486 BONES AND MUSCLES OF THE LOWER EXTREMITY. 
forwards. The knee is a good example of a pure hinge-joint, 
all its movements being restricted to one plane. 
647. The leg, although containing two bones like the fore¬ 
arm, does not in Man possess the peculiar movement which 
characterises it. One of these hones, called the Tibia , is much 
larger than the other which is called the Fibula ; and it is 
the former alone on which the thigh-hone rests, and which 
itself rests upon the foot, so that no movement of rota¬ 
tion is permitted in the leg. In fact, the fibula, which is 
a long slender hone running nearly parallel with the tibia 
(fig. 223), looks like a mere appendage or rudiment, and 
serves only for the attachment of muscles. The upper end of 
the tibia is broad, and has two shallow excavations, in which 
the condyles of the femur are received. Upon the front of 
the knee-joint we find a small separate bone, the patella or 
knee-pan; the purpose of this is to change the direction of 
the tendons that come down from the front of the thigh to be 
attached to the tibia ; in such a manner as to enable them to 
act more advantageously, upon the principle formerly stated 
(§. 611). In the elbow-joint, this change was not required ; 
since the ulna projects sufficiently far backwards to afford ad¬ 
vantageous attachment to the tendon of the extensor muscle. 
—The very powerful muscles which tend to straighten the 
knee-joint, arise from the front of the pelvis and from the 
femur itself; and they form the fleshy mass of the front 
of the thigh. On the other hand, those which bend the knee 
arise from the lower border of the pelvis and from the back of 
the thigh-bone, and pass downwards to be inserted into the 
sides of the tibia and fibula a little below the knee, their 
tendons forming the two strong cords known as the hamstrings. 
The articulating surface at the lower extremity of the leg, 
which enters into the ankle-joint, is principally formed by the 
tibia; but its outer border is formed by the fibula, which 
there makes a considerable projection that can be felt through 
the skin.—In the Quadrumana, and in a less degree in some 
other Mammals, the two bones of the leg resemble those 
of the fore-arm ; and are so articulated as to give to the foot 
a power of rotation corresponding with that of the hand. 
648. The Foot is composed, like the hand, of three distinct 
portions, which are called the tarsus , metatarsus , and phalanges. 
There are seven bones in the tarsus, all of which are larger 
