706 



REPRODUCTION AND GROWTH 



and the next day a man or woman ; but are recognized by the characteristics 

 which plainly prevail, once they are fully established. These periods are : 



(1) Period of the Newborn, from .birth to the loss of the umbilical cord, 

 which usually takes place in four or five days. 



(2) Period of Infancy, up to the appearance of the first teeth, from the 

 seventh to the ninth month. 



(3) Later Childhood, up to the appearance of the permanent teeth about 

 the seventh year. 



(4) Age of Boyhood and Girlhood, up to the beginning of puberty, thir- 

 teenth to fourteenth years. 



(5) Age of Youth or Adolescence, up to the time of bodily maturity, 

 nineteenth to twenty-first year. 



(6) Age of Maturity, up to the prime of life (climacteric in the woman), 

 forty-fifth to fiftieth year. 



(7) Old Age. 



The first five periods are the ones of particular interest here because they 

 cover almost the entire period of growth. The sixth period is the age within 

 which man reaches his full physical and mental development. During the 

 seventh period various disorders, caused more or less by chronic ailments of 

 one kind or another, gradually encroach upon the normal functions, and the 

 physical and mental powers are on the wane. 



In turning now to the subject of the size relations of the body, let it be 

 understood that the data presented are average results and that many indi- 

 vidual variations from them are to be observed. In any exhaustive discussion 

 of the subject it would be necessary to consider these variations and their 

 meaning, but it will be impossible to do so here. The following is a concrete 

 example of the method employed in arriving at average results. 



Quetelet and Altherr carried out a series of observations on the weight of 

 the newborn child, and found the mean value, irrespective of sex, to be 3,100 g. 

 The extremes however were very considerable, for among the children weighed 

 there were some under 1.5 kg., and some over 4.5 kg. In order to get a general 

 view of the variations and thus to be able to grasp the significance of the mean 

 value more correctly, the entire series of observations may be divided into 

 groups according to weight: 1.0-1.5, 1.5-2.0, etc., and the proportion of indi- 

 vidual cases belonging to each calculated in percentages of the entire number. 

 We get in this way the following table, the results of which might be made still 

 more clear by a graphic representation like that in Fig. 302. 



