Page:Popular Science Monthly Volume 71.djvu/113

Rh 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. 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 tissues which are known by the technical name of epithelia. You can notice immediately in the figures from the skin 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 with the various figures now before you, of the fact that these epithelia are unlike one another. The figures represent epithelium, 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. We turn now to a representation of a section of one of the orbital glands. This is very instructive because we see not only that the cells which compose the gland have acquired a special character of their own, but also that they are not uniform in their appearances. This lack of uniformity is due chiefly to the fact that the cells change their appearance according to their functional state. We can actually see in these cells under the microscope the material imbedded in their protoplasmic

 A. with the material to form the secretion accumulated within the cells. B. after loss of the material through prolonged secretion. From R. Heidenhain after Lavdowsky.