THE SUPPORTING TISSUES 51 



milk should form a large part of a child's diet. Many children 

 though given food abundant in quantity are really starved, since 

 their food does not contain in sufficient amount the mineral salts 

 requisite for their healthy development. 



At birth even those bones of a child which are most ossified are 

 often not- continuous masses of osseous tissue. In the large bone 

 of the arm, the humerus, for example, the shaft of the bone is well 

 ossified and so is each end, but between the shafts and each of the 

 articular extremities there still remains a cartilaginous layer, and 

 at those points the bone increases in length, new cartilage being 

 formed and replaced by bone. The bone 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. Never- 

 theless here as elsewhere 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 overexert ion. 



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 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 (p. 66) joins the 

 shaft by bone from the seventeenth to the nineteenth year, and 

 the lower end during the twentieth. In the tibia (p. 66) 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 (p. 66) the upper end joins the shaft in the twenty- 



