48 THE HUMAN BODY 



The final knitting together of the bony articular ends with the 

 shaft of many bones takes place only comparatively 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 ex- 

 tremity 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 continuity in 

 the twentieth year. The upper end of the femur joins the shaft by 

 bone from the seventeenth to the nineteenth 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 the 

 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 

 lads of sixteen or seventeen row and take other exercise in plenty 

 is one thing, and a good one; but to allow them to undergo the 

 severe and prolonged strain of training for and rowing a long race 

 is quite another, and not devoid of risk. 



