994 DISEASES OF THE HOCK. 



from the articular surface to the periosteum or fibrous covering of 

 the free margins of the bones. 



(3) Changes in the ligaments and synovial membrane. Both the 

 villi and borders of the synovial membrane seem thickened and very 

 vascular. Sometimes the secretion of synovia is excessive. The 

 ligaments of the joint, and the connective tissue surrounding it, 

 are also thickened. 



(4) In advanced stages of the disease, the smaller joints may be 

 obliterated, especially those between the scaphoid and cuneiform 

 bones, and between the cuneiform and metatarsus. This anchylosis 

 may either be central or peripheral. The ligaments are then often 

 ossified, and so surrounded by new growths of bone that they can 

 scarcely be recognised. Sometimes the cunean sheath of the flexor 

 metatarsi tendon becomes diseased. Petit states never having 

 found this part of the hock -joint diseased even when extensive exostoses 

 existed. 



In many cases pathological processes are confined to the lower 

 and inner portions of the hock, though not infrequently they extend 

 further, invading the cuneiform bone, the inner small metatarsal, 

 and even the astragalus, cuboid and external surface of the joint. 

 French observers, therefore, distinguished " eparvin tarso- 

 metatarsien," as opposed to the disease of the cuneiform bones and 

 metatarsus, which is termed " eparvin metatarsien." 



The appearance of the above-described changes, either alone 

 or in combination, is easily explicable on the theory that the point 

 of origin of the disease varies. Menveux made a collection of hocks, 

 showing widely varying anatomical changes, from horses, all of 

 which had clinically been recognised as suffering from " spavin." 



Causes. In no other disease is the division of causes into causa 

 externa and causa interna so well warranted and so practical as in 

 spavin. The causa interna is a predisposition partly dependent 

 on the conformation of the hock, partly on that of other portions 

 of the body. Whilst the complicated mechanism of the hock pre- 

 disposes to disease, and especially to inflammation, this tendency 

 is enormously increased by defective formation of the joint. It is 

 scarcely needful to say that, under the powerful action of the muscles 

 of the hind limb, small, cramped joints are more likely to suffer than 

 those having well-developed, broad and ample articular surfaces. 

 Defective development of the lower portions of the hock and of the 

 upper end of the metatarsus, the condition described by horsemen 

 as " tied-in hock," is particularly disastrous, while a distinct curvature 

 (sickle-shaped hock) is almost as bad. In this formation the action 



