160 MUSCULAR SYSTEM. 



dyle. Inferiorly. it is continued over the hock, down the posterior 

 part of the leg, to be fixed to the os coronne. 



Relations. — Posteriorly, with the gastrocnemius externus and 

 skin ; anteriorly, with the stifle joint, the muscles of the posterior 

 deep crural region, tendons of the gastrocnemius externus, and 

 flexor perforans. The first crural nerve runs also along the front 

 side. 



Peculiarities. — Remarkable for the little difference in volume 

 there exists between the fleshy belly and its tendon ; and in 

 being entirely tendinous until it has descended to the stifle. 



Structure.— The belly of this muscle is of considerably less 

 volume than that of the gastrocnemius externus, but it has more 

 tendon in its composition. About half way down the thigh it 

 becomes entirely tendinous ; and its tendon, which is flattened, 

 turns round that of the gastrocnemius externus in approaching 

 the hock : so that the relative position of the two muscles be- 

 comes reversed in their tendons. In passing over the point of 

 the hock the tendon expands into a sort of cap or theca, which 

 is so fixed and closed in around its border, by cellular substance, 

 to the bone, that a complete ball-and-socket joint is formed be- 

 tween them, circumscribed within the space of about an inch from 

 its centre all the way round, and lined by a thin delicate mem- 

 brane secreting synovia into the interspace. It constitutes, in 

 fact, altogether, what is denominated a bursa mucosa ; but is 

 one that, from its size and disposition to take on disease, pecu- 

 liarly merits the attention of the anatomist. From this bursal 

 formation it proceeds down the posterior part of the leg; and 

 from the place at which it leaves the hock takes the name of 

 tendo perforatus. Its subsequent course and attachment, to- 

 gether with its relations to the tendo perforans, answer to the 

 descriptions given of the tendons of the same name in the fore 

 extremity. 



Action. — To extend the hock ; but to flex the fetlock and 

 pastern. 



PLANTARis. — Peroneo-calcaneus. 



Situation. — Postero-external part of the thigh. 



Form. — Elongated ; thin ; slender. 



Jttachment. — Superiorly, to the head of the fibula; inferiorly, 

 (in union with the tendon of the gastrocnemius externus) to the 

 OS calcis. 



Relations. — Externally, with the skin ; internally and poste- 

 riorly, with the gastrocnemius externus; anteriorly, with the 

 deep posterior crural muscles. The third popliteal nerve crosses 

 its superior extremity. 



