352 HEALTH AND DISEASE 



Causes. The early occurrence of the displacement of this bone in 

 foals, and its persistence, in the absence of any obvious cause, led the late 

 Professor Varnell to attribute such cases to a congenital smallness or want 

 of development of the outer ridge referred to above, which permits the 

 bone in certain movements to be displaced outwardly by the pull of the 

 biceps femoris. 



Post-mortem examinations on animals so affected have convinced the 

 writer of the accuracy of this conclusion, and further that, associated with 

 this deficiency of development of the articulation, there is also a corre- 

 sponding attenuation and weakness of its connecting ligaments, which 

 favours the displacement of the bone. 



General weakness is no doubt a cause of luxation of the patella, as 

 evidenced by its frequent occurrence in poor, weakly, overgrown foals, and 

 after such debilitating diseases as influenza and strangles. 



The displacement usually occurs in an upward and outward direction, 

 and although it is impossible to say precisely and fully how this comes 

 to pass, there can be no doubt that it is in great measure determined by 

 the smallness of the outer ridge of the trochlea of the thigh-bone, and the 

 outward pull of the biceps femoris muscle. 



As a result of violence, it is seen to follow upon some extraordinary 

 twist of the limb in the act of slipping, jumping, or rearing. 



Symptoms. These will vary somewhat with the nature of the case. 

 When the mishap is the effect of weakness the displacement is usually 

 sudden and transient, coming and going without any apparent reason, and 

 being repeated at varying intervals again and again, each time passing 

 away without assistance. The affected limb is noticed to be suddenly 

 jerked backward and held for a moment in an extended position, and 

 then to resume its place again. This may be confined to one leg, but it 

 is commonly the case that both are more or less affected. Here the dis- 

 placement is only partial, and although it may be repeated from time to 

 time for some days or weeks, it ultimately passes away without assistance 

 as the animal acquires strength. 



In more complete displacement the leg is fully extended backward 

 (fig. 379) and cannot be advanced, and the animal stands in this position 

 helpless and immovable. The displaced bone may be felt on the upper 

 and outer side of the joint. 



Treatment. Replacement of the bone may sometimes be caused to 

 take place by a mere cut of the whip. For the most part, however, it is 

 difficult to effect in cases of forcible displacement. Before attempting to 

 reduce the dislocation the extended limb must be brought forward by 

 means of a rope attached to the pastern, and then passed through a collar 



