Page:Popular Science Monthly Volume 70.djvu/491

Rh a curious change going on, a change of which old age presents to us the culminating record. In order to study the weight of the brain, it is necessary to compare people of the same size, for the size and weight of the brain depend somewhat upon the size of the individual. Now it has been discovered by careful examination of persons of similar size that the brain begins relatively early to diminish its weight. Thus in persons of a height of 175 centimeters, and over, of the male sex, it is found that in a period of from twenty to forty years the brain weight is 1,409 grams. But from forty-one to seventy years it has sunk to 1,363, and in persons of from seventy-one to ninety it has shrunk to 1,330. Women of corresponding size are not easily found, and a more average height for women is 165 centimeters; a woman of such a height is likely to have—among the white races, be it always understood—a weight of brain of 1,265 grams, at forty to seventy years a brain of 1,200, and at seventy-one to ninety years a brain of only 1,166 grams.

I give these figures because they show that there is no guessing, but a definite, positive knowledge, proving that soon after the maturity of life in the individual is reached, the shrinkage of the brain begins, and then continues almost steadily to the very end of life.

It is not only the anatomist, but it is perhaps almost equally the physiologist who gives us insight into the changes, which go on in the old. I spoke a few moments ago of the pulse rate, and of the change which that offers. At first sight it seems as if a greater pulse rate indicated an improvement, but if you recall the explanation which I have given you, you will acknowledge that this is by no means an acceptable interpretation, but that on the contrary the change is a clear mark of enfeeblement. In the respiration, also, we observe a like change. Here the comparison is not quite so easy as we should at first imagine, because there is a relation between the size of the individual and the respiration. The respiration, as you all know, frees the body from the products of combustion, particularly from that product which we know as carbon dioxide. The result of the combustion going on in the body (which in its end term appears to us as carbon dioxide expelled from the lungs) is to produce heat, to develop the necessary warmth for the maintenance of the proper