PRACTICAL CONSIDERATIONS : THE KNEE-JOINT. 409 



forward, the outer part of the external swings back. This motion occurs below the 

 external semilunar cartilage. Flexion begins with a corresponding inverse rotation 

 of the tibia. While the knee is straight the tibia is hrmly fixed, so that in rotation 

 of the limb at the hip the bones move as one. The long lateral ligament and that 

 part of the capsule called the internal ligament are placed so far back that they are 

 relaxed in flexion but become tense in extension. Both the crucial ligaments 'are 

 always nearly tense, especially the posterior. The anterior is quite tense in exten- 

 sion, the posterior in flexion. The latter prevents forward displacement of the 

 femur on the tibia when, as in alighting from a leap, the whole weight is carried for- 

 ward by the impetus, the knee being flexed. Another rotation on a vertical axis 

 through the middle of the joint may occur when the knee is flexed. The motion 

 is between the femur and the internal semilunar cartilage, and both above and below 

 the external one. This motion is chiefiy passive, — i.e., imparted by another person 

 twisting the leg when the muscles are relaxed. It probably, however, can be exe- 

 cuted actively to some extent. It is very slight in less than semiflexion of the knee, 

 and diminishes as flexion becomes more extreme. The precise angle at which it is 

 greatest seems uncertain. Rotation of the tibia outward, tending to untwist the 

 crucial ligaments, is resisted by neither, but by the internal lateral ligament. Rota- 

 tion inward is resisted by both crucials, especially the anterior, and by the external 

 lateral. The posterior ligament is made tense in life in positions in which it would 

 otherwise be lax by the action of the semimembranosus. It is tense in extension. The 

 front part of the capsule is tense in flexion and relaxed in extension, but its condi- 

 tion in the latter state is considerably modified by the degree of contraction of the 

 quadriceps extensor. 



Movements of the Patella. — The patella in the upright position, when the 

 muscles are relaxed, has the lower part of the articular surface resting against the 

 top of that of the femur. When the muscle is contracted the former is drawn 

 entirely above the latter. As flexion begins the lower zone of the articular surface 

 fits into the groove on the femur, the two upper and the internal strip not being in 

 contact with it. In semiflexion the knee-pan has passed below the patellar surface 

 of the femur, and the middle zone rests on the front of the outer condyle and on a 

 small part of the inner. As flexion becomes extreme the patella follows the outer 

 condyle, resting on its under side by its superior zone, the convex portion is in the 

 notch, and only the strip along the inner edge is in contact with the outer side of the 

 internal condyle. In the latter part of the movement the mucous ligament becomes 

 tense, and through it, and still more by atmospheric pressure, the alar ligaments are 

 brought close in to fill the chink between the femur and the tibia. 



PRACTICAL CONSIDERATIONS. 



The Knee-joint. — The anatomical conditions which should render the knee- 

 joint peculiarly subject to dislocation are as follows : i. Its situation between the 

 longest bones of the skeleton and its consequent exposure to tremendous leverage. 

 2. Its similar exposure to frequent strain and traumatism. 3. The extensive and 

 varied character of its movements. 4. The absence of bony prominences, which 

 could effectively strengthen the joint, upon either the articular surface of the lower 

 f end of the femur or the shallow upper surface of the tibial tuberosities. 



The ability of the joint to resist dislocation, which is of very rare occurrence, 

 lies in («) the strength of the ligaments, especially the crucial : {b) the expansions 

 of the quadriceps tendon on the front of the joint ; {c) the reinforcement of the 

 posterior ligament by the semimembranosus tendon ; (a?) the similar relation of the 

 internal lateral ligament to the semimembranosus, and of the external lateral to the 

 tendons ot the biceps and popliteus ; {e) the power thus conferred upon strong 

 muscles to meet and modify or resist sudden strains by varying the tension of the 

 capsule and even of the ligaments ; {f) the deepening of the tibial cup by the seftii- 

 lunar cartilages, and the adaptation of the latter to the varying positions of the bones 

 so that the contact between and pressure upon the joint-surfaces are as extensive and 

 as uniform as the shape of the condyles will permit. 



Dislocations of the knee may be antero-posterior or lateral in direction. The 



