152 



Anatomy of Skeleton 



The time of appearance of the centre for the lower end seems to be somewhat 

 variable : as a rule it appears shortly before birth, but in a small number of cases it 

 may be found as early as the eighth month, and in others, very rarely, it may be 

 delayed even as long as the second year after birth. 



PATELLA, 



A bone situated in front of the knee joint, somewhat triangular in shape, with 

 rounded margins and its apex pointing downwards. It may be looked on as a large 

 sesamoid developed in the tendon of the Quadriceps, receiving the insertion of these 

 muscles on its upper margin and sides and being attached below by the Ligamentum 

 patellae or patellar tendon to the tubercle of the tibia. The aponeurotic fibres of the 

 extensor group in which it exists are in fact continued over its surface into the tendon, 



outer. 



f. 



FIG. 126. Left patella. Back and front views, i, medial vertical facet, usually 

 considered to rest against the condylar trochlear surface in extreme flexion, but 

 certainly covered by fatty synovial pads in large part ; 2, surface overlaid by 

 fatty folds below ; 3, 3, lower facets in contact with femur in extension ; 

 4, 4, upper facets in contact with femur in flexion. Other references given 

 in text. To tell left from right, hold the bone so that the larger articular 

 area is external and behind, with the " apex " down. 



so that it is sheathed in front by fibrous tissue that separates it from subcutaneous 

 tissue and skin, and, in its lower part, from a subcutaneous bursa. 



But its deep or posterior surface forms part of the front wall of the joint cavity, 

 wherefore it carries a cartilage-covered articular surface. This moves up and down 

 on the trochlear surface of the femur as the knee is moved, being always separated 

 from the tibial tubercle by the length of the tendon. 



The front surface of the bone is longitudinally ridged, indicating that the ossifica- 

 tion has involved some of the deeper fibres of the aponeurotic tissue that covers its 

 surface. 



The margins show rough markings of Vasti and Crureus, and the top margin also 

 receives the Rectus in front of Crureus. The outer margin is occasionally the seat of 

 a rough process, constituting an " emarginated " bone, where ossification has extended 

 into the tendon of the Vastus externus. 



The deep surface of the bone is shown in Fig. 126. Observe that the articular 

 area that rests on the outer condyle is larger than the inner one : this, in conjunction 

 with the apex pointing downward, enables one to recognise the side to which the bone 

 belongs. 



The ridge between the two surfaces lies in the hollow of the trochlear surface of 



