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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



into finer twigs, forming the terminal arborizations. Contrasted with this 

 principal outgrowth are the other branches of the cell, which are individu- 

 ally much less extensive and which divide dichotomously at frequent in- 

 tervals. From the tree-like form which they thus acquire they have been 

 designated <lni<lrites. 



The accompanying illustration (Fig. 67) shows the cell-bodies of several 

 neurones, together with the beginning branches, and also gives some idea of 

 the variations in the size of the cell-bodies as found in man. The nerve-cell 

 body is at first ovoid in shape, although this type is much modified in many 

 cases. As will be seen from Fig. 67, the diameters of nerve-cells range 

 from 4 ft to 65 //,' though in some instances in the spinal cord cell-bodies of 

 even larger diameter are found. 



Peculiarities of Nerve-cells.— As compared with the other cells of the 

 body, the best developed nerve-cells are of large size, but the nucleus, pro- 

 portionately to the remainder of the 

 neurone, is not large. Moreover, its pro- 

 portional value decreases with the increase 

 in the size of the entire cell. The most 

 striking feature of the nerve-cell, how- 

 ever, is the great length to which its chief 

 branch, the axone, may attain, for in no 

 other tissue does anything like so great a 

 proportion of the cell-substance occur as 

 a branch. Since the axone is the direct 

 outgrowth of the cell-body and can have 

 attained its Jength only gradually, it is 

 not surprising to find that all varieties of 

 length occur. 



The form of cell represented in Fig. 

 68 is one in which the axone shows a 

 very short stem between the cell-body and 

 its terminal twigs. In such an instance 

 the entire extension of the axone may be 

 less than a millimeter. With this are to 

 be contrasted those forms in which the 

 axone is very long and its mass great. 

 What its greatest length may be is easily determined. Within the central 

 system there are cells whose axones extend from the cerebral cortex to the 

 lumbar enlargement (60 centimeters), and again in the peripheral system 

 there are cell-bodies in the lumbar enlargement of the spinal cord, the axones 

 of which extend to the skin and muscles of the foot, a distance of 100 centi- 

 meters or more. Among the neurones found in the spinal ganglia of the 

 lumbar region, some cells send one axonic branch to the level of the nuclei 

 of the dorsal funiculi in the bulb and the other branch to the skin of the 



1 u = 0.001 of a millimeter. 



Fig. 68.— A cell, with a short axone, 

 Rivinp off many branches. In such a cell 

 the axone is less in volume than the cell- 

 body. This is the extreme form of the 

 "central cell" of Golgi (Ramon y Cajal). 

 /' . di mlrites ; N., axone. 



