47. The Thigh. The longest and strongest of all the bones is the
femur, or thigh bone. Its upper end has a rounded head which fits into the
_acetabulum_, or the deep cup-like cavity of the hip bone, forming a
perfect ball-and-socket joint. When covered with cartilage, the ball fits
so accurately into its socket that it may be retained by atmospheric
pressure alone (sec. 50).

The shaft of the femur is strong, and is ridged and roughened in places
for the attachment of the muscles. Its lower end is broad and irregularly
shaped, having two prominences called _condyles_, separated by a groove,
the whole fitted for forming a hinge joint with the bones of the lower leg
and the knee-cap.


48. The Lower Leg. The lower leg, like the forearm, consists of
two bones. The tibia, or shin bone, is the long three-sided bone
forming the front of the leg. The sharp edge of the bone is easily felt
just under the skin. It articulates with the lower end of the thigh bone,
forming with it a hinge joint.

The fibula, the companion bone of the tibia, is the long, slender
bone on the outer side of the leg. It is firmly fixed to the tibia at each
end, and is commonly spoken of as the small bone of the leg. Its lower end
forms the outer projection of the ankle. In front of the knee joint,
embedded in a thick, strong tendon, is an irregularly disk-shaped bone,
the patella, or knee-cap. It increases the leverage of important
muscles, and protects the front of the knee joint, which is, from its
position, much exposed to injury.