Definition and Measurement of Senescence 13 



Ectopic flexure lines form on those ])arts of the body where 

 the skin is habitually creased or folded, particularly on the 

 face. A single creasing of the skin is insuttieient to form an 

 overt flexure line, but presumably even one creasing leaves 

 some trace in the dermal fibrous skeleton, because if the skin 

 is creased sufficiently often, a flexure line will eventually 

 appear. Old people have acquired lined faces for two clearly 

 distinguishable reasons: {a) because an innate deterioration 

 of their skin has made it more susceptible to the engravery of 

 habitual usage, and {b) because they have smiled or frowned 

 or raised their eyebrows more often than their juniors. There 

 can be little doubt that the first factor, the true "innate" 

 deterioration of the skin, by far outweighs the latter, and 

 that it represents the process which most people understand 

 by senile change, but both agencies are at work: the per- 

 sistent and cumulative effects of recurrent stress, and the 

 progressive decline of power of the skin to resist its action. 



This is a trivial example, but it would be idle to deny that 

 purely contingent factors influence the vulnerability of 

 animals, particularly when the organ or part of the body that 

 is affected is constitutionally incapable of repair. For a higher 

 animal to lose a limb may not in itself be fatal, but if it does 

 so it will certainly be more vulnerable to the hazards which 

 limbs are normally used to escape from. It may be that the 

 actual life-span of wild carnivores is as closely bounded by the 

 life of their teeth as by any other single factor. Teeth may 

 become more fragile with increasing age; but whether they 

 do or not, it is quite certain that one good reason why an old 

 animal should have fewer sound teeth than a young one is 

 because time alone, unconnected with senile decay, has given 

 the older animal more opportunity to break them. In short, 

 risks are not all of an all-or-nothing character; there is a 

 gradient of efi'ect between being killed (which alone qualifies 

 one for admission to the life table) and being left unscathed. 

 An injury which falls short of being mortal does not necessarily 

 leave an animal as well equipped as before to escape a hazard 

 of the same or of another kind. The residue of injuries that 



