320 



CHAPTER XV. 



Synovial Enlaegbments. 



general rbmaeks windqalls of the fetlock bog spavin wind- 

 gall below the hock thoroughpin windgall op the knee 



capped knee capped withers synovial capped hook. 



General Remarks. 



ANATOMY. — ^In order to diminish friction between certain structures, 

 closed membranous sacs, containing an oily fluid called synovia (joint oil), 

 are placed between them. Prom mjury, and sometimes, apparently from 

 mere weakness, there may ensue an effusion of serum into'one or more of 

 these sacs. Thus we have in bog spavin an enlargement of the synovial 

 membrane which lines the capsular ligament of the true hock joint ; in 

 capped knee, we find distension of the synovial bursse which, facilitate the 

 movement of the extensor tendon over the front of the knee: and in 

 thoToughpin, there is distension of the synovial sheath which allows the 

 perforans tendon to glide smoothly over the os calcis (Figs. 98 and 130). 

 These instances afford us examples of the three actions respectively per- 

 formed by the three classes of synovial membranes, viz. : to line the 

 capsular ligaments of joints ; to allow one surface to glide over another ; 

 and to line a canal or sheath through which a tendon passes. If the 

 effusion takes place into the joint, thereby causing distension of the capsular 

 ligament, we have " dropsy of the joint." When one of the other kinds of 

 synovial sacs becomes thus affected, the condition is termed " dropsy of the 

 bursa." 



With the exception of capped knee, which is the result of 

 injuries, these enlargements are, as a rule, signs of work in old 

 horses, and of weakness in young ones. In the great majority of 

 . cases, the superabundant fluid remains unchanged, and the animal 

 suffers little or no inconvenience from its accumulation. Heat 

 and hardness are the two chief signs by which the fact is recog- 

 nised that an enlargement of this kind interferes with the sound- 

 ness of the animal. As long as it remains cool and soft, its 

 existence will, as a rule, be of but little detriment. The hardness, 

 owing to the enlargement becoming converted into fibrous tissue 

 or bone, may remain after all heat has disappeared, in which 

 case, the presence of the deposit may interfere with the action of 

 joint, tendon, or ligament. Although synovial enlargements may 



