310 GENERAL ZOOLOGY 



spinal cord. A child at birth is rather helpless and does 

 not correlate its movements well, but is nourished, cared 

 for, and protected during this critical period. It also 

 receives education, which even in the most savage races far 

 exceeds that obtained by any other animal. It is taught 

 to walk, talk, get food, care for itself, and to make imple- 

 ments. No wonder that man has been able to dominate 

 the earth. 



The most important structural changes which occur in a 

 growing child after it ceases to be nourished by its mother 

 are in the teeth and reproductive organs. A child usually 

 acquires part of its first set of teeth before it is weaned 

 (six to eighteen months) ; but children are rarely born with 

 all their milk teeth, and others do not acquire any for more 

 than a year. The second set of teeth usually appears 

 between the ages of seven and twelve years, except the 

 " wisdom" teeth which generally break through the gums 

 before the twenty-fifth year but sometimes fail to emerge at 

 all. The reproductive organs become functional in both 

 sexes between thirteen and fifteen ; the voice becomes deeper 

 in the male and there are other accompanying changes. 

 The ovaries cease their activities between the ages of forty 

 and fifty, but the male reproductive organs may remain 

 functional throughout life. 



