198. Besides the bones already described, there are other small bones, of the size and figure of flattened peas, found in certain parts of the extremities, never in the trunk, called sesamoid, from their resemblance to the seed of the sesamum. They belong rather to the tendons of the muscles than to the bones of the skeleton. They are embedded within the substance of tendons, are found especially at the roots of the thumb and of the great toe, and are always placed in the direction of flexion. Their office, like that of the patella, which is, in truth, a bone of this class, is to increase the power of the flexor muscles by altering the line of their direction, that is, by removing them farther from the axis of the bone on which they are intended to act.
199. The ligaments which connect the bones of the lower extremities are the firmest and strongest in the body. Of these, the fibrous capsule of the hip-joint (fig. XCIV. 1), which secures the head of the femur in the cavity of the acetabulum (fig. XCIV.), is the thickest and strongest. It completely surrounds the joint (fig. XCIV. 1). It arises from the whole circumference of the acetabulum, and, proceeding in a direction outwards and backwards, is attached below to the neck of the femur (fig. XCIV. 1). It is thicker, stronger, and much more closely attached to the bones than the fibrous capsule of the shoulder-joint (144), because the hip-joint is formed, not like the shoulder-joint, for extent of motion, but for strength. Its internal surface is lined by synovial membrane, and its external surface is covered and strengthened by the insertion of muscles that move the thigh-bone. The joint is strengthened by another ligament, which passes from the inner and fore part of the cavity of the acetabulum (fig. XCV.) to be inserted into the head of the femur (fig. XCIV.), called the round ligament, the office of which obviously is to hold the head of the femur firmly in its socket.
1. The fibrous capsule of the hip-joint, laid open and
turned aside to show, 2. the round ligament in its natural
position.
A view of the head of the femur drawn out of its socket,
and suspended by the round ligament, to show more clearly
the action of the ligament in retaining the head of the
femur in its socket.
200. Numerous and complicated ligaments connect the bones that form the knee-joint (fig. XCVI.), and the strength of these powerful bands is greatly increased by the tendons that move the leg (fig. XCVI. 5), which pass over, and more or less surround, the joint.
General view of the ligaments of the knee-joint. 1. Lower
end of the femur; 2. upper end of the tibia; 3. upper end
of the fibula; 4. the patella; 5. united tendons of the
extensor muscles; 6. ligaments of the patella; 7. the capsular
investment of the knee; 8. the internal lateral ligament;
9. the external lateral ligaments; 10. the posterior
ligament; 11. the ligament connecting the tibia and fibula;
12. a portion of the interosseous ligament.