But this condition of things has its dangers.  The fact that a child’s bones bend easily, also renders them liable to permanent change of shape.  Thus, children often become bow-legged when allowed to walk too early.  Moderate exercise, however, even in infancy, promotes the health of the bones as well as of the other tissues.  Hence a child may be kept too long in its cradle, or wheeled about too much in a carriage, when the full use of its limbs would furnish proper exercise and enable it to walk earlier.

57.  Positions at School.  Great care must be exercised by teachers that children do not form the habit of taking injurious positions at school.  The desks should not be too low, causing a forward stoop; or too high, throwing one shoulder up and giving a twist to the spine.  If the seats are too low there will result an undue strain on the shoulder and the backbone; if too high, the feet have no proper support, the thighs may be bent by the weight of the feet and legs, and there is a prolonged strain on the hips and back.  Curvature of the spine and round shoulders often result from long-continued positions at school in seats and at desks which are not adapted to the physical build of the occupant.

[Illustration:  Fig. 29.—­Section of the Knee Joint. (Showing its internal structure)

  A, tendon of the semi-membranosus muscle cut across;
  B, F, tendon of same muscle;
  C, internal condyle of femur;
  D, posterior crucial ligament;
  E, internal interarticular fibro cartilage;
  G, bursa under knee-cap;
  H, ligament of knee-cap;
  K, fatty mass under knee-cap;
  L, anterior crucial ligament cut across;
  P, patella, or knee-cap
]

A few simple rules should guide teachers and school officials in providing proper furniture for pupils.  Seats should be regulated according to the size and age of the pupils, and frequent changes of seats should be made.  At least three sizes of desks should be used in every schoolroom, and more in ungraded schools.  The feet of each pupil should rest firmly on the floor, and the edge of the desk should be about one inch higher than the level of the elbows.  A line dropped from the edge of the desk should strike the front edge of the seat.  Sliding down into the seat, bending too much over the desk while writing and studying, sitting on one foot or resting on the small of the back, are all ungraceful and unhealthful positions, and are often taken by pupils old enough to know better.  This topic is well worth the vigilance of every thoughtful teacher, especially of one in the lower grades.