-^. -? ~~^~7 '/** 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 conditions that deter- 

 mine their migration as well as upon the 

 normal character of these directing influ- 

 ences (mechanical strain; 1 chemotaxis ; nu- 

 tritive attraction or electrical influences). 3 

 But with so much liberty of movement and 

 with directing influences that are so compli- 

 cated, the chances for deviation from a fixed 

 arrangement are much enhanced. 



Second, very early in the history of the 

 neuroblast the point on the cell-body from 

 which the axone will grow appears in many 

 x lioodiametera (His) .a, germinal ceil; cases to fa determined, and the cell is thus 



A', neuroblasts. 



physiologically polarized. 3 This polarity being 

 established, the direction in which the axone first grows is determined, and 

 where the cells are misplaced this polarization can lead to a confusion of 

 arrangement. 



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

 blast was found by His' to be 697 cubic fi in a human fetus (embryo It- 

 length 5.5 millimeters), aged 3-3.5 weeks ; and it can be shown that the mature 

 neurone must often attain a volume more than 50,000 times that of the orig- 

 inal neuroblast. 



1 His: Unsere Kin- perform, 1874. 



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

 Hcrl.st: Biologiaekea Centralblatt, 1894, Bd., xiv.; H. Strasser: Ergebnme der Anatamie u. Ent- 

 wickclunr/.tr/esrfiirlite, Merkel and Bonnet, 1891, Bd. i. S. 731. 



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

 ' Arrhir fur Anatomie mid Pkysiologie, 1889. 



Fig. 69.— Portion of developing medul 

 lary tube (human) seen in frontal section 



