OBSERVATIONS ON THOROUGH-PIN. 
219 
invariably present, which commences a little above the protuber¬ 
ance of the tibia, investing the latter, and passes down over a 
portion of the capsular ligament of the hock-joint, which lies 
directly in front of and a little below it; and the cartilage is dis¬ 
continued when it has partly lined the groove on the inner side of 
the calcis. The bursa of the tendon is reflected over the surface 
of the cartilage, which lies in connexion with the tendinous rope. 
This bursa is a bursa of itself, and not at all connected with the 
true hock-joint. 
Notwithstanding all these admirable adaptations for the preven¬ 
tion of disease in this locality, yet practical experience has clearly 
proved that these beautifully arranged structures are unable to 
compete with the stress entailed upon them in the domesticated 
horse from undue exertion. An increased secretion of synovia is 
often effused in this bursa, either from injury inflicted on the 
bursal membrane, or from the tendon itself becoming strained; 
and in proportion to the extent of the injury, so do we find the 
amount of distention of the bursa, both as regards size and extent. 
Hence is produced that anormal condition, known, I feel con¬ 
vinced, by the profession generally, under the name of thorough- 
pin. But this diseased state Mr. Percivall seems not to have the 
slightest knowledge of, inasmuch that he never hints in the least 
about it. In some cases, when thorough-pin arises from this 
cause, the tumour is circumscribed; though in the generality 
of cases it is more lengthened in form, taking its course in the 
direction of the tendon. 
It will be now perceived, that I believe thorough-pin exists 
under two different diseased states, two different structures being 
involved; the one from disease of the hock-joint, the other from 
unnatural distention of the bursa of the flexor pedis; both these 
synovial sacs being distinct cavities, in a healthy condition of the 
parts: nor did I ever see or hear of there being any communication 
between them in the worst cases of disease, though such a thing is 
possible, but not at all probable. 
Allowing, then, that thorough-pin may be dependent on two 
different causes, a question now arises, Is it possible to detect on 
which of these two causes an existing thorough-pin depends! I 
answer, Yes: a correct diagnosis may be adduced, by remember¬ 
ing the anatomy of the parts implicated. 
If thorough-pin is caused by distention of the capsular liga¬ 
ment of the hock joint, bog-spavin is invariably a concomitant 
symptom. On the other hand, in the case of thorough-pin arising 
from an unnatural distention of the bursa of the flexor pedis, it 
does not necessarily follow that bog-spavin must simultaneously 
exist. But there is no doubt that the same peculiar kind of hocks 
