104 THE SURGICAL ANATOMY OF THE HORSE 



the projecting portion of the inner lip and gains its upper surface 

 which is almost at right angles to the vertical surface. 



As the patella ascends, the tension on the internal lateral patellar 

 ligament progressively increases until the patella reaches the line of 

 division between what we may appropriately term the vertical and 

 superior surfaces of the inner trochlear ridge. This ligament, therefore, 

 tends to restrain the extensor muscles from pulling the patella above 

 the angle where these two surfaces meet. When the ligament is over- 

 powered the patella passes over this angle and gains the upper surface 

 of the ridge, when the tension on the ligament named is suddenly 

 relaxed. The consequence is that the patella becomes fixed on the 

 superior surface of the inner lip for a period of time which varies 

 considerably in duration, and it is only by violent exertion of the 

 muscles which flex the stifle that it can be replaced on the front of 

 the trochlea again. 



This condition is frequently termed Subluxation of the Patella and it 

 is characterised by the peculiar snatchy flexion of the joint each time 

 the patella passes on to the front of the trochlea, a symptom which is 

 not uncommon. Owing to the fact that the numerous muscles which 

 play over the stifle, e.g., flexor metatarsi, gastrocnemius, extensor pedis 

 and flexor perforatus, act also on the other joints, we find that in any case 

 of interference with the movements of the patella, the remaining joints 

 of the limb below the stifle become afl^ected and lameness results. 



The condition just described occurs most frequently in loose-jointed, 

 weak young animals owing to the relaxation of the parts surrounding 

 the joints generally, and in animals in poor condition owing to the 

 diminution in the quantity of fat, which forms a kind of packing for the 

 ligaments of the patella, and which is present in well-nourished animals. 

 Owing to the increase in the length of muscles when their bulk diminishes 

 through inactivity, we also find it a common sequel to severe attacks of 

 depressing diseases such as influenza. 



In young animals and those afi?"ected during convalescence it is 



