CENTRAL NERVOUS SYSTEM. 



617 



Changes in the Cytoplasm. While the nerve-cell is passing from the 

 immature to the mature form, increasing in mass and in the number of its 

 branches, as well as acquiring its medullary sheath, it is also undergoing vari- 

 ous chemical changes. The chromatic substance in the cytoplasm becomes 

 more abundant at maturity and the pigment-granules increase in quantity. 1 



Old Age of Nerve-cells. But the nerve-cell, though possessing, in most 

 cases, a life-history co-extensive with that of the entire body, eventually exhibits 

 regressive changes. These changes of old age consist, in some measure, in a 

 reversal of those processes most evident during active growth. The cell-body, 

 together with the nucleus and its subdivisions, becomes smaller, the chromatic 

 substance diminishes, the pigment increases, the cytoplasm exhibits vacuoles, the 



c D 



FIG. 150. To show the changes in nerve-cells due to age : A, spinal ganglion-cells of a still-born male 

 child ; B, spinal ganglion-cells of a man dying at ninety-two years ; n, nuclei. In the old man the cells 

 are not large, the cytoplasm is pigmented, the nucleus is small, and the nucleolus much shrunken or 

 absent. Both sections taken from the first cervical ganglion, X 250 diameters ; C, nerve-cells from the 

 antennary ganglion of a honey-bee, just emerged in the perfect form ; D, cells from the same locality of an 

 aged honey-bee. In Cthe large nucleus (black) is surrounded by a thin layer of cytoplasm; in D the 

 nucleus is stellate, and the cell-substance contains large vacuoles with shreds of cytoplasm (Hodge). 



dendrons atrophy, and the neurons also probably diminish in mass. In some 

 instances the entire cell is absorbed. Some of these facts are illustrated by the 

 observations of Hodge 2 on the spinal ganglion-cells of an old man of ninety- 

 two years as compared with those of a new-born child (see Fig. 150). The 



1 Vas: Archiv fur mikroskopische Anatomic, 1892. 

 J Journal of Physiology, 1894, vol. xvii. 



