Page:Popular Science Monthly Volume 70.djvu/490

486 and greater effort of the heart to drive the blood through them, and in response to this new necessity, the heart becomes enlarged in an effort of the organism to adapt itself to the new unfavorable condition of the circulation established by age. But the power of the heart becomes inferior along with this hypertrophy or enlargement, and we see that in the old, in order to make up for the feebleness of the enlarged heart, it beats more frequently. In other words, the pulse rate in the old person increases. We find, for instance, that at the time of

birth the pulse rate is at the rate of 134 beats to a minute. It rises slightly during the first three months of infancy until at the end of the third month it reaches some 140 beats a minute; it soon falls off, however, and at the end of the first year it has sunk to 111; at five or six years it becomes 98, and at twenty-one years it has sunk to 71 or 72. There are thereafter certain minor fluctuations in the rate of the heart-beat with advancing age, but generally it may be said that this value of 72 beats a minute is characteristic of adult life. But when a person becomes eighty years old, it has been found that upon the average the rate of the heart-beat rises and becomes 79 a minute. Hence it is clear that though the heart is larger, it has to make a greater effort, that is to say a more frequent beat, in order to maintain the necessary circulation of the blood. We see also, as we go back to the anatomical examination of the body, that those important structures which we call the germ cells, upon which the propagation of the race depends, which present under the microscope certain clearly recognized characteristics by which they can be distinguished from all other cells of the body, that these germ-cells cease their activity altogether in the very old, and one of the great functions of life is thus blotted out altogether from the history of the individual.

Turning now to the yet nobler organs, especially the brain, we see