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412 HUMAN ANATOMY. 



merits then occurring between the tibia and the cartilages — one of them is fixed 

 between the corresponding condyle and the tibia which rotates beneath it ; the 

 remaining cartilage, especially if the rotation is marked, may be dragged or squeezed 

 so that it is nipped between the tibia and femur. Thus the contraction of the biceps 

 which effects outward rotation of the leg brings more closely together the external 

 tuberosity of the tibia and the external condyle, and the outer cartilage is held firmly 

 between them. This increases slightly the distance between the internal condyle and 

 the head of the tibia, leaving the internal cartilage freer to move into an abnormal 

 position. When the popliteus, semitendinosus, and semimembranosus contract to 

 rotate the leg inward, they, in like manner, fix the internal cartilage and allow of 

 increased mobility of the external cartilage. 



Subluxation of the inner cartilage is the more frequent because (i) outward 

 rotation of the leg is far more common than inward rotation ; (2) the muscle chiefly 

 concerned in effecting inward rotation, — the popliteus, — when it contracts, steadies 

 and supports the external cartilage by pressure against its outer margin (Morris) ; 

 no corresponding support is given the internal cartilage during outward rotation ; 

 (3) the anterior crucial ligament is attached somewhat in front of, and often directly 

 to the inner cornu of the external cartilage, tending to limit its forward motion. It 

 is altogether behind the internal cartilage ; (4) the external cartilage has a strong 

 attachment to the femur through the ligament of Wrisberg posteriorly. 



The displacement is forward in the majority of cases. The symptoms are pain, 

 from the pressure on the cartilage itself, increased by, reflex spasm of the muscles 

 moving the joint, and followed by a synovitis. The edge of the cartilage may often 

 be felt. 



Disease of the knee-joint is of great frequency on account of its exposure to (a) 

 direct violence and to cold and wet, by reason of its superficial position, and (/^) to 

 strains and wrenches through the leverage of the femur and tibia. The factors 

 competent to resist luxation are not able to protect it from minor injuries. It is a 

 favorite seat, therefore, of traumatic synovitis, and — on account also of its complexity, 

 its large size, and the difficulty in keeping it at absolute rest — disease, if acute, is 

 apt to be severe and threatening ; if subacute, tends to become chronic or to recur. 

 All the above reasons, combined with its inclusion of the lower femoral epiphysis 

 and its close relation to the upper tibial epiphysis, — the seats of the chief growth of 

 the lower limb, — make it also one of the joints most commonly subject to tuberculous 

 disease, while gout, rheumatism, and syphilitic and gonococcic infection are often 

 localized in it. 



Most of the chronic diseases due to infection, as well as those directly following 

 traumatism, begin in the synovial membrane because of the large superficial expanse 

 of that membrane. The intra-articular effusion — whether "simple," from hyper- 

 semia, or inflammatory, from infection — causes the knee to assume the position of 

 moderate flexion because (i) its capacity is then greater than in full extension or 

 full flexion, and maximum capacity is equivalent to minimum pressure ; (2) flexion 

 relaxes the densest and most resistant ligaments, — the posterior and the lateral (as 

 they are attached behind the centre of the bone) and (if moderate) the posterior 

 crucial. It is resisted only by the ligamentum patellae, which is in less close rela- 

 tion to the joint (being separated by the pad of fat on which it lies), and by the 

 thinner and more extensible anterior portion of the capsule ; (3) the joint is inner- 

 vated in accordance with the general law that the same nerves which supply the 

 interior of an articulation supply also both the muscles moving it and the skin over 

 the insertion of those muscles (Hilton). The knee-joint is acted on by ten muscles, 

 four of which are extensors and six flexors. The latter are not only numerically in 

 excess, but are also the more powerful and the more favorably situated for acting 

 upon the joint. Therefore, when the articular twigs of the obturator, sciatic, and 

 anterior crural nerves are irritated by disease, and both the anterior and posterior 

 groups of muscles contract reflexly, the flexors predominate. The principle is of 

 wide-spread application, and should be considered in reference to the position of 

 most joints, at least in the early stages of disease. « 



Later in knee-joint disease the softening and elongation of the ligaments permit 

 the pull of the flexors to produce posterior displacement of the bones of the leg 



