148 AGE-INCIDENCE OF CAUSES OF INFANT MORTALITY 



Experience acquaints us with other amounts of prenatal sick- 

 ness becoming lethal in postnatal time. These, with postnatal 

 sickness, amount to 40 lethal doses in the first month. Two hun- 

 dred such amounts may be delivered in the first year of life, 

 and, in the eighth year 5 lethal doses per 1,000 persons living. 

 The numbers which we call mortality units 5 are characteristic 

 of the first, the eighth, or other year to which they correspond. 

 Every remoter value being greater, and every nearer value being 

 less, the characteristic mortality value which we observe de- 

 scribes the integral part of all the sickness possible to observe 

 in that period. The sickness equivalent of a given mortality 

 unit is a lethal dose, characteristic of the age at which it is 

 regularly observed; enough to kill five persons in the eighth 

 year, or two hundred persons in the first year. Non-lethal sick- 

 ness (continuing through or recovering in the year) must be 

 measured as an addition to lethal sickness, starting a new series 

 of denominate numbers which in their sum may equal any frac- 

 tion but not the whole of the characteristic lethal dose. Lethal 

 sickness is the sum of the superior values of sickness down to 

 and including the minimum fatal dose. Non-lethal sickness is 

 the sum of a maximum non-lethal dose and all inferior magni- 

 tudes of sickness; a mantissa therefore of the lethal unit. With 

 equal distribution of the causes of sickness throughout the 

 years of life, the sickness curve should obey the mortality curve, 

 and should always lie between it and the vitality curve (the 

 probability of living one year). 



But the causes of sickness are not equally distributed. The 

 diseases of infancy and childhood include most of those which, 

 once survived, make no second attack. In the very early 

 years, when the curves of life and death are diverging, sickness 

 is at its highest frequency and briefest duration. Its frequency 

 diminishes and duration increases with age, and when the 

 curves converge in later life, the sickness curve loses signifi- 

 cance, until at length, and well within the century mark, the 

 vitality curve becomes a sickness curve. When the probability 

 of living a year and the probability of dying within a year 

 reach, as they do at 86, the same relations which they had at 

 birth, the sickness curve becomes superfluous. The vitality curve 

 becomes thereafter a curve of permanent sickness; so that when 

 observed sickness in old age is described by a curve attaining 

 its maximum nine years earlier and ending 15 years earlier than 

 the death curve, it proves no more, perhaps, than that life ends 

 in a twilight as baffling as that in which it begins. 



The probability of dying within a year. 



