On the flexor muscles of the lower leg and their action about the knee. Tab. III, Fig. 4.
The tibia flexes about the round-shaped epiphysis of the femur as does the ulna about the cylindrical epiphysis of the humerus. There exists, however, a difference. In the forearm the flexor muscles are inserted almost only in a median point in the vicinity of the proximal extremity of the ulna. The muscles flexing the lower leg are inserted at two points on the opposite sides of the proximal epiphyses of the tibia and fibula like the reins of horses. Three muscles, the semitendinosus, semimembranosus and gracilis, are inserted on the medial side of the tibia and one, the biceps, laterally in the proximal end of the fibula. But this discrepancy does not disturb the nature and properties of the lever. The main discrepancy with the elbow is completely different and consists in the difference of structure of the knee joint CB and elbow joint. In the elbow the proximal extremity of the ulna tightly surrounds the distal epiphysis of the humerus about which it rotates. The knee is different. The femur AB ends distally in a large basis BH which is not really cylindrical. It is somewhat flattened resulting in an elliptical or spiral curvature. This epiphysis is not tightly contained in the tibial epiphysis CH which is as large as the femoral extremity but flatter. The femoral condyles are not deeply buried in corresponding cavities of the tibia. They are separated from the tibia by thick and hard semilunar cartilages which act as a cushion and provide cavities with some depth.
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