Fox shank and foot
Photos and text: Nick Zolkowski and Elizabeth Farrell
The tarsus has much the same function as the wrist, connecting the most distal aspects of the limb with the more proximal. In the shank, the proximal bones, the tibia and fibula, are rigidly bound together near the tarsal joint, comprised of the talus, the calcaneus, and the tarsal bones. Distal to the tarsal bones are the metatarsals and phalanges. The fox is anatomically adapted for cursorial movement, with elongated limb bones and muscles that become tendinous distally. As a result, the fox's tarsus is restricted from rotating the pes, limiting eversion and inversion. Foxes are also digitigrade, which enables them to cover more ground with each stride [1].
We began the dissection by removing fascia from the distal thigh, starting at the knee joint. The first muscle we identified was the gastrocnemius. Its mass was very concentrated proximally, with a long tendon inserting distally into the calcaneal tendon. We continued to remove fascia from the lateral side, and located the tibialis cranialis, which is the most cranial muscle. The extensor digitorum longus muscle is caudal to the tibialis, and the two are very closely associated. The fibularis longus muscle and the flexor hallucis longus muscle are both caudal to the to the extensor digitorum longus and have shorter muscle bellies. We also observed the fibular nerve, which originates from the thigh, and dives between the fibularis and flexor hallucis muscles. After we cut the biceps femoris and reflected it dorsally, we saw the flexor digitorum superficialis, the most caudal of the muscles we observed on this side.
In the medial view there were fewer muscles to identify, as most were identified in the lateral view. Near the thigh at the knee joint, we found a muscle much smaller than those previously viewed, the popliteus. It does not have a long tendon like the previously observed muscles. Additionally, we located the flexor digitorum longus, distal and caudal to the popliteus.
We were not able to trace the tendons of the lower pelvic limb muscles as we had in the forelimb, as the phalanges were missing. We cut through the top layer of the retinaculum to confirm that the tendons continued at this point. On the medial side, the tendon of flexor digitorum superficialis runs behind the medial malleolus. On the lateral side, the tendons of the tibialis cranialis and extensor digitorum longus run over the lateral malleolus, while the tendons of fibularis longus and flexor hallucis longus run behind the lateral malleolus.
The fox does not have a soleus or a plantaris muscle. This occurrence was unusual since most mammals possess these muscles. The muscles, if present, would have been associated with the gastrocnemius.
| Muscle | Origin | Insertion | Action |
| Gastrocnemius | popliteal surface of femur | calcaneal process | extend tarsus |
| Fibularis longus | lateral condyle of tibia | fourth tarsal and proximal ends of metatarsals | flex the tarsus, turn plantar surface lateral |
| Tibialis cranialis | Muscular groove and articular margin of tibia | proximal end of second metatarsal bone | rotate foot laterally, flex tarsus |
| Flexor hallucis longus | proximal half of fibula | base of each distal phalanx | flex digits |
| Flexor digitorum superficialis | popliteal surface femur | calcaneal processes of second phalanges of digits II-V | flex first two digital joints of digits II-V |
| Flexor digitorum longus | proximal two-thirds of tibia | base of each distal phalanx | flex digits |
| Extensor digitorum longus | extensor fossa | extensor tubercles of third phalanges of digits II-V | extend digits II-V |
| Popiletus | lateral epicondyle of femur | proximal third of tibia | flex the stifle |
Adapted from Miller (1952)
1. Alderton, David. 1994. Foxes, Wolves, and Wild Dogs of the World. Facts on File, New York.
2. Miller, Malcolm E. 1952. Guide to the Dissection of the Dog. (3rd Ed). Edward Brothers Inc., New York.