302 SENESCENCE AND REJUVENESCENCE 



than that of other organs. Even in starvation the nervous system 

 shows little or no reduction. There is, therefore, some reason for 

 believing with Muhlmann and Ribbert that death from old age 

 uncomplicated by disease or incidental factors is primarily a death 

 of the nervous system, and both the histological characteristics of 

 the nerve cells and the physiological condition of the nervous system 

 in cases of extreme old age afford support to this view. Even in 

 invertebrates as low in the scale as annelids, Harms ('12) has 

 observed that the first structural changes preceding natural death 

 occur in the cephalic portion of the central nervous system. 



But death from old age alone without any complicating factors 

 is undoubtedly rare, and it is very difficult to determine in any 

 given case whether complicating factors are present or not; con- 

 sequently it is not possible to assert positively that natural death 

 is in all cases death of the brain or nervous system, although the 

 evidence points in that direction. 



In various insects and in certain fish, e.g., the salmon, death 

 occurs almost at once after extrusion of the sexual products. In 

 such cases the factor immediately concerned in bringing about 

 death is probably exhaustion rather than old age, although the 

 organism is undoubtedly in an advanced stage of senescence when 

 sexual maturity is attained. In certain insects and some other 

 invertebrates which do not feed in the adult stage natural death 

 is probably a death from starvation. 



The natural length of life of organisms must depend on a variety 

 of factors, such as specific constitution of protoplasm, rate of senes- 

 cence, continuity of progressive development, or in other words the 

 degree of rejuvenescence during the life history, functional activity, 

 perhaps the amount and in some forms probably also the character 

 of food. In general it represents the length of time from the 

 beginning of senescence in the early stages of development to the 

 stage where the rate of metabolism is so low that the physiological 

 mechanism disintegrates. Commonly the life of the organism is 

 very much longer than that of many of its constituent cells, but it 

 is probable that the extreme limit of life of the individual is deter- 

 mined by the length of life of its shortest-lived essential organ or 

 tissue, and this must be the organ or tissue which is least subject to 



