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.

58. The Bones in After Life. Popular impression attributes a less
share of life, or a lower grade of vitality, to the bones than to any
other part of the body. But really they have their own circulation and
nutrition, and even nervous relations. Thus, bones are the seat of active
vital processes, not only during childhood, but also in adult life,
and in fact throughout life, except perhaps in extreme old age. The final
knitting together of the ends of some of the bones with their shafts does
not occur until somewhat late in life. For example, the upper end of the
tibia and its shaft do not unite until the twenty-first year. The separate
bones of the sacrum do not fully knit into one solid bone until the
twenty-fifth year. Hence, the risk of subjecting the bones of young
persons to undue violence from injudicious physical exercise as in rowing,
baseball, football, and bicycle-riding.

The bones during life are constantly going through the process of
absorption and reconstruction. They are easily modified in their growth.
Thus the continued pressure of some morbid deposit, as a tumor or cancer,
or an enlargement of an artery, may cause the absorption or distortion of
bones as readily as of one of the softer tissues. The distortion resulting
from tight lacing is a familiar illustration of the facility with which
the bones may be modified by prolonged pressure.