SENESCENCE, DECADENCE, AND DEATH 



421 



well-defined areas of arteriosclerotic atrophy and soften- 

 ing. Metchnikoff has observed that in old parrots the 

 nerve cells become surrounded by phagocytic cells neu- 

 rophages that gradually encroach upon and destroy 

 them. Such destruction of the nervous tissues inevit- 

 ably results in changes in the psychic condition of the 

 individual. 



FIG. 147. Nerve cells surrounded by neurophages phagocytic cells by 

 which they are gradually destroyed. This form of phagocytic activity only 

 occurs during old age. (Metchnikoff.) 



These anatomical and histological changes amply 

 explain the defective physiology of senility. The cardiac 

 weakness and defective vessels are inadequate to pro- 

 vide that free circulation by which alone the integrity 

 of the tissues can be maintained, and there is a tendency 

 for widespread calcification to make its appearance. 



