220 
OBSERVATIONS ON THOROUGH-PIN. 
are predisposed to those diseases. Mr. Percivall has correctly 
stated, that “ straight hocks are more liable to bog-spavin and 
thorough-pin than those of an opposite formation.” But why are 
upright hocks predisposed to the above diseased collections'! I con¬ 
ceive that the “straight hock,” from its want of angularity, pre¬ 
vents, to a certain degree, the joint possessing the same amount of 
elasticity which is present in well-formed tarsi; hence, concussion 
is apt to occur in the hock joint, and a consequence of the bad 
effects produced by that concussion is bog-spavin. Again, in the 
upright hock, the tendon of the flexor pedis has to perform its func¬ 
tions under a disadvantage, attributable to a want of leverage, 
which the tendon has afforded it in the angular tarsus; and from 
this circumstance there is a liability to disease in that tendon. 
When the bursa of the flexor pedis is over-distended, the tu¬ 
mour is chiefly, at all events primarily, evidenced on the inner 
side; and the tumefaction may be traced easily for some distance 
along the course of the tendon. In some cases it may appear as 
much on the outer as on the internal side of the hock, from the 
fact of the distended bursa gaining room there also to lodge itself. 
Providing there is no disease of the hock joint, there will be no 
bog-spavin; and how does this fact arise? Because the pad of 
cartilage (already referred to) in that situation will not yield or 
give to the pressure, and consequently has not to impart it to the 
capsular ligament of the hock joint, with which it is so intimately 
connected: and, supposing this pad to be absent, it is very unlikely 
that bog-spavin would be produced from pressure by the gorged 
theca; for the adaptation of the bones forming the hock joint im¬ 
mediately in front of the bursa would not admit of it. 
Again, in thorough-pin, arising from diseased tarsal joint (as I 
above stated) bog-spavin is certain to be also there; and the tumour 
at the posterior of the hock is more circular in form than in the 
other case: it is most evident on the outside, from its having less 
resistance there; the swelling on the inner side is of a more definite 
character. But it may be asked, how can the tumour present it¬ 
self on the inside, when the pad of cartilage must be there 1 In 
this instance the cartilage allows of this, in a manner precisely ana¬ 
logous to that in which a valve permits water to run in one direc¬ 
tion, but obstructs the passage of the fluid if it attempts to flow in 
an opposite course. I stated that the cartilage was laid on the ex¬ 
ternal part of the bones; hence, if pressure comes from without (as 
in disease of the flexor pedis), the cartilage does not bulge; on the 
contrary, if pressure is imparted to it from within (as in bog-spa¬ 
vin), it is able to recede, at all events so to alter its position, thaC 
thorough-pin is produced. 
It may be as well to remark at this part of the subject, that in 
