160 Anatomy of Skeleton 



(Fig. 127), so that it prevents over-extension,* also when it is tight there is only possible 

 a very slight rotation outwards of the tibia on the attachment of the ligament, helped 

 by the shape of the articular surface of the femur, so that the presence of this band 

 is probably primarily responsible for the terminal " locking " rotation of complete 

 extension. 



The posterior crucial is made tense by movement backwards of the tibia, but it is 

 doubtful if it limits flexion : probably it holds the bones together, while the smaller 

 curve of the back part of the condyles enables flexion to take place without increasing 

 its tension. 



The back of the condylar cavities is covered in, between the crucial region and the 

 lateral ligaments, by vertical capsular fibres which afford extensive origin to Gastro- 

 cnemius and Plantaris (Fig. 120). The same figure shows how the upward expansion 

 (ligam. posticum) from Semimembranosus covers in the crucial ligaments behind, pass- 

 ing to the inter-condylar ridge and to the outer condylar capsule. All these fibres 

 when tense will limit extension. 



FIG. 131. Right knee and upper end of tibia. Top left figure is a diagram of the synovial sac from the front, 

 showing the trochlear part of the sac continued below each condyle into the condylar recesses, which 

 are partly subdivided by the projection into them of the wedge-like semilunar cartilages. The synovial 

 sac does not come down lower than the cleft between femur and tibia, whereas the " capsule," shown 

 by the black lines, is brought down on the tibia in front to take in the tubercle and to be attached along 

 the ligamentum patellae. Thus the area A is included on the front of the bone between the sac and the 

 capsule, behind the patellar tendon, the interval being filled by the infrapatellar pad and bursa (see 

 Fig. 127). The front and side parts of the capsule are made by the aponeuroses of Vasti and Crureus, 

 extending on each side to the lateral ligaments, and the deep fascia is blended with their superficial 

 surfaces. Thus on the tibia, in the figure next below, the capsular attachment is represented by the 

 Vasti, the lig. patellae, and the ilio-tibial band, while the infrapatellar bursa lies in position deep to 

 these. On the right, the back of the joint is seen ; the tendon of Semimembranosus has been cut and 

 turned back and its expansion (see Fig. 120) removed to show the crucial ligaments which occupy the 

 notch between the condylar recesses and capsules ; the position of the tendon and expansion are 

 indicated by interrupted lines. The opening for Popliteus exposes the outer meniscus, and the bursa 

 round the tendon is carried on a cartilage-covered area on the tibia ; the muscle itself is not shown, 

 but the expansion over it from Semimembranosus is seen. Below this the areas occupied by these 

 several structures are shown on the tibia ; notice that, internal to insertion of Popliteus, fibres of the 

 expansion from Semimembranosus reach the bone and roughen it, the upper ones passing under the 

 ligament and the lower ones joining it. The capsule round the fibular facet is weak above, where the 

 bursa may perforate it. The epiphysial line is shown as a thin green line in the middle figures. The 

 lowest figures show the upper tibial surface. On the right the crucial ligs. and semilunar cartilages 

 are in situ, cut away from the femur. Each cartilage lies on the tibia, the infrachondral part of the 

 cavity being between them and the bone ; their thick margins are attached by coronary fibres to the 

 bone, as shown in the middle figures, such fibres being, of course, blended with those of the condylar 

 capsule behind. The free edges of the menisci rest on the tibia, their position being indicated by the 

 interrupted lines in the left-hand figure, in which their cornual attachments and those of the crucial 

 ligs. are also shown. ISC, ESC, inner and outer semilunar cartilages. At X a part of the supra- 

 chondral cavity reaches the bone between the post, crucial and internal posterior cornu. 



The Ligamentum patellae is really the tendon of the Quadriceps, and extends from 

 the patella to the tibial tubercle : it therefore comes down to a much lower level than 

 the synovial membrane which passes directly from the patella to the top of the tibia. 

 Thus between this level and the tubercle the tendon lies in front of the triangular area 

 here on the tibia, and it is separated from this by the infrapatellar pad of fat and bursa. 



The tendon has the aponeurotic expansions of the Vasti inserted into it and 

 surrounding it, so that it and the patella are held more firmly by this means in their 

 central position, and possibly on account of this the tendon may be considered to have 

 some value as a means of securing the bones during movement of the joint : otherwise 

 it is difficult to imagine that it could exercise any restraining influence on lateral 

 movements or indeed on antero-posterior movements of an abnormal sort. 



Speaking generally, the articular surface on the femur (for the tibia) shows a 

 decreasing curve as it is followed forward : that is, there is a segment of a small circle 



* In cases of genu recurvatum the ligament is stretched or wanting. 



