minute apartments, in the rock-like portion of the temporal bone. The
socket for the eye has a jutting ridge of bone all around it, to guard the
organ of vision against injury. Grooves and canals, formed in hard bone,
lodge and protect minute nerves and tiny blood-vessels. The surfaces of
bones are often provided with grooves, sharp edges, and rough projections,
for the origin and insertion of muscles.

[Illustration: Fig. 28.--External Ligaments of the Knee.]

56. The Bones in Infancy and Childhood. The bones of the infant,
consisting almost wholly of cartilage, are not stiff and hard as in after
life, but flexible and elastic. As the child grows, the bones become more
solid and firmer from a gradually increased deposit of lime salts. In time
they become capable of supporting the body and sustaining the action of
the muscles. The reason is that well-developed bones would be of no use to
a child that had not muscular strength to support its body. Again, the
numerous falls and tumbles that the child sustains before it is able to
walk, would result in broken bones almost every day of its life. As it is,
young children meet with a great variety of falls without serious injury.

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