184 



NERVE-CENTRES 



[CH. XVI. 



but one process, and one process only, of each cell becomes the axis 

 cylinder of a nerve-fibre. 



Passing next to the central nervous system, we here again find 

 the multipolar cell is the principal kind present. 



N.S. 



FIG. 178. Bipolar nerve-cells. A. From the Gasserian ganglion of the pike (after Bidder). B. From a 

 spinal ganglion of a 4k weeks' human embryo (after His). C. Adult condition of the mammalian 

 spinal ganglion cell : N. S. nucleated sheath ; only the nuclei seen in profile are represented. T. is 

 the T-shaped junction (after Retzius). 



Fig. 180 shows one of the typical multipolar cells of the 

 spinal cord. Here again, only one process (a) becomes the axis 

 cylinder of a nerve-fibre, and the others break up into arborisa- 

 tions of fibrils. The cells have a finely fibrillar structure, and the 

 fibrils can be traced into the axis cylinder process and the other 

 branches of the cell. Between the fibrils the protoplasm of the cell 

 contains a number of angular or spindle-shaped masses, which have 

 a great affinity for basic aniline dyes like methylene blue. They are 

 known as Nissl's granules. These nerve-cells often contain, especi- 

 ally in the adult, granules of pigment, usually yellow, the nature of 

 which has not been determined. 



