MUSCLES OF THE POSTERIOR LIMBS. 371 



sheath, named the tarsal sheath. It glides in the interior of this canal by 

 means of a very extensive bursa, which extends upwards on the posterior liga- 

 ment of the tibio-tarsal articulation, and is prolonged inferiorly to the middle 

 third of the metatarsus. The tendon of the perforans afterwards descends, 

 vertically, behind the suspensory ligament, receiving from it a strong fibrous 

 band (the suhtarsal ligament), analogous to that of the fore limb, but less 

 voluminous ; it then passes through the annular portion of the perforatus, is 

 inflected with that muscle over the sesamoid groove, glides on the posterior 

 articulating surface of the second phalanx and that on the navicular bone, 

 thinning out into a plantar aponeurosis, which is provided with a phalangeal 

 reinforcing sheath, and finally terminates on the semilunar crest of the os pedis. 

 This tendon, therefore, on leaving the tarsus, comports itself exactly like that of 

 the anterior limb. 



Relations. — Outwards, with the lateral extensor of the phalanges, the soleus, 

 and the tibial aponeurosis. Inwards, with this aponeurosis and the oblique flexor. 

 Behind, with the gastrocnemius, the perforatus, and the fibrous band of the tendon 

 of the hock. In front, with the tibia. 



The tendon glides in the tarsal sheath by means of a very extensive bursa that 

 ascends on the posterior ligament of the tibio-tarsal articulation, behind which it 

 appears as a soft tumour (thoroughpin) when it is distended by synovia (Fig. 

 204, 3). The bursa descends to about the lower third of the metatarsal region. 



Action. — This muscle flexes the phalanges on one another and on the meta- 

 tarsus. It may also extend the foot in pressing, during its contraction, behind 

 the tibio-tarsal articulation. In addition to this, its tendon acts, while the 

 animal is standing, as a mechanical support to the phalanges and the articular 

 angle of the fetlock. 



In the Ass and Mule, the tendon of the perforans does not receive any sub- 

 tarsal ligament, this being absent in them. 



6. Oblique Flexor of the Phalaijges (Flexor Accessorius) 

 (Fig. 203, '22) 



Synonyms. — Peroneo-phalangeus— G/rard. The tibialis posticus of Man. (Small tihio- 

 phalangeua — Leyh.) 



Situation — Direction — A muscle situated behind the tibia, between the pop- 

 liteus and the perforans, in a direction slightly oblique downwards and inwards. 



Form — Structure. — It is composed of a fleshy fusiform body, intersected by 

 numerous fibrous bands, and provided with a funicular tendon inferiorly 



Attcichments. — The superior extremity arises behind the external tuberosity 

 of the tibia — origin. The tendon is united, by its inferior extremity, to that of 

 the perforans towards the upper third of the metatarsal region — termination. 



Relations. — The muscular portion responds : in front, to the perforans, the 

 popliteus, and the posterior tibial artery ; behind, to the gastrocnemius and the 

 perforatus. The tendon, at first lodged in a muscular channel in the perforans 

 and covered by the tibial aponeurosis, afterwards enters a tortuous sheath at the 

 inner side of the tai-sus, and which is formed by the groove that inchnes behind 

 the infero-internal tuberosity of the tibia. 



Action. — It is a congener of the deep flexor. 



