128 THE SURGICAL ANATOMY OF THE HORSE 



loosely applied to the back of the joint in order that the bones may 

 move freely. The result is that it readily permits of distension of the 

 membrane posteriorly. This enlargement is frequently referred to as 

 articular thoroughpin. It must not be confused with distension of the 

 sheath of the perforans tendon (tendinous thoroughpin). The enlarge- 

 ment in the latter case is situate more posteriorly and is elongated from 

 above to below. Moreover in these latter cases there will usually 

 appear a bulging at the postero-inferior aspect of the joint at the edge 

 of the perforans tendon, due to distension of the lower end of the tendon 

 sheath. Franck states that there is often a communication between the 

 sheath of the flexor perforans tendon and the synovial capsule of the 

 joint. Chauveau does not mention anv such communication, and 

 Franck's statement is probably due, either to a confusion of the two 

 enlargements, or to cases in which there has been a rupture of the 

 posterior common ligament. This ligament, it will be remembered, 

 separates the two sheaths, and the separation is made more complete by 

 the presence on the back of the ligament of a fibro-cartilaginous 

 thickening over which the perforans tendon plays. 



That the posterior bulging is due to distension of the synovial 

 capsule of the joint may frequently be demonstrated by the application 

 of pressure, when it will diminish in size and bring about a corresponding 

 increase in the size of the anterior enlargement. 



From our anatomical study of the joint it will be readily understood 

 that the condition will be most common in those joints which are 

 upright — i.e., those in which the angle formed by the longitudinal axes 

 of the tibia and large metatarsal bone is the largest. Young animals are 

 more particularly affected owing to the greater elasticity of the 

 anatomical structures of the joint. It is also common in animals in 

 which great strain is thrown upon the hocks, such as entire horses. 



The condition is generally attributed to a chronic inflammation of 

 the synovial membrane as a result of severe strain. The inflammation 

 in some cases is acute, when the enlargement is hot, tense, and painful. 



