410 OLD AGE. [BOOK iv. 



by the respiratory and urinary excreta, is also not only absolutely 

 but relatively to the body -weight less, and her blood is not only 

 less in quantity but also of lighter specific gravity and contains a 

 smaller proportion of red corpuscles. Her strength is to that of 

 man as about 5 to 9, and the relative length of her step as 1000 to 

 1157. 



978. From birth onward (and indeed from early intra- 

 uterine life) the increment of growth as we have seen, though 

 undergoing certain variations, continues to diminish. At last a 

 point is reached at which the curve cuts the abscissa line, and 

 the increment becomes a decrement. After the culmination' of 

 manhood at forty and of womanhood at the climacteric, the 

 prime of life declines into old age. The metabolic activity of 

 the body, which at first was sufficient not only to cover the daily 

 waste but to add new material, later on is able only to meet the 

 daily wants, and at last is too imperfect even to sustain in its 

 entirety the existing frame. Neither as regards vigour and 

 functional capacity, nor as regards weight and bulk, do the 

 turning-points of the several tissues and organs coincide either 

 with each other or with that of the body at large. We have 

 already seen that the life of such an organ as the thymus is 

 far shorter than that of its possessor. The eye is in its dioptric 

 prime in childhood, when its media are clearest and its muscular 

 mechanisms most mobile, and then it for the most part serves 

 as a toy ; in later years, when it could be of the greatest service 

 to a still active brain, it has already fallen into a clouded and 

 rigid old age. The skeleton reaches its limit very nearly at the 

 same time as the whole frame reaches its maximum of height, the 

 coalescence of the various epiphyses being pretty well completed 

 by about the twenty-fifth year. Similarly the muscular system in 

 its increase tallies with the weight of the whole body. The brain, 

 in spite of the increasing complexity of structure and function to 

 which it continues to attain even in middle life, early reaches its 

 limit of bulk and weight. At about seven years of age it attains 

 what may be considered as its first limit, for though it may 

 increase somewhat up to twenty, thirty, or even later years, its 

 progress is much more slow after than before seven. The vascular 

 and digestive organs as a whole may continue to increase even to 

 a very late period. From these facts it is obvious that though the 

 phenomena of old age are, at bottom, the result of the individual 

 decline of the several tissues, they owe many of their features to 

 the disarrangement of the whole organism produced by the 

 premature decay or disappearance of one or other of the con- 

 stituent bodily factors. Thus, for instance, it is clear that were 

 there no natural intrinsic limit to the life of the muscular and 

 nervous systems, they would nevertheless come to an end in 

 consequence of the nutritive disturbances caused by the loss of 

 the teeth. And what is true of the teeth is probably true of 



