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.

[Illustration: Fig. 23.--Patella, or Knee-Cap.]

49. The Foot. The bones of the foot, 26 in number, consist of
the tarsal bones, the metatarsal, and the phalanges. The
tarsal bones are the seven small, irregular bones which make up the
ankle. These bones, like those of the wrist, are compactly arranged, and
are held firmly in place by ligaments which allow a considerable amount of
motion.

One of the ankle bones, the _os calcis_, projects prominently backwards,
forming the heel. An extensive surface is thus afforded for the attachment
of the strong tendon of the calf of the leg, called the tendon of
Achilles. The large bone above the heel bone, the _astragalus_,
articulates with the tibia, forming a hinge joint, and receives the weight
of the body.

The metatarsal bones, corresponding to the metacarpals of the hand,
are five in number, and form the lower instep.

The phalanges are the fourteen bones of the toes,--three in each
except the great toe, which, like the thumb, has two. They resemble in
number and plan the corresponding bones in the hand. The bones of t