724 
DISEASES OF THE HOCK. 
peipendicularly 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 oil 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 accord¬ 
ingly is most exposed to strain of its lateral ligaments and fibrous 
capsule. Irritation and inflammation are thus caused in the neighbour¬ 
ing peiiosteum, and produce disease of the articular cartilage. Depend¬ 
ing 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 
m 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 (Dieckerlioff), or even on & the 
surface of the joint. Moller’s and Petit’s investigations, however, showed 
t le 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. 
. Symptoms ' (1) Spavin lameness. The diagnosis of arthritis chronica 
in small, “ clean” hocks offers no difficulty, provided disease processes 
have extended beyond the joint, and exostoses exist. Until they develop 
however, diagnosis remains uncertain, for the lameness is not sufficiently 
characteristic to form the basis of a decided opinion. It is just on this 
account that errors so frequently occur, and that other lamenesses even 
foot-lameness, are mistaken for spavin. But other injuries to the’hock, 
