364 Harris M. Benedict 



proceed the cell is being gradually entombed by an inevitable decrease in 

 the permeabihty of its protoplasm. 



While decreasmg permeabihty furnishes a possible explanation of the 

 more obvious symptoms of senility, it cannot be the only degeneration of 

 first rank. All protoplasmic functions must be involved. Underlying 

 these primary causes of senile degeneration there must be some general 

 fundamental cause from which they spring. This fundamental cause may 

 well be the colloidal nature of protoplasm. While the physical attributes 

 that characterise the colloidal condition of matter are not thoroughly 

 understood, the crude generalization may perhaps be advanced that the 

 unit of structure of the colloid is a molecular complex while that of the 

 crystalloid is a single molecule. The physical changes which colloids 

 undergo when exposed to external forces may be referred to rearrange- 

 ments in the positions of the molecules in the unit complexes. The rise 

 of the melting point of paraffin on repeated applications of heat, the 

 reversible or irreversible changes from sol to gel under different degrees 

 of heat, and other modifications of colloidal response produced by repeated 

 apphcations of a given force, are examples of colloidal changes probably 

 due to rearrangement of the molecules in the complex. 



This intricate interrelation between the constituent parts of the colloidal 

 unit of structure is subject to progressive changes in many non-Hving colloids, 

 and these, with the passage of time, exhibit changes which might be called 

 senile degenerations. The tendency to a closer approxunation of the 

 units of colloidal suspensions, as tune goes on, is an illustration of the 

 instabihty of the colloidal condition. Since the relatively simple relations 

 in the nonhving colloidal complex tend to change on the application of 

 external forces, or even with long standing, it seems inevitable that the 

 extremely complex colloidal states constituting protoplasm should be 

 progressively modified by the activities of life and the impact of external 

 forces. The wonder is not that protoplasm is subject to senile change, 

 but that the change is so slow. 



If one of the changes in the colloidal units is a closer aggregation of the 

 constituent parts, the probable effect on protoplasm will be to make it 

 less permeable, lessen the quantity of imbibed water, and obstruct the 

 other physical activities of life. Whatever the changes in the colloidal 

 complex may be, they are the fundamental cause of seniUty. 



