In
very early life, the bones consist chiefly of the animal part, and are
then soft and pliant. As the child advances in age, the bones grow
harder, by the gradual deposition of the phosphate of lime, which is
supplied by the food, and carried to the bones by the blood. In old
age, the hardest material preponderates; malting the bones more brittle
than in earlier life.
The bones are covered with a thin skin or membrane, filled with small
blood-vessels which convey nourishment to them,
Where the hones unite with others to form joints, they are covered
with _cartilage,_ which is a smooth, white, elastic substance. This
enables the joints to move smoothly, while its elasticity prevents
injuries from sudden jars.
The joints are bound together by strong, elastic bands called
_ligaments,_ which hold them firmly and prevent dislocation.
Between the ends of the bones that unite to form joints are small sacks
or bags, that contain a soft lubricating fluid. This answers the same
purpose fur the joints as oil in making machinery work smoothly, while
the supply is constant and always in exact proportion to the demand.
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