spavin. 997 



starting from the inner side, runs backwards and outwards. As 

 a consequence, the two tibiae are not parallel, but converge from 

 above downwards, whilst the metatarsal bones of the two limbs 

 are parallel with each other when the limbs are in their normal position. 

 It therefore results that the power does not act perpendicularly to 

 the articular axis of the astragalus, and consequently, when weight 

 is placed on the limb, the muscles do not simply extend the joint, 

 but also tend to rotate it. This arrangement certainly assists the 

 transference of the body-weight to the opposite limb and ensures 

 stability ; but the consequent rotary movement of the limb 

 necessarily involves displacement of the bones of the hock-joint, 

 and thus favours strain of the ligaments and disease of the joints. 

 The tibio-tarsal joint is protected against injurious action of this 

 kind by its strong lateral ligaments, and by the prominences on the 

 astragalus ; whilst the inner portion of the lower joint, which can 

 easily be shown by anatomical preparations to possess the least 

 power of resistance to such rotary movements, has therefore to take 

 the chief share in them and accordingly is most exposed to strain 

 of its lateral ligaments and fibrous capsule. Irritation and 

 inflammation are thus caused in the neighbouring periosteum, and 

 produce disease of the articular cartilage. Depending on whether 

 the cartilage and synovial membrane become affected early or late, 

 lameness is either present from the first, or only occurs when new 

 bony growths have formed outside the joint. This, again, depends 

 principally on whether strain is severe from the first, or whether 

 it is only gradually exerted. 



This theory seems to explain in a satisfactory way both the manner 

 of origin and many of the symptoms of spavin. Anything which 

 impedes this physiological rotation of the hock-joint, such as working 

 in heavy ground or on rough pavements, must favour the production 

 of disease. It seems tolerably certain that spavin cannot be referred 

 to any single cause, but that the initial disease may start in one of 

 several structures ; and therefore, while the formation of the joint 

 on which Peters thus lays stress is probably one of the most important 

 factors, it would be unwise to deny the possibility of the process 

 originating in disease of the bursa of the flexor metatarsi (Dieckerhoff), 

 or even on the surface of the joint. Moller's and Petit's investi- 

 gations, however, showed the bursa to be frequently intact, even 

 when disease of the joint was advanced. Nor does co-existent disease 

 of the bursa necessarily prove the latter to have been the point of 

 origin, for in many cases it becomes affected secondarily, by 

 inflammation extending to it from the joint. 



