218 
OBSERVATIONS ON THOROUGH-PIN. 
inner parietes of the hock-joint are defended only by the capsular 
ligament and integument; and from this fact, if a superabundant 
or unnatural increase of synovia is poured out, this part admits to 
a certain extent of the distention of the sac, the lateral and upper 
walls of the joint being unyielding to the pressure. 
On dissecting the hock another part of the joint is also found to 
be enclosed only by its capsular ligament and skin, and this is at 
its posterior boundary, where the inferior extremity of the tibia is 
connected with the trochleated surface of the astragalus. In this 
situation we find the capsular ligament not lying on bone, but 
loose, yet not so loose as at the anterior part of the joint: nor 
does it bag so much as Mr. Percivall seems desirous for us to sup¬ 
pose ; for if it did, the loose portion would become pinched between 
the lower end of the tibia and upper surface of the astragalus, at 
that time when the joint is in a state of extension, and the healthy 
condition of the parts would not last long : in other words, the 
loose portion would become constricted and bruised, as it does in 
cases of relaxation from disease. In this case it is found that tho¬ 
rough-pin is the effect of bog-spavin; or, perhaps, we ought more 
correctly to state, that the latter disease presents itself in the site 
of the former one. It is in such a case that, “ felt on either side, it 
has all the sensible characters of windgall; and the fluid it con¬ 
tains is readily made, by pressure or pulsation with the fingers, to 
fluctuate from one tumour to the other, shewing that free communi¬ 
cation exists between them.” 
Yet, as I have already stated, thorough-pin may exist inde¬ 
pendently of bog-spavin; and how is this “ seeming paradox” to be 
explained 1 Any person who has ever dissected, or who has the 
most vague idea of the anatomy of, the tarsus, must be aware that 
the flexor pedis, as it proceeds from its origin to its insertion, 
passes over a protuberance at the inferior extremity of the tibia, 
at its posterior and towards its inner aspect; also immediately 
afterwards it runs within a groove over the inner side of the calcis. 
In this spot is found a tendon passing at an angle over bones, and, 
wherever this takes place, it is well known that a bursa is 
generally present; at the same time experience has proved that, in 
such situations, disease is very liable to occur. Such is the case 
with the tendon in this particular part; and from the great extent 
of osseous tissue it has to play over, we find it is supplied with 
a bursa of very large dimensions, it being from five to six inches 
in length in an ordinary-sized horse. Nature is not content with 
granting to the tendon in this part this structure to facilitate 
its action—also acting in some measure as a preventive to dis¬ 
ease—but, in order to render less the probability of friction arising 
between the bones and the tendon, a pad of cartilage is found 
