154 Anatomy of Skeleton 



comes in contact with the area b and is attached to it. Trace c round the bone : it is 

 found to be continuous with the narrow strips d, d along the edges, seen on the dorsal 

 aspect. These in their turn are continued into the wider area at the upper end of the 

 dorsal surface, which can be subdivided as a rule without much difficulty into 

 an upper e and a lower /. The strips d, d are for the Vasti, e for the Crureus, and / for 

 the Rectus femoris : between / and the apex the bone is subcutaneous, although 

 it is covered by a thick fibrous periosteum continuous with the aponeurosis, and 

 the upper end of the bursa on the patellar tendon may lie on the lower part of this 

 subcutaneous area. 



Fig. 127 represents a vertical section through the middle of the right knee in which 

 the cavity has been partly filled with gelatine. The infrapatellar pad is well seen : 

 observe that it not only has a tongue-like projection backward over the tibia, but also 

 overlaps the lower part of the articular surface of the patella. There is another similar 

 but much smaller overlapping fold above the bone, but it is not well shown in the 

 section. Notice also the appearance of the areas just dealt with when they are seen 

 on section ; they are lettered to correspond with Fig. 126. 



The whole circumference of the patella is covered along its articular margins by 

 overlapping fatty synovial folds similar to that seen in Fig. 127. The folds are smallest 

 along the upper edge, but well developed along the remainder of the margin. Along 

 the inner and lower edges there are definite areas where the articular cartilage is in 

 contact with these synovial folds : the areas are shown in Fig. 126. These fatty folds 

 are continuous below and laterally with the ligamenta mucosum et alaria respec- 

 tively : the former ligament extends back from the lower end of the articular surface 

 of the patella to the intercondylic notch of the femur. 



In the early condition the ligamentum mucosum forms a complete septum below 

 the level of the patella between the two halves of the lower (condylar) parts of the joint ; 

 the septum is generally broken through at birth, leaving the upper part only as a 

 band connecting patella and femur. Fig. 124 is a drawing of a case in which the septum 

 remains complete. The ligamenta alaria are remains of the former division 

 between the condylar and trochlear parts of the articulation. 



The patella is held in position partly as a result of the tension of the Quadriceps 

 muscles retaining it in its groove, and partly owing to the attachments of the aponeu- 

 roses that lie beside it and fasten it to the tissues in the neighbourhood of the lateral 

 ligaments : some fibres are described as lying more transversely deep to the aponeu- 

 rotic sheets and attached to the femur, known as the " retinacula patellae." When 

 the knee is flexed the tension of the muscles and patellar tendon keep the bone firmly 

 against the trochlear surface of the femur, and its articular surface rests on the femur 

 by its upper areas when the knee is fully flexed, whereas its lower areas only are in 

 contact with that bone when the joint is extended. 



When the limb is extended the upper edge of the bone is about two or three 

 fingers' breadths below the upper limit of the synovial cavity, and its lower end 

 is a little above the upper surface of the tibia. The synovial membrane is 

 reflected backwards and downwards from this end to the tibia. Its lower limit 

 therefore lies somewhat below the lower end of the patella, perhaps a finger's 

 breadth below it. 



Development. 



The patella is preformed in cartilage that is apparent in the third month. It is 

 cartilaginous at birth, and a bony centre appears in the third year and extends slowly, 

 completing the bone about puberty. 



