THE DURATION OF LIFE. 21 



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 epithelial 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. multicellular) animals 

 are accompanied by a renewal of the morphological 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 repro- 

 duction. Death takes place because a worn-out tissue cannot for 

 ever renew itself, and because a capacity for increase by means of 

 cell-division is not everlasting, but finite 1 . This does not however 

 imply that the immediate cause of death lies in the imperfect re- 

 newal of cells, for death would in all cases occur long before the 

 reproductive power of the cells had been completely exhausted. 

 Functional disturbances will appear as soon as the rate at which the 

 worn-out cells are renewed becomes slow and insufficient. 



But it must not be forgotten that death is not always preceded 

 by senility, or a period of old age. Tor instance, in many of 

 the lower animals death immediately follows the most important 

 deed of the organism, viz. reproduction. Many Lepidoptera, all 

 may-flies, and many other insects die of exhaustion immediately 

 after depositing their eggs. Men have been known to die from 

 the shock of a strong passion. Sulla is said to have died as 

 the result of rage, whilst Leo X succumbed to an excess of joy. 

 Here the psychical shock caused too intense an excitement of the 

 nervous system. In the same manner the exercise of intense effort 

 may also produce a similarly fatal excitement in the above- 

 mentioned insects. At any rate it is certain that when, for some 



1 See Appendix, note 8, p. 59. 



