Page:Popular Science Monthly Volume 71.djvu/120

114 in the size of cells is a great advantage, and enables us to speak in general terms in regard to the growth of cells, and renders it superfluous to stop and discuss for each part of the body the size of the cells which compose it, or to seek to establish different principles for different animals because their cells are not alike in size.

Now we pass to a totally different aspect of cell development, that which is concerned with the degeneration of cells. For we find that,

after the differentiation has been accomplished, there is a tendency to carry the change yet further and to make it so great that it goes beyond perfection of structure, so far that the deterioration of the cell comes as a consequence. Such cases of differentiation we speak of as a degeneration, and it may occur in a very great number of ways. Very frequently it comes about that the alteration in the structure of the cell goes so far in adapting it to a special function that it is unable to maintain itself in good physiological condition, and failing to keep up its own nourishment it undergoes a gradual shrinkage which we call atrophy. A very good illustration of this, and a most important one, is offered us by the changes which go on in the nerve cells in extreme old age. This is beautifully illustrated by the two pictures which are now before us, copied from investigations of Professor Hodge, of Clark University. The two figures represent human nerve cells taken from the root of a spinal nerve. The left-hand figure shows these cells as they exist in their full maturity; the right-hand figure, as they appear in a person of extreme old age. In the latter you will readily notice that the cells have shrunk and no longer fill