NERVE-CENTRES. 



[CH XVII. 



into contact with the branches of other nerve-cells. When the 

 nerve-cells degenerate as they do in some cases of brain and cord 

 disease, there is a reversal of this process ; just as in a dying tree 

 the terminal branches, those most distant from the seat of nutri- 

 tion, are the first to wither, so it is in the degenerating nerve-cell. 

 If one traces the structure of nerve-cells throughout the zoological 

 series, there is also seen an increase in their complexity, and the 



number of points of contact produced 

 by an increase in the number and 

 complexity of the branches multiplies. 

 The simplest nerve-cells known 

 are termed bipolar. In the lower 

 animals the two processes come off 

 from the opposite ends of the cells ; 

 the cell, in other words, appears as 

 a nucleated enlargement on the 

 course of a nerve-fibre. Fig. 197 (A) 

 shows one of these nerve-cells from 

 the Gasseriaii ganglion of the pike. 

 The cells of the Gasserian and spinal 

 ganglia in the mammalian embryo 

 are also bipolar, but as development 

 progresses, the two branches become 

 fused for a considerable distance, so 

 that in the fully-formed animal the 

 cell appears to be unipolar. This 

 is shown in a more diagrammatic 

 way in fig. 180, p. 169. The bifur- 

 cation of the nerve-fibre is spoken of 

 as a T~ s haped junction. As will be 

 seen in fig. 197 (C), the nerve process 

 has a convoluted course on the sur- 

 face of the cell before it bifurcates. 

 In these ganglia it should be also 

 noted that each cell is enclosed within 

 a connective tissue sheath, and the nuclei seen are those of the 

 connective tissue corpuscles. 



In the sympathetic ganglia, the cells may have a similar 

 structure, and here also the nucleated sheath is seen. In some 

 cases, however, when there appear to be two fibres connected 

 to a cell, one of them is really derived from another cell, and is 

 passing to end in a ramification which envelopes the ganglion 

 cell; it may sometimes, as in fig. 198, be coiled spirally around 

 the issuing nerve-fibre. 



Fig. 198. Sympathetic ganglion cell 

 of a frog, highly magnified. 

 (Beale.) 



