LOWER LIMB.] the femur, whiUt the condyles of the femur roll upon the semilunar cartilages and articular surfaces of the tibia. A powerful investing ligament encloses the articular sur faces. This ligament is subdivided into bands, one on each side of the joint the internal and external lateral liga ments a posterior and an anterior. The anterior extends from the patella to the anterior tubercle of the tibia, and serves both as a ligament and as the tendon of insertion of the extensor muscles of the leg. Within the investing ligament two interarticular or crucial ligaments pass from the inter-condyloid fossa to the upper surface of the tibia ; and interposed between the tibia and femoral condyles are two menisci, which from their shape are called the semi- lunar cartilages. The synovial membrane not only lines the investing ligaments, but covers the front of the femur for some distance above the trochlea, and forms folds or pads within the joint itself, which in certain movements are interposed between the articular surfaces of the bones. The movements at this joint are those of flexion and exten sion. The flexors are the three great muscles on the back of the thigh, called the ham-strings ; they all arise from the ischial tuberosity, and are inserted the biceps into the head of the fibula, the semi-tendinosus and semi-mem- branosus into the upper end of the tibia. The extensors form the fleshy mass on the front and outer side of the thigh ; one muscle, the rectus, arises from the ilii:m the others, the vasti, from the shaft of the femur ; and they are all inserted by a powerful tendon into the patella, arid through the anterior ligament of the knee into the tibia. The patella is indeed a sesamoid bone, developed in the tendon of these muscles (Fig. 18). The knee can be bent so that the calf can touch the back of the thigh, and in this position the patella is drawn down in front of the joint, as in kneeling. The articular surface of the patella is divided into seven areas or facets, and in passing from the bent to the extended position of the joint, these facets come successively into contact with the articular surface of the femur, until, when the leg is fully extended on the thigh, the whole of the patella is raised above the femoral trochlea, except the lowest pair of narrow facets. It is in order to provide a smooth surface for the patella in this position that the synovial membrane of the joint covers the front of the lower end of the femur. At the com mencement of flexion a slight rotation inwards of the leg and foot takes place through the action of the sartorius, gracilis, and semi-tendinosus, which are inserted close together into the tibia ; whilst the extensor muscles cause, at the completion of extension, a slight rotation outwards of the leg and foot. The movements of flexion and exten sion are not simply in the antero-posterior plane, but along oblique paths which are determined by the screwed configuration of the femoral condyles. In complete exten sion of the leg the joint is " screwed home ;" and as this position is necessary for the preservation of the erect attitude, the lateral, the posterior, and the anterior crucial ligaments are then all tense, to prevent displacement of the bones. The muscles which rotate the leg and foot inwards initiate the act of flexion by unlocking the joint. The Tibio-filular Joints are found between the upper and lower ends of the bones, and in addition a strong interosseous membrane fills up the interval between their shafts. The movement between the two bones is almost inappreciable. ints of The Ankle Joint is formed by the convex upper and the kle and lateral surfaces of the astragalus fitting into the concavity formed by the lower end of the tibia and the two malleoli. An investing ligament, lined by synovial membrane, encloses the joint; the lateral portions of this ligament form distinct bands, and are much stronger than the anterior and posterior fibres. A diarthrodial joint also 841 exists between the astragalus and oa calcis, between which bones a powerful interosseous ligament passes. Between the astragalus and scaphoid, and the os calcis and cuboid, important diarthrodial joints are found, which are enclosed by ligamentous bands. The remaining tarsal bones arc connected together usually by dorsal, plantar, and inter osseous ligaments, and a similar mode of union is found between the distal row of tarsal bones and the metatarsals, except between the great toe and ento-cuneiform, where there is no interosseous ligament. The four outer meta tarsals are also connected at their proximal ends by distal, plantar, and interosseous ligaments; and further, a trans verse metatarsal ligament passes between the distal ends of all the metatarsal bones. The metatarsal bones articu late with the phalanges, and the phalanges with each other, in a similar manner to that described in the corresponding bones of the hand. At the ankle joint movements of flexion and extension take place. The dorsum of the foot is bent towards the front of the leg by the direct action of the muscles on the front of the leg, more especially the tibialis anticus, inserted into the ento-cuneiform and metatarsal of great toe, and the peroneus tertius, inserted into the metatarsal of little toe ; the opposite movement, the so-called extension of the foot, is due to the action of the gastrocnemius and soleus, the great muscles of the calf of the leg, which are inserted by the Tendo Achillis into the posterior prominence of the os calcis or heel. This movement is made at every step in walking or running, and the great size of the calf-muscles is in relation to their use in the act of progression. The foot cannot, however, be drawn so far back as to be brought into direct line with the leg. In standing erect the foot is at right angles to the axis of the leg, the astragalus is locked in between the two malleoli, and the fibres of the lateral ligaments are tense, so as to check movement forwards or backwards, and prevent displacement. Between the several bones of the tarsus a certain amount of gliding is permitted, more especially between the QS calcis and cuboid and the astragalus and scaphoid, so that it is possible to invert or evert the foot, i.e., to raise its inner or outer borders from the ground. The inversion is performed by the tibialis anticus and by the tibialis posticus, which latter is inserted into the scaphoid bone ; the eversion by the peroneus longus and brevis muscles, situated on the outer side of the leg, the tendons of which pass behind the outer malleolus, the brevis to be inserted into the metatarsal bone of the little toe, the longus into the plantar surface of the metatarsal bone of the great toe. The individual toes are bent on the sole by the action of the flexor muscles inserted into the plantar surface of the phalanges, and they are straightened by the extensor muscles inserted into their dorsal surfaces ; the toes also can be drawn asunder or abducted, and drawn together or adducted, chiefly by the action of the interossei muscles. The hallux or great toe is the most im portant digit; a line prolonged backwards through it to the heel forms the proper axis of the foot, and the sole chiefly rests upon the pads of integument situated beneath its metatarso-phalangeal joint and the heel. The hallux is much more restricted in its movements than the thumb : the configuration of its tarso-metatarsal joint and the attachment of the transverse metatarsal ligament prevent the great toe from being thrown across the surface of the sole as the thumb is thrown across the palm in the move ment of opposition ; an object can, however, be grasped between the hallux and second toe by the action of its adductor muscles, and persons can be trained to write with a pen or pencil held in this position. The act of walking consists in the movement forwards of the trunk by the alternate advancement of the lower
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