Page:Popular Science Monthly Volume 71.djvu/365

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Ladies and Gentlemen: In order to present the subject of this evening, I will take a few brief moments at the beginning to review the results reached in the previous lecture. In the last lecture I spoke of the phenomena of growth, and endeavored then to make clear to you what I consider the fundamental conception of this study—that the decline in the growth power is extremely rapid at first and slow afterwards. This change in the rate of growth is of course due to things in the animal body itself. It is a logical conclusion for us to draw that if we are to study out the cause of the loss of growth power, we should do it rather at that period of development when the change in the rate of growth is most rapid, for then we should expect those modifications to exhibit themselves most clearly because the magnitude of cause is likely to be proportionate to the magnitude of result, and when the decline is most rapid, then we must expect to find the alterations which cause that decline in the organism to show themselves most conspicuously. You will remember, further, that we spoke of growing old as being a much more complicated question than one of growth alone, and that there occur, as the years advance, changes in the structure of the body. It is convenient to use one collective term for all these phenomena of becoming old, and that term, established by long usage, is senescence, the becoming old. What, therefore, we have to search for at present is a cause, a proximate cause at least, of senescence. In order to make the view I am to bring forward this evening quite clear to you, I must first of all take advantage of your kindness and recapitulate briefly what I said in regard to cells, for you will remember that the cell is the foundation and unit of organic structure. With your permission I should like to recall more exactly to your minds what I said of the cells by having thrown upon the screen the slide which we saw before and which we used as an illustration of the cell. Here is the picture. Above we see the typical cell from the oral epithelium of the salamander, and you remember in the center this more conspicuous body with a granular and reticulated structure which we called the nucleus, and surrounding it is this mass which we called the body of the cell, or the protoplasm. Here is