288 



SENESCENCE AND REJUVENESCENCE 



of age. Under such conditions, or where the rate of metabolism 

 has fallen below a certain level in consequence of age, the break- 

 down and elimination of the substratum is not compensated by the 

 synthesis of new substance, consequently a decrease in size and 

 finally cell death occur. Atrophy in the higher animals differs 

 from reduction in the lower forms in that, while decrease in size 

 occurs, there is little or no dedifferentiation. The cell has appar- 

 ently become so highly differentiated that it has lost the capacity 

 for synthesizing a substratum adequate in quantity or constitution 



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FIGS. 115, 116. Cells from the first cervical ganglion of man at different ages: 

 Fig. 115, from fetus killed by accident of birth; Fig. 116, from man dying of old age 

 at ninety-two years, showing on the left two cells shrunken and undergoing atrophy 

 and on the right the outlines of spaces formerly occupied by cells now degenerated. 

 After Hodge, '94. 



to carry on metabolism. Consequently the losses from degradation 

 and breakdown of the existing substratum are not compensated by 

 the synthesis of new substratal substance, and sooner or later the 

 fundamental mechanism of the cell is destroyed and degeneration 

 and death occur. The atrophy of old age in organs of such funda- 

 mental importance as the nervous system indicates that there is 

 some truth in the statement, so often made, that the later stages of 

 senescence are a " wearing out" of the physiological mechanism or 

 some essential part of it. Apparently the nerve cells or some of 

 them do "wear out" because they are no longer able to synthesize 



