For comparison, see shank and foot dissections of a bat, a cat, a fox, a rabbit, and a tree shrew.

Muscles of the Ferret Shank and Foot

Adapted from Klingener, 1979.

We began this dissection by removing the skin from the knee joint to the footpads on the left leg. We removed a layer of heavy fascia and fat just underneath the skin to observe the underlying musculature. We first identified the superficial muscles on the lateral side of the leg, using a dull probe to separate the muscle bellies from one another at their fascial boundaries.

The broadest and thickest superficial muscle is the gastrocnemius, although only its lateral head is visible on this side. This is the only flexor muscle visible on the lateral side. The majority of this muscle's mass is in the proximal half of the leg. The muscle begins to taper off in the distal half to form a thick tendon that inserts into the caudal edge of the calcaneus.

The fibularis longus is adjacent to the lateral head of the gastrocnemius. This extensor muscle is of uniform thickness throughout its length and runs along the midline of the lateral side of the leg. The tendon of the fibularis longus passes posterior to the lateral malleolus of the fibula, wraps around the bottom of the foot, and inserts into the metatarsal of digit I. The fibularis brevis is deep to the fibularis longus. It is much shorter than the fibularis longus and its tendon runs posterior to the lateral malleolus in a sheath that also contains the tendon of the extensor digitorum lateralis. The tendon of the fibularis brevis inserts into the metatarsal of digit V, while the tendon of the extensor digitorum lateralis passes over the dorsum of the digit and inserts into the phalanx of digits II through V [1].

The extensor digitorum longus is anterior to the group of three fibularis muscles. This muscle is long and thin, and only the distal half of the muscle is visible; the proximal half of the muscle belly is covered by the tibialis cranialis. To view the tendons of this muscle, we removed a thick retinaculum that crosses the anterior ankle.

To remove the retinaculum, we cut it parallel to the tendons of the muscles to avoid damaging them. Once we removed the retinaculum, the tendinous insertions were much easier to see. The tendons of the extensor digitorum longus insert on the phalanges of digits II through V. This is the only muscle that has tendons that inserts on these four digits of the dorsum of the pes, similar to the extensor digitorum communis of the manus. The tendon of the extensor hallucis longus, a very slender muscle deep to the extensor digitorum longus and the tibialis cranialis, inserts into the hallux.

The tibialis cranialis is located on the anterior border of the tibia, and we could see it on both the lateral and medial sides. This muscle originates on the surface of the tibia, and we could not separate it from the bone. The tibialis cranialis is the largest extensor muscle, and its tendon passes under the retinaculum at the ankle and inserts into metatarsal I.

The popliteus is the only muscle that we observed in the leg that is neither a flexor nor an extensor, but a rotator of the shank. It runs along the midline of the medial aspect of the shank, just posterior to the tibia and anterior to the medial head of the gastrocnemius. This fan-shaped muscle originates at the lateral epicondyle of the femur, crosses the knee joint, and inserts into the posterior surface of the tibia.

The flexor digitorum longus and flexor hallucis longus are posterior and distal to the popliteus. Together, these two muscles make up the flexor digitorum profundus. The tendons of both muscles pass together behind the medial malleolus and then separate to their respective insertions. As their names imply, the flexor hallucis longus inserts into digit I while the flexor digitorum longus inserts on digits II through V.

The medial head of the gastrocnemius is the broadest and thickest muscle on the medial side of the leg. It joins the lateral head just above the calcaneus to form the calcaneal tendon. After we had observed all of the superficial muscles of the leg, we cut this tendon to see the deep flexors, the plantaris and the soleus. The plantaris (also sometimes called the flexor digitorum superficialis) is a thick muscle just deep to the gastrocnemius. It runs down the entire length of the leg along the midline of the gastrocnemius. The tendon of the plantaris runs medially around the calcaneal tendon, behind the calcaneus, and splits to insert on the ventral side of the foot into digits II through V. We then reflected the plantaris and immediately identified the soleus, which is much shorter and thinner than the gastrocnemius and plantaris. The muscle belly is in the proximal half of the leg, and a long tendon continues down the leg to insert into the calcaneus.

An easy way to identify a muscle as an extensor or a flexor is its position relative to the tibia. Muscles that are anterior to the tibia are extensor muscles, while those posterior to the tibia are flexor muscles. We could clearly see the tibia on the medial side of the leg between the tibialis cranialis and the popliteus. This is an important landmark to identify the muscles of the leg. The flexor mass of the leg is much greater than the extensor mass. This is an important characteristic in ferret locomotion. The flexors are larger than the extensors because they are used in the power stroke of locomotion, while the extensors are used in the recovery stroke and do not need to be as great [1]. This is characteristic of the fox and the cat as well.

A key difference between the ferret and the other carnivores is the distribution of the musculature of the leg. The ferret's muscle bellies run almost the entire length of the leg and have relatively short tendons. In contrast, the fox and cat's musculature are concentrated in the proximal half of the leg and have much longer tendons. Another important difference between the ferret and the other carnivores is the length of the calcaneal tuber. The ferret's calcaneal tuber is longer relative to its foot length than the cat's. In the ferret, the calcaneal tuber is one-ninth of its foot length, while the cat calcaneal tuber is one-twelfth of its foot length. The larger in-lever provided by the lengthened calcaneal tuber gives the ferret relatively more power in its hindlimbs [2]. The shorter calcaneal tuber of the cat sacrifices power but improves its speed.

References

1. Klingener, D. 1979. Laboratory Anatomy of the Mink (2nd Ed.). William C. Brown, Dubuque, Iowa.

2. Hildebrand, Milton. 1995. Analysis of Vertebrate Structure, (4th Ed.). John Wiley & Sons, Inc. New York, Chichester, Brisbane, Toronto, Singapore. 657 pp.

Links

to Comparative Mammalian Anatomy

to mammalian shank and foot