110 THE HUMAN BODY. 



increases in thickness by new osseous tissue formed beneath 

 the periosteum. The same thing is true of the bones of the 

 leg. On account of the largely cartilaginous and imperfectly 

 knit state of its bones, it is cruel to encourage a young child 

 to walk beyond its strength, and may lead to " bow-legs" 

 or other permanent distortions. Nevertheless here as else- 

 where in the animal body, moderate exercise promotes the 

 growth of the tissues concerned, and it is nearly as bad to 

 wheel a child about forever in a baby-carriage as to force 

 it to walk beyond its strength. 



The best rule is to let a healthy child use its limbs when 

 it feels inclined, but not by praise or blame to incite it to 

 efforts which are beyond its age, and so sacrifice its healthy 

 growth to the vanity of parent or nurse. 



The final knitting together of the bony articular ends 

 with the shaft of many bones takes place only compara- 

 tively late in life, and the age at which it occurs varies 

 much in different bones. Generally speaking, a layer of 

 cartilage remains between the shaft and the ends of the 

 bone, until the latter has attained its full adult length. To 

 take a few examples: the lower articular extremity of the 

 humerus only becomes continuous with the shaft by bony 

 tissue in the sixteenth or seventeenth year of life. The 

 upper articular extremity only joins the shaft by bony con- 

 tinuity in the twentieth year. The upper end of the femur 

 joins the shaft by bone from the seventeen th to the nine- 

 teenth year, and the lower end during the twentieth. In 

 the tibia the upper extremity and the shaft unite in the 

 twenty-first year, and the lower end and the shaft in the 

 eighteenth or nineteenth: while in the fibula the upper end 

 joins the shaft in the twenty-fourth year, and the lower 

 end in the twenty-first. The separate vertebrae of the 

 sacrum are only united to form one bone in the twenty-fifth 

 year of life; and ti.e ilium, ischium, and pubis unite to 

 form the os innominatum about the same period. Up to 

 about twenty-five then the skeleton is not firmly "knit," 

 and is incapable, without risk of injury, of bearing strains 

 which it might afterwards meet with impunity. To let 



