The external lateral ligament ( lig. laterale externum) arises from the tuberosity on the external condyle of the femur, and descends, inclining backwards, partly covered by the tendon of the biceps, to be inserted with it into the head of the fibula ; the attachment of its upper extremity is immediately above the origin of the popliteus tendon, which it crosses in its descent, so that this tendon enveloped by its synovial sheath is situated between the liga ment and the joint. The deviation of this ligament from the perpendicular direction is perceived most distinctly in the state of exten sion ; when the joint is flexed, the upper at tachment of the ligament is brought more into the perpendicular over its fibular attachment, the ligament is relaxed and assumes the per pendicular direction ; hence, in the flexed con dition of the joint, the external condyle of the femur, or the tibia on it, admits of a more free motion. This ligament is contrasted by its less length and more rounded form with the internal lateral, and is composed like it of shining tendinous fibres ; a still shorter set of fibres sometimes passes more posteriorly from the condyle to the head of the fibula, or from the sheath of the popliteus tendon, and has been called the short external lateral liga ment.
The posterior ligament (lig. posticum Win slawii) is a portion of the tendon of the semi membranosus muscle which is given off' near its insertion at the posterior and inner margin of the head of the tibia; the portion under con sideration forms a flat and dense fascia which passes upwards and outwards to the external condyle, where it becomes adherent to the sy novial capsule and minglingwith the tendinous origin of the outer head of the gastrocnemius : posterior to it lie the popliteal vessels, and in front of it there is a quantity of firm granulated fat, into which some of its fibres penetrate.
When the posterior ligament and the fat just spoken of are removed, and the joint is ex tended, the two crucial ligaments ligamenta eructate) are brought into view ; they may be seen on the anterior aspect by dissecting down the patella from the fore part of the joint, and putting it in a state of flexion ; in the former view, the posterior crucial ligament is best seen ; in the latter, the anterior : the upper extremities of both are fixed in the fossa be tween the condyles of the os femoris; their lower extremities are attached to the head of the tibia between the two articular surfaces.
The anterior crucial ligament passes from the inner and back part of the outer condyle downwards and forwards to the depression in front of the spine of the tibia, where some portion becomes continuous with the anterior extre mity of the internal semilunar cartilage. The
posterior extends from the fore and outer part of the internal condyle downwards and back wards to the notch at the posterior margin of the head of the tibia, where it becomes like wise attached to the posterior extremity of the external semilunar cartilage. The crucial liga ments thus derive their name from decussating one another like the strokes of the letter X ; the crossing is, however, considerably above their centre : the anterior passes to the outer side of the posterior.
The synovial capsule-I- entirely surrounds the joint, and there are good reasons for affirming that it is continued in a highly attenuated state over all the interarticular and incrusting carti lages, giving them their smooth and secreting surfaces. (See ARTI C U LA1 ION .) The remain der of its extent may be traced in the following manner : from the upper edge of the patella it ascends behind the common extensor tendon, and is loosely reflected upon the thigh-bone two or three inches above the trochlea in the extended position of the limb; from each side of the patella it passes backwards in a broad sheet, whose lower margin is attached to the edge of the semilunarcartilage, and thence goes to the tibia, while above it is loosely reflected on to the condyles of the os femoris, at the distance of nearly an inch from their cartila ginous surfaces; from the back part of the con dyles these two lateral portions pass into the fossa and join to cover the anterior surface of the crucial ligaments. From the lower edge of the patella the synovial membrane descends to cover the fatty body which is placed in that part of the joint, and it accompanies a small prolongation from that body which frequently passes across the joint to the lowest portion of the trochlea of the os femoris, forming what has been named the mucous ligament; this structure however is not always present. There is some discrepancy in the descrip tions of different anatomists as to the alar ligaments, which are described as folds at the sides of the patella, and it seems altogether unnecessary to distinguish these lateral portions by name from the other parts of the synovial capsule. They are simply folds of the syno vial membrane projecting into the articular cavity, and obviously destined to increase the extent of synovial surface for a greater amount of secretion. This membrane has a dense cel lular tissue on its outer surface, by which it is connected firmly to the posterior surface of the extensor tendons and fascia lata. It possesses some degree of elasticity, but its chief power of accommodation to the motions of the joint is derived- from its lax connection with sur rounding parts.