THE NATURAL HISTORY OF MAN 67 



cranium, grow by additions along their edges which form the 

 sutures. Beginning at about forty years of age, when the brain 

 stops growing, the cranial sutures disappear. In the long limb 

 bones in early postnatal life, one or more centers of ossifica- 

 tion appear in the terminal cartilages and form caps of bone. 

 The growth in length occurs in the cartilage under the bony 

 caps. This was shown experimentally many years ago by 

 John Hunter, who inserted shot in the shaft and in the end 

 cap of a femur in a young animal, and later found that the dis- 

 tance between them had increased. Usually by the twenty-first 

 year these caps have joined to the shaft thus making any 

 further growth in length impossible. The growth in the cir- 

 cumference of a bone takes place by additions under the outer 

 fibrous membrane, known as the periosteum. This has been 

 shown experimentally by feeding growing animals with madder, 

 which gives a yellowish stain to the new bone. 



The child, like the young of all mammals, is born without 

 teeth. The first set, twenty in number, begins to erupt about 

 the sixth month after birth and is completed when the child is 

 nearly two and one half years of age. During the next few 

 years the permanent teeth, thirty-two in number, are forming 

 beneath the temporary teeth and also behind them, and, by the 

 absorption of their roots, replace them. The first permanent 

 tooth to erupt is the first molar which appears behind all the 

 temporary teeth at the sixth year. By thirteen years of age 

 all of the permanent teeth have erupted except the third 

 molars, or wisdom teeth, which appear about the seventeenth 

 year. The latter may be small in size, be abnormally placed, 

 appear late, or even fail entirely to appear. 



A study of the growth curves for height and weight (Fig. 

 25), which have been plotted by averaging the results obtained 

 from weighing and measuring many children at various ages, 

 but mostly over five years of age, shows a general agreement 

 of results. There is a loss of weight for the first few days 



