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. [Illustration: Fig. XCIV. 1. The fibrous capsule of the hip-joint, laid open and turned aside to show, 2. the round ligament in its natural position.] [Illustration: Fig. XCV. 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. [Illustration: Fig. XCVI. 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.] 201. Strong ligaments maintain in their proper position the bones that form the ankle-joint (fig. XCVII.), connect the bones of the tarsus and metatarsus with one another (fig. XCVIII. 1), and articulate the several phalanges of the toes (fig. XCVIII. 2). [Illustration: Fig. XCVII. General view of the posterior ligaments of the ankle-joint. 1. Lower end of the tibia; 2. lower end of the fibula; 3. astragalus; 4. os calcis; 5. ligament between the tibia and fibula; 6. ligament passing from the fibula to the astragalus; 7. ligament passing from the fibula to the os calcis; 8. ligament passing from the tibia to the astragalus.] [Illustration: Fig. XCVIII.