Tree shrew shank and foot
Photos and text: Terence Mitchell
Removing the skin - Peel the skin covering the shank from an existing edge using the rat-tooth forceps. The skin should come off easily, leaving only a layer of deep fascia. The deep fascia will prevent a clear view of the thigh and gluteal muscles, and should be removed with a sharp probe and rat-tooth forceps.
Tibialis anterior - This large and superficial muscle arises from the anterolateral tibial shaft along the tibial crest. This muscle is tendinous distally and passes deep to the extensor retinaculum to insert on the medial cuneiform. Inverts the foot and dorsiflexes at the ankle joint. Probably plays a large role in pedal reversal at the ankle joint.
Extensor digitorum longus - Lateral to tibialis anterior, this muscle
arises from the femur's lateral condyle. This muscle is tendinous distally and
passes deep to the extensor retinaculum to insert on the distal phalanges of
digits II-V. Extends the toes and dorsiflexes the foot at the ankle joint.
Extensor hallucis longus - Deep to tibialis anterior and extensor digitorum
longus, this muscle arises along the anteromedial fibular shaft. This muscle
is tendinous distally and passes deep to the extensor retinaculum to insert
on the medial cuneiform. Extends the hallux and dorsiflexes the foot at the
ankle joint.
Lateral compartment
Note: All of the lateral compartment muscles are visible in Figure 2. The fibularis muscles are on the lateral aspect of the shank.
Fibularis longus - Arises from the lateral side of the fibular head. This muscle becomes tendinous and passes posterior to the lateral malleolus to wrap around the dorsum of the foot and attach on the metatarsal of the hallux. Everts the foot at the ankle joint.
Fibularis brevis - Arises from the lateral surface of the fibular shaft. This muscle becomes tendinous and passes posterior to the lateral malleolus to wrap around the dorsum of the foot and attach on fifth metatarsal. Fibularis brevis has a large muscle body that is continuous with fibularis longus proximally. Everts the foot at the ankle joint.
Fibularis digiti (quinti or quarti) - Deep to fibularis longus and brevis on both the right and left leg, a third muscle was tightly bound by deep fascia. I assume the third muscle is one of the two other fibularis muscles, fibularis digiti quinti and quarti, found in the literature. During separation of these three muscles, the origin and insertion of the third muscle were sufficiently damaged to render it unrecognizable. I did not see a fourth fibularis muscle muscle on either leg. Both muscles evert the foot at the ankle joint.
Posterior compartment
Gastrocnemius - Arises from the medial and lateral knee joint and femoral head, and inserts on the calcaneus via the calcaneal tendon. Plantarflexes the foot at the ankle joint.
Plantaris - Arises from the lateral knee joint and from the dorsal surface of the gastrocnemius. The muscle becomes tendinous distally, and runs between the soleus and gastrocnemius muscles. Beginning at the calcaneus, it broadens to form the plantar aponeurosis. Plantarflexes the foot at the ankle joint.
Soleus - Arises from the head of the fibula and medial fibular shaft, and inserts on the calcaneus via the calcaneus tendon. Plantarflexes the foot at the ankle joint.
Popliteus - Arises from the femur's lateral epicondyle, and inserts near the midline on the posterior tibial shaft. Flexes the leg at the knee joint.
Flexor digitorum fibularis - Arises along the posterior fibular shaft with some fibers taking origin off of the tibia and interosseus membrane. See flexor digitorum tibialis for insertion. Flexes the digits of the foot and plantarflexes at the ankle joint.
Flexor digitorum tibialis - Arises on the medial condyle of the tibia and the tibial shaft. Both flexor digitorum fibularis and tibialis become tendinous distally. The two tendons fuse in the foot deep to flexor digitorum brevis. It was difficut to determine which tendons were fusing and giving rise to other tendons. However, it appeared that the common tendon mentioned above gave rise to five tendons for digits I-V. Flexes the digits of the foot and plantarflexes at the ankle joint.
Tibialis posterior - Arises on the posterior side of the medial fibular shaft and the dorsal surface of flexor digitorum fibularis. This muscle becomes tendinous distally, passes distal to the medial malleolus, and then inserts on the ventral side of the medial cuneiform. Plantar flexes and inverts at the ankle joint.
Many mammals, particularly those that are scansorial or fully arboreal, possess
the ability to rotate the foot 180 degrees while maintaining a tight grip on
the substrate [1]. This specialization is accomplished by alterations to the
articulations in the ankle and muscles of the shank. In fully arboreal species
this function is used when descending a trunk, branch, vine, etc. head first
or during hind limb suspension. Small animals often develop this ability, as
they are required to climb obstacles that do not present problems for larger
animals. Their small size and varied substrate use require that tree shrews
frequently make use of the hind foot reversal mechanism.
Similar to Didelphis, Felis, Sciurus, and many other mammals,
tupaiids have evolved hind foot reversal with only minor structural alterations.
Several authors have cited various joints as producing the 180 degree of rotation
necessary for full reversal. Lateral rotation of the thigh in rodents and supination
at the talocalcaneonavicular in tree shrews and tree squirrels have all been
invoked to explain at least part of this rotation [2,3]. Most authors recognize
a large degree of rotation occurring at tarsal joints as well. However, in a
study of radiographs taken of live animals performing full reversal Jenkins
and McClearn demonstrated that all of the necessary rotation occurs at the tarsal
joints [1]. In tree shrews about half of the reversal takes place at the transverse
tarsal joint by supination about the calcaneocuboid and talonavicular articulations.
The foot is thereby rotated to a medially facing position. This movement is
augmented by an additional 90 degrees of rotation at the subtalar joint produced
by inversion about the calcaneotalar articulation. If the foot is already placed
in a medially facing orientation by movement at the transverse tarsal joint,
the subtalar inversion brings the foot around to face completely dorsally. Finally,
hind foot reversal is usually accompanied by moderate to extreme plantarflexion,
which takes place at the crurotalar joint. In fact, in many animals, including
tree shrews, hind foot reversal is precluded by a posture involving pronounced
dorsiflexion.
The tibialis anterior muscle is responsible for producing much of the force
necessary to bring about hind foot reversal. In the more terrestrial tree shrews,
tibialis anterior crosses the dorsum of the foot to insert on the ventrum of
the medial cuneiform [4]. Such an attachment should be sufficient to produce
much of the necessary inversion. However, in the more arboreal Tupaia minor
and Ptilocercus lowii the insertion as migrated to attach on the hallucal
metatarsal. This more distal positioning should give these species more power
and range during hind foot reversal. The role of tibialis anterior is also supported
by studies showing that electrical stimulation of this muscle produce 90 degrees
of inversion in tree squirrels [1]. It is interesting to note that all of the
above specializations are in fact minor alterations to existing mammalian patterns
and not newly derived behaviors or structures.
While all mammals do have some degree of mobility at these joints, most eutherians are restricted to predominantly plantarflexion/dorsiflexion. This is probably a derived condition, as early mammalian ancestors such as cynodonts show a high degree of mobility, with a convex talus and flat distal tibia. The restricted movement among most eutherians probably represents an adaptation for stability during rapid locomotion.
1 Jenkins, F.A. and D. McClearn. 1984. Mechanisms of Hind Foot Reversal in
Climbing Mammals. J. Morphol. 182:197-219.
2 Landry, S.O. 1965. The basic adaptation of scansorial rodents. Am. Zool.
5:681.
3 Cartmill, M. 1974. Pads and claws in arboreal locomotion. In F.A. Jenkins,
ed.: Primate Locomotion. New York: Academic Press. pp. 45-83.
4 George, R.M. 1977. The limb musculature of the Tupaiidae. Primates 18:1-34.
Davis, D.D. 1938. Notes on the anatomy of the tree shrew Dendrogale.
Field Mus. Publ. Chicago Zool. 20:383-405.
George, R.M. 1977. The limb musculature of the Tupaiidae. Primates 18:1-34.
Le Gros Clark, W.E.1924. The myology of the tree shrew (Tupaia minor).
Proceedings of the Zoological Society of London 1924: 559-567.
Le Gros Clark, W.E.1926. On the anatomy of the pen-tailed tree shrew (Ptilocercus
lowii). Proceedings of the Zoological Society of London 1926: 559-567.