POSTERIOR MEMBER. 271 



serious blemish in pleasure-horses, on account of the deformity which it produces 

 and the tenacity with which it resists the means employed for its resolution. 

 It must not be confounded with the purely accidental oedema of this part in 

 horses standing in the stable for a long time, which disappears promptly under 

 the influence of exercise. 



c. Tendons and Tendinous Synovials. — We will mention, in passing, 

 the rupture of the tendinous portion of the flexor metatarsi, of which we have 

 already spoken in the article on the leg, and which gives rise to a lameness of a 

 special character. 



We will also name an accident, — an exceptional one, indeed, — luxation of the 

 calcanean attachment of the perforatus. In the two known cases, the tendinous 

 portion of this muscle had ruptured its insertioil on this bone from a very 

 violent effort of traction, was dislocated externally to the point of the hock, and 

 produced so visible a deformity that the animal could not have been offered for 

 sale. 



It is the same with the ossification of this calcanean cap, as well as of a 

 portion of the tendon itself, which we have observed, on one occasion only, in an 

 old horse that was sacrificed for dissection, and whose hock we deposited in the 

 Department of Collections at the Alfort School. 



These lesions have no other interest than their rarity. A different case, how- 

 ever, is that of the abnormal dilatations of the synovial sheaths which facilitate 

 the gliding of the tendons in the region of the hock. These are, on the contrary, 

 most common, always very grave (some of them at least), with regard to the de- 

 preciation in the value of the animal. They are known under the generic name 

 of tendinous hygroma or thorough-pin. 



The most serious among them is undoubtedly that of the tarsal sheath, which, 

 for this reason, has received the name of tarsal or tendinous thorough-pin. It is 

 characterized by subcutaneous tumors, which appear at the superior or at the 

 inferior part of the hock, at the points where the synovial membrane has no 

 support. The superior dilatation is situated in the hollow of the hock, immedi- 

 ately in front of the tendo Achillis or cord, and along its course. More salient 

 on the internal side, it is sometimes bilobed, and may reach the inferior fourth of 

 the leg. When it is very voluminous, pressure exercised • upon it will not force 

 the liquid to the anterior face of the hock, because the tarsal sheath does not 

 communicate with the articulation ; but such pressure, being transmitted to the 

 swelling at the inferior part of the hock, renders them more apparent. 



These tumors, less developed than the preceding one, and less distinctly out- 

 lined in consequence of the thickness of the walls of the sheath, follow the 

 course of the flexor tendons to the superior third of the canon. 



"Tarsal thorough-pin," says H. Bouley, " is susceptible of acquiring enormous 

 dimensions, particularly on the internal side. It has been seen to be enlarged 

 in such proportions that the space between the two members was no longer suffi- 

 cient for their normal movements, and the skin over its surface was excoriated 

 from the constant friction against the opposite hock during the movements of 

 locomotion." 



The synovial bursa which facilitates the gliding of the expansion of the 

 perforatus upon the summit of the calcaneus is also capable of becoming dilated 

 to an abnormal degree. Although very strongly attached here, it can yield to 

 the pressure of the liquids from within, forming along the cord of the hock an 



