282 Harris M. Benedict 



senility that are more or less fanciful have been based on conditions found 

 in extremely senile animals. 



It is now generally recognized that the forces causing senility and 

 natural death are not dormant in early hfe and later roused to action, 

 but instead are at work from the time when embryonic development 

 begins until death occurs, and, in fact, as Minot (1908) ^ has so efficiently 

 emphasized, are most active at the beginning. Therefore, in view of the 

 difficulty involved in the investigation of extremely senile tissue and of 

 the constant presence of senile changes throughout life, the importance of 

 comparison between younger stages is manifest; and in the accompanying 

 summary of the results of investigations, first rank is given to those char- 

 acteristics that can be shown to undergo continuous change from the 

 beginning. The results of the various essential researches into senility 

 in animals are here classified under three headings: (1) Senile changes 

 in physiological activities; (2) Senile changes in the structure of animal 

 organs; (3) Senile changes in the structure of animal cells. It is evident 

 that in this classification there will be some overlapping, but its advan- 

 tages in permitting a concise and clear-cut arrangement of the various 

 results of different investigators outweigh this defect. 



This very condensed summary of the results of investigations on animal 

 senility gives at least a conception of the difficulties involved. Evidently, 

 however, a start has been made in the various avenues of approach to the 

 problem. 



SENILE CHANGES IN PHYSIOLOGICAL ACTIVITIES 



Deterioration evident from the beginning 

 Decrease in rate of growth. — The most obvious characteristic of senility 

 is a decrease in rate of growth. The rate of decrease found in animals 

 thus far investigated may be illustrated by figures obtained from the 

 study of human development. Hertwig (1906) estimates that the human 

 ovum has a volume of 0.004 cubic millimeter, while the size of the child 

 at birth is 4,000,000 cubic millimeters — an increase of 100,000,000,000 

 per cent in nine months. According to Minot (1908) the gain in weight 

 during the same period is 500,000,000 per cent. It is evident that the 

 rate of growth of the embryo is extraordinary. Miihlmann (1900 and 

 1901) states that the rate of growth of the embryo is greater during the 



2 Dates in parenthesis refer to bibliography, page 365. 



