SENESCENCE IN HIGHER ANIMALS AND MAN 273 



as one hundred and forty calories; in medium-sized, ninety-nine 

 cr,lories per square centimeter of body-surface. In other words, 

 the rate of metabolism is determined by age, rather than by surface. 

 According to the data compiled by Magnus-Levy from various 

 authors, the amount of proteid necessary to keep old persons in 

 health is less than that necessary in early life. After "a certain 

 time old persons in general take less food than is necessary to 

 maintain their weight, and a gradual loss of weight occurs which 

 varies in rate and amount according to various conditions. More- 

 over, the whole course of the life history from youth to old age with 

 its decrease in bodily activity and in rate of growth, and its advan- 

 cing differentiation and accumulation of structural substance points 

 very clearly to a decreasing rate of metabolism per unit of weight. 

 It may also be noted that the process of chemical differentiation of 

 the brain of the white rat during growth indicates that the rate 

 of metabolism is decreasing during this period (W. and M. L. 

 Koch, '13), and Dr. S. Tashiro kindly permits the statement from 

 unpublished data that in the horseshoe crab, Limulus polyphemus, 

 the production of carbon dioxide per unit of weight in the nervous 

 system decreases as the weight of the nervous system increases; 

 apparently, the larger and older the animal, the lower the rate of 

 carbon-dioxide production in the nervous system. 



THE RATE OF GROWTH 



The rate of growth also shows, in general, a decrease from early 

 stages of development onward; although in many cases periodic 

 or occasional increases in rate of greater or less magnitude occur. 

 The decrease in the rate of growth during development in man and 

 the higher vertebrates has been demonstrated beyond all question 

 from a great variety of data, and its significance for the problem of 

 senescence has been so ably presented by various authors 1 that only 

 a brief consideration is necessary at this time. It must be remem- 

 bered that, as Minot ('91) pointed out, absolute increments of 

 weight, volume, length, or any other component of growth during 

 equal successive periods are not measures of the rate of growth, for 



1 See particularly Donaldson, '95, the chapters on growth; Minot, '91, '08, 

 chap, iii, "The Rate of Growth"; Muhlmann, 'oo. 



