58 



THE NERVOUS SYSTEM 



7 mmm. in diameter. In the prolongations from certain ganglion-cells 

 even this covering is apparently lacking. The non-medullated fibers 

 have a grayish color. They are generally in the sympathetic system. 



Nerve-cells are the other elements of the nervous system besides the 

 conducting fibers or fibrils. They vary greatly in size, for while the 

 motor cells of man's spinal cord may be 150 mmm. in diameter (ranging 

 upward from 75), some of the nerve-cells in the granular layer of the 

 cerebellum have a diameter of not over 4 mmm., while others are twice 

 as large. Nerve-cells have large nuclei and nucleoli, but relatively little 

 chromatin. There is usually about the periphery of the cell a non-granular 



FIG. 26 



Motor nerve-cells from the anterior horns of the spinal cord: a, nucleus with its conspicuous 

 nucleosus; 6, stainable substance of Nissl; c, neurite; d, implantation cone. Note the neuro- 

 fibrils. (Bates.) 



layer, and probably a similar clear layer around the nucleus. The prin- 

 cipal fiber, the neuraxis, or axis-cylinder, joining a nerve-cell spreads out 

 into a cone-shaped mass (the implantation cone) in the cell, which is also 

 free of granules. In other parts of the cells there are fibrils doubtless 

 continuous with those of the prolongation from the cell; very fine, highly 

 refractive biogenic granules, and the so-called chromatophile granules 

 (Nissl bodies, tigroid substance), which are coarse granules or flakes 

 especially abundant about the nucleus, and continuous into some of the 

 cell's branches. About these three elements of the nerve-cells, and others 

 (e. g., pigment) which it is needless to mention, there is at present no 

 little discussion among histologists. Their exact respective functions are 

 still more obscure; in fact, little is definitely known of the use of any of 

 the many parts of a nerve-cell. 



In the brain the cells of the cerebellum, of the retina, and of many 

 other parts are small. So are those of the posterior horns of the cord, 

 while those of the anterior horns, as already stated, are many times 

 larger. Some competent neurologists (e. g., Campbell) claim that on 



