LAMENESS IN HORSES. 
131 
plains why the swellings disappear at the time the hock is flexed, 
and re-appear the moment the act of setting the foot down upon the 
ground causes extension of the joint. Flexion occasions tensions 
of the capsule and pressure of the fluid (synovia) into the interior 
of the joint; extension, on the contrary, relaxes the capsular liga¬ 
ment, while the reflux of the fluid into it occasions the bulging ; 
the bulging taking place at the sides for the reason of there being 
thereabouts no ligaments or tendons to oppose the protrusion. We 
now perceive the reason, too, why thorough-pin has no existence 
independently of bog spavin. Both anormalities consisting in 
distention of the hock-joint with synovia, the pressure of the fluid 
being greatest below, and the capsular ligament being least sup¬ 
ported on the inner side, the bulging will take place there—in the 
site of bog spavin—the first; and when that part has become so 
distended that resistance begins to be set up—from the skin, and 
perhaps the vein, as well as the capsule itself—then does the fluid 
(supposing accumulation still to be going on) make its way into 
the upper compartment of the joint, and produce thorough-pin. 
Consequently, in the normal or ordinary state of parts, thorough- 
pin must for its presence be completely dependent upon bog spavin, 
though bog spavin does not necessarily entail thorough-pin. 
Although we feel no apprehension about this account being per¬ 
fectly intelligible to persons in the profession who are acquainted 
with the structure of the hock-joint, yet we are apprehensive it 
may turn out in some respects not to be altogether so to persons 
out of the profession ; to whom all that we can, by way of solution, 
recommend is, a view of the parts themselves, either in skeleton 
or preparation. The fluctuation felt in either tumour while the 
one opposite is being tapped with the fingers is now likewise per¬ 
fectly comprehensible. There is evident communication between 
the swellings, and this we now know to be through the interven¬ 
tion of the hock-joint. 
Thorough-pins, chronic in their nature, existent in hocks under¬ 
going, or that have undergone, great and continued stress from 
work, with time experience changes, which, if not alike in degree 
or intensity, are similar to those we have detected in bog-spavin. 
Old thorough-pins, under circumstances stated, lose their pliancy of 
feel, their elasticity and their fluctuation; they acquire a substan¬ 
tiality of feel they never possessed before, and are evidently suf¬ 
fering from inward deposition approaching to consolidation. To 
what extent such chang s of structure have gone, or may go, we 
must ask those who have had opportunities of dissecting thorough- 
pinned hocks advanced in disease to kindly inform us. 
Of Lameness from pure Thorough-pin we know of no 
example on record ; 
Treatment, therefore, will hardly be called for. 
