CII. XVII.] 



NERVE-CELLS 



193 



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. 206 (A) shows one of these nerve- 

 cells from the Gasserian 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 

 each cell appears to be unipolar. This is shown in a more diagram- 

 matic way in fig. 187, p. 168. The bifurcation of the nerve-fibre is 



Fio. 205. Diagram after Ramon y Cajal to show the ontogenetic (or embryological) and phylogenetic 

 (i.e. in the animal series) development of a neuron. A, cerebral cell of frog ; B, newt ; c, mouse ; D, 

 man. As the place in the zoological series rises, the neuron increases in complexity and in the 

 number of points of contact ; this is produced partly by an increase of the dendrons, partly by an 

 increase in the side branches or collaterals of the axon. a, 6, c, d, e, show the early stages in the 

 development of a similar cell in the human embryo ; the first branch of the cell to appear (in a) is 

 the axon ; the dendrons are later outgrowths. The reversal of this process takes place in primary 

 degeneration. 



spoken of as a T-shaped junction. As will be seen in fig. 206 (C), 

 the nerve process has a convoluted course on the surface 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 ramifi- 

 cation which envelopes the ganglion cell ; it may sometimes be coiled 

 spirally around the issuing nerve-fibre. 



N 



