32 THE SURGICAL ANATOMY OF THE HORSE 



A large muscle is applied to the posterior surface of the tibia. This 

 is the muscular portion of the deep flexor of the digit (flexor perforans) 

 which arises from the roughened ridges found on the back of this 

 bone. 



Inwardly the muscle is subcutaneous, whilst outwardly it comes 

 into contact with the peroneus. On the posterior aspect of this 

 muscle and towards the inner side of the median line is a depression, 

 which accommodates the flexor accessorius muscle, a section of 

 which approaches the elliptical in outline. Behind the outer third 

 of the flexor accessorius muscle a large vein will be observed. 

 This is the upward continuation of the external metatarsal vein, 

 which, having passed through the tarsal sheath, forms the posterior 

 root of the internal saphena vein. Slightly more deeply seated 

 than this vein, and a short distance behind it, is the posterior tibial 

 artery, which has emerged from beneath the flexor accessorius 

 muscle a little higher up the limb. The artery is accompanied by 

 two small veins. 



The posterior tibial nerve will be found behind the posterior tibial 

 artery. The two structures are well separated from one another by a 

 quantity of fatty areolar tissue. It will be observed that it is a very 

 large nerve, and that it is placed in a slightly more superficial plane 

 than the artery. 



On the outer aspect of the limb, and almost in a direct line with 

 the posterior tibial nerve, is the external saphena nerve. This is a 

 small nerve, and it occupies a very superficial position. In front of 

 the external saphena nerve is the vein of the same name. 



The posterior tibial nerve will be observed to lie in front of a large 

 tendon. This is the tendon of the flexor perforatus muscle. It should 

 be noticed that the nerve and tendon are not in immediate relationship 

 to one another (/.e., the nerve does not lie on the tendon). The tendon 

 inclines towards the inner side of the limb, and is here almost 

 circular in outline. The posterior surface of the tendon of the flexor 



