COOKDINATION 75 



they belong; that is, the fibers of the roots do not come from 

 higher or lower parts of the cord or from the brain. It has 

 been found that when these roots are stimulated they throw 

 muscles into contraction and produce effects on the blood 

 vessels and glands, but they do not give rise to sensations 

 or produce other effects in the cord itself. In other words, 

 the fibers of the ventral root conduct impulses from the cells of 

 the spinal cord outward; they do not conduct impulses from 

 outside into the spinal cord. Hence they are known as 

 efferent fibers (Latin ex, " out of " ; ferre, " to carry " ). 



The nerve cells from which these fibers arise consist of a 

 mass of cytoplasm around the nucleus and of one or more 

 outgrowths of this cytoplasm, usually more or less branched. 

 These outgrowths of the cytoplasm divide and subdivide, ulti- 

 mately forming in the gray matter exceedingly fine terminal 

 branches like those of a tree in the air. Such processes are 

 known as dendrites (Greek dendron, " a tree " ). The nerve 

 cells in question have numerous dendritic processes ; in other 

 nerve cells there may be but one, and still others possess 

 no den'dritic processes at all. In all cases the general appear- 

 ance of the cell depends largely upon the number and man- 

 ner of branching of these dendrites. Thus it happens that 

 nerve cells differ from one another in appearance just as 

 a Lombardy poplar, an oak, an elm, and a maple differ, 

 although all show the fundamental characteristics of a tree 

 (Fig. 41; see also Figs. 109, 110, and 111). 



In subsequent portions of this work it is unnecessary for 

 us to go into the details of the form of the nerve cells to 

 any extent ; the student need only understand henceforward 

 that nerve cells consist of a central mass of nucleated cyto- 

 plasm from which proceed outgrowths, or processes, which 

 are of two kinds: (1) those which become axons of nerve 

 fibers and which form an essential part of all nerve cells ; 

 and (2) the dendrites, which are usually but not always 

 present. The whole structure, including the central cell 



