I.] THE DURATION OF LIFE. 21 



senility so-called, — have been most accurately studied among 

 men. We know that with advancing age certain alterations 

 take place in the tissues, by which their functional activity is 

 diminished ; that these changes gradually increase, and finally 

 either lead to direct or so-called normal death, or produce 

 indirect death by rendering the organism incapable of resisting 

 injuries due to external influences. These senile changes have 

 been so well described from the time of Burdach and Bichat to 

 that of Kussmaul, and are so well known, that I need not enter 

 into further details here. 



In answer to an inquiry as to the causes which induce these 

 changes in the tissues, I can only suggest that the cells which 

 form the vital constituents of tissues are worn out by prolonged 

 use and activity. It is conceivable that the cells might be thus 

 worn out in two ways ; either the cells of a tissue remain the 

 same throughout life, or else they are being continually replaced 

 by younger generations of cells, which are themselves cast off 

 in their turn. 



In the present state of our knowledge the former alternative 

 can hardly be maintained. Millions of blood corpuscles are 

 continually dying and being replaced by new ones. On both 

 the internal and external surfaces of the body countless epithe- 

 lial cells are being incessantly removed, while new ones arise in 

 their place ; the activity of many and probably of all glands is 

 accompanied by a change in their cells, for their secretions 

 consist partly of detached and partly of dissolved cells ; it is 

 stated that even the cells of bone, connective tissue, and muscle 

 undergo the same changes, and nervous tissue alone remains, 

 in which it is doubtful whether such a renewal of cells takes 

 place. And yet as regards even this tissue, certain facts are 

 known which indicate a normal, though probably a slow 

 renewal of the histological elements. I believe that one might 

 reasonably defend the statement,— in fact, it has already found 

 advocates,— that the vital processes of the higher (i. e. multi- 

 cellular) animals are accompanied by a renewal of the morpho- 

 logical elements in most tissues. 



This statement leads us to seek the origin of death, not in the 

 waste of single cells, but in the limitation of their powers of 

 reproduction. Death takes place because a worn-out tissue 

 cannot for ever renew itself, and because a capacity for increase 



