192 NERVE-CENTRES [CH. XVII. 



the pia mater. The other fibres of the tissue are cell processes of 

 the neuroglia or glia cells proper, or spider cells as they are some- 

 times termed (see fig. 204). 



Neuroglia is thus a connective tissue in function, but it is not 

 one in origin. Like the rest of the nervous system, it originates 

 from the outermost layer of the embryo, the epiblast. All true 

 connective tissues are mesoblastic. 



Chemically, it is very different from connective tissues. It con- 



FIG. 204. Branched neuroglia-cell. (After Stohr.) 



sists of an insoluble material called neuro-keratin, or nerve-horn, 

 similar to the horny substance, keratin, which is found in the surface 

 layers of the epidermis. 



Structure of Nerve-Cells. 



Nerve-cells differ a good deal both in shape and size. 



In the early embryonic condition, the future nerve-cell is a small 

 nucleated mass of protoplasm without processes. As development 

 progresses branches grow, and by this means it is brought into con- 

 tact 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 nutrition, 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 (fig. 205). 



