54 AGE, GROWTH, AND DEATH 



by another method which brings out very clearly to 

 the eye the fact that in the protoplasm of the cell 

 there are scattered spots of substance of a special 

 sort. No such spots can be demonstrated in the 

 elements of the young embryonic nerve cells. To 

 some fanciful observers the spots, thus microscopically 

 demonstrable in the nerve cells, recall the spots which 

 appear on the skin of leopards, and hence they have 

 bestowed upon these minute particles the term tigroid 

 substance. The bottom figures represent the kind of 

 nerve cells which occur upon the roots of the spinal 

 nerves, and each of which is surrounded by a special 

 protective envelope of small non-nervous cells. It is 

 unnecessary to dwell upon their appearance, as the 

 mere inspection of the figures shows at once that they 

 differ very much indeed from the other nerve cells 

 we have considered. 



We pass now to another group of structures, the tis- 

 sues which are known by the technical name of epithe- 

 lia. You can notice immediately in the figures on the 

 plate (Fig. 14) that the appearances are very different 

 from those we have encountered in contemplating the 

 cells of the nervous system, and you can readily satisfy 

 yourselves, by the comparison of the various figures 

 now before you, of the further fact that these epithelia 

 are unlike one another. The figures represent epi- 

 thelium, respectively, first from the human ureter ; 

 second, from the respiratory division of the human 

 nose ; third, from the human ductus epididymidis ; and, 

 fourth, from the pigment layer of the retina of the cat. 



