DISEASES OF THE HOCK. 311 



DISEASES OF THE LIGAMENTOUS AND TENDINOUS STRUCTURES OF 

 THE HOCK. 



Thorough-pin is a bursal enlargement situated on the inferior 

 lateral aspect of the thigh and upper and posterior part of the 

 hock, arising from disease of the tendon of the flexor pedis per- 

 forans muscle, which is enclosed in a synovial sheath, on the 

 inner side of the os calcis, or from dropsy of the sheath, without 

 disease of the tendon. The fluid which fills it may, by pressure, 

 be forced from one side to the other ; hence the term thorough- 

 pin, or running through from side to side. This may be 

 described as true thorough-pin, in contradistinction to that 

 associated with very large bog-spavins. 



A thorough-pin, however large, does not cause bog-spavin, as 

 there is no real channel of communication between the true hock- 

 joint and the bursa of the perforans tendon ; but the capsule of 

 the joint swells upwards and backwards, bulges into the bursa 

 of the tendon, and a large bog-spavin may thus cause the dis- 

 tension of the bursa, and the appearance of thorough-pin. 



Thorough-pins are generally found in short, fleshy, upright 

 hocks, where the os calcis is short and ill developed. We can 

 account for such hocks being subject to them by the knowledge 

 that the lever of the limb being a short one, more strain is 

 thrown upon the flexor tendons, the flexors of the foot being 

 extensors of the hock. 



Fig. 54.— Spring truss for thorough-pin and bog-spavin, designed 

 by Mr. Broad, Bath. 



Eailway shunt horses are very liable to fall unsound from 

 thorough-pin, in consequence of the very heavy truck loads they 

 have to start. 



In the treatment of thorough-pins and bog-spavins, rest, the 



