82 THE ANATOMY OF THE HORSE. 



FEMOEO-PATELLAR ARTICULATION. 



Movements. — This joint is commonly classified as an arthrodia. The 

 movements (see page 43) of the patella on the trochlea, however, are not 

 those of simple gliding, but of gliding with coaptation. In the latter 

 movement, while the patella moves as a whole upwards or downwards, 

 successive areas of its articular surface come into contact with the 

 trochlea. These movements take place at the same time as the move- 

 ments in the femoro-tibial articulation. In complete extension of that 

 joint the patella lies at the upper part of the trochlea, and the three 

 straight patellar ligaments are tense. When flexion takes place, these 

 ligaments become relaxed, and the patella descends over the trochlea 

 till it rests at its lower part. 



The ligaments of the joint are — one capsular, two lateral, and three 

 straight. 



The Capsular Ligament is loose and membranous, and it supports 

 the synovial membrane. It is attached, on the one hand, to the margin 

 of the patellar articular surface, and, on the other, at the periphery of 

 the trochlea. 



The Lateral Ligaments are two thin, riband-shaped bands, stretching, 

 one on each side of the joint, from the femur to the patella. They serve 

 to strengthen the capsular ligament, from which they are not distinct. 



The Straight Patellar Ligaments. These correspond to the single 

 ligamentum patellce of the human knee. They are three in number, and 

 are distinguished as external, middle, and internal. All three ligaments 

 are attached superiorly to the anterior surface of the patella, the inner 

 one having a fibro-cartilaginous thickening which extends the articular 

 surface of the patella, and glides on the inner ridge of the femoral 

 trochlea. The middle ligament lies on a deeper plane than the other 

 two, and rests inferiorly in the vertical groove on the anterior tuberosity 

 of the tibia. It is inserted into the lower part of this groove, and a 

 small synovial bursa is developed between the ligament and the bone 

 above the point of insertion. The external and internal ligaments are 

 inserted into the same tuberosity, one on each side of the attachment of 

 the middle ligament. These three ligaments may be regarded as the 

 terminal tendon of the quadriceps extensor cruris, whose action they 

 transmit tc the bones of the leg. 



Synovial Membbanb. This will be exposed by incising the capsular 

 ligament. It lines the inner surface of that ligament, and extends 

 upwards beyond the trochlea, forming a protrusion under the quad- 

 riceps extensor cruris. Inferiorly it is in contact with the synovial 

 membranes of the femoro-tibial joint, and sometimes it communicates 

 with them. 



It is a point worthy of notice in connection with the interior of the 

 joint, that the inner ridge of the femoral trochlea is much higher than 



