SENESCENCE IN HIGHER ANIMALS AND MAN 271 



rate of metabolism and the rate of growth decrease, the water- 

 content of the body likewise decreases, and the tissues become 

 denser. But the condition known as old age or senility accom- 

 panied by atrophy of tissues, which is well marked in man and has 

 also been observed in various mammals, is either less clearly defined 

 in the lower forms or else is not usually reached because advancing 

 senescence induces reproduction and rejuvenescence. 



Because of the absence of agamic reproduction and the limited 

 capacity for regression and reduction in these forms, they consti- 

 tute much less favorable material than the lower forms for study 

 and analysis of age processes, and theories of senescence based only 

 or chiefly on data obtained from the higher forms have in most 

 cases but little general biological value. Much of the literature 

 of the subject belongs primarily to the medical field and throws 

 little light upon the general biological problem of senescence, but 

 various attempts have been made to formulate general theories of 

 senescence from the study of the higher animals and man alone.' 



In the following sections of this chapter the chief characteristics 

 of senescence in the higher forms are briefly considered and the 

 bearing of some of the recent experimental work upon the problem 

 is discussed. 



THE RATE OF METABOLISM 



Most authbrities agree that the rate of metabolism in man and 

 mammals, so far as determined, undergoes in general a decrease 

 with advancing age.^ Rubner has attempted to show that in warm- 

 blooded animals the rate of metabolism per unit of surface of the 



' The following references are selected from the more recent literature dealing 

 primarily with senescence and old age in man: Bilancioni, '11, with bibliography; 

 Demange, '86; Friedmann, '02; Lorand, '11, with bibliography; Metchnikoff, '03, 

 '10; Ribbert, '08; Rubner, '08. Recent more general considerations of the problem 

 of senescence, but concerned chiefly with man and the higher animals, are Dastre, '03; 

 Muhlmann, '00, '10; Minot, '08, '13. 



' The article by Magnus-Levy on "Metabolism in Old Age" with bibliography, 

 in the Anglo-American issue of von Noorden's Metabolism atid Practical Medicine 

 (1907), is a valuable general survey of our knowledge on the subject. See also Miihl- 

 mann, '00 (p. 164). Among special papers dealing with the question of metabolic 

 changes in relation to age in man and mammals may be mentioned A. V. and A. M. 

 Hill, 13; von Hosslin, '88; Kovesi, '01; Magnus-Levy and Falk, '99; Rubner, '83, 

 '8s, '08, '09; Sonden and Tigerstedt, '95; Speck, '89. 



