Bat shank and foot
Text: Sarah Winstanley and Kathy Coulombe
The epidermis was removed from the shank and foot of the specimen. At the tarsal joints, the skin becomes difficult to separate from the deep tendons and musculature, and is almost impossible to remove from the phalanges. The plagiopatagium and uropatagium wing membranes around the lower limb were carefully dissected with a needle point to observe the tensor plagiopatagii, and intrinsic wing muscle that originates from the distal half of the tibia and inserts into the wing membrane cranio-distally.
The pes in Tadarida braziliensis is naturally dorsiflexed and inverted. This position of the foot is very unusual among mammals; inversion of the foot in mammals that have joint flexibility in the foot occurs more commonly with plantarflexion (e.g., the squirrel). This orientation of the foot is necessary during the day, when the bats rest, clinging by their feet to branches or the roofs of caves. Therefore, the relaxed state of the foot is inverted so that they can hang without using great muscle effort to maintain that position. This orientation of the foot is aided by the adaptations of the phalanges. All five toes are the same size (approximately 4 mm). This is unusual compared to most mammals that have differing lengths of toes. Bats do not have a specialized opposable hallux for grasping. A grasping hallux would require bats to exert muscular energy in their feet when they hold a branch. When hanging upside-down during their daily rest, they rely on the their claws to hold onto the branch. There are claws on all five digits of each lower limb. The claws measure 4mm and are extremely curved. This curvature acts as a hook to maintain the foot's grip on the branch. Further, the major tendons in each digit are attached to the phalanges in such a way that the weight of a hanging bat keeps the claws upright and oriented properly to maintain grip on the roosting substrate. Also, the skin on the phalanges is hairy, with longer hairs on the dorsal side (10mm) than on the ventral side (3mm).
The gastrocnemius is the most superficial muscle in the posterior compartment of the shank. On our dissection, we noticed that this muscle had small black specks that could not be removed and seemed to be imbedded into the muscle. This unusual appearance of the muscle may have been a bruise, with tiny flecks of blood, or the result of a parasitic infection.
Bats do not have heels, as they do not use their feet for quadrupedal or bipedal locomotion except in very rare circumstances. The large tendon of the gastrocnemius muscle broadens into a plantar aponeurosis, which joins the tendon of flexor digitorum fibularis. The primitive condition
in terrestrial mammals is for the plantaris tendon to divide and give rise to a deep and a superficial aponeurosis. However, bats have a derived condition in which the superficial plantar aponeurosis is absent, the plantaris is not the origin for flexor digitorum brevis, the deep plantar aponeurosis is fused distally to that of the flexor digitorum fibularis, and the ventral surface of plantar aponeurosis serves as the origin of the lumbricales.[1]
| Muscle | Origin | Insertion | Action |
| Shank and Foot Extensors: | |||
| extensor digitorum longus [1, p. 179-80] | lateral condyle of the femur, and the anterior portion of extensor digitorum longus | distal phalanges of digits one to five | extension of the digits |
| extensor digitorum brevis [1, p. 180 ] | This muscle is made up of seven slips: medial slip, distal portion of the fibulalateral slip, proximal portion of the calcaneus; remaining five slips: dorsal projection of the distal portion of the calcaneus | medial slip: medial surface of the first phalanx of the first digit; lateral slip: lateral surface of the first phalanx of the fifth digit; remaining five slips: distal phalanges of digits one to five | extension of the digits |
| peroneus longus [1, p. 180-1 ] | head and lateral surface of the proximal portion of the fibula | ventral base of the third metatarsal | rotation of the foot dorsad and laterad |
| Remarks: The actions of peroneus longus and brevis, as well as those of gastrocnemius, aid in terrestrial locomotion by giving the final push at the end of the stride. | |||
| peroneus brevis [1, p. 181 ] | middle portion of the lateral surface of the fibula | dorsal surface of the fifth metatarsal | Along with peroneus longus, peroneus brevis is also responsible for dorsad and laterad rotation of the foot. |
| Shank and Foot Flexors: | |||
| gastrocnemius [1, p. 185 ] | medial head: posterior surface of the femur, just proximal to its medial condylelateral head: posterior surface of the head of the fibula and lateral condyle of the femur | proximal end of the calcaneus | flexion of the foot |
| Remarks: Gastrocnemius has both a lateral and a medial head near its origin. | |||
| plantaris [1, p. 185 ] | proximal portion of the posterior surface of the tibia, and the adjacent surface of the proximal portion of the fibula | ventral surfaces of the distal phalanges of digits one to five. | flexion of the digits of the foot, and extension of the ankle |
| Remarks: The fused tendons of plantaris and flexor digitorum fibularis act together to form the strongest muscular unit in the shank, and probably help the claws to grip substrate during terrestrial locomotion. | |||
| flexor digitorum fibularis [1, p. 186 ] | nearly the entire length of the medial surface of the fibula | the ventral surfaces of the distal phalanges of digits one to five | flexion of the digits of the foot |
| Remarks: The aponeurosis of flexor digitorum fibularis and that of plantaris are fused at their distal ends. | |||
| flexor digitorum brevis [1, p. 186 ] | proximal portion of the calcaneus | lateral surfaces of the second phalanges of digits two to four | flexion of digits two to four |
| Remarks: The identification of this muscle can be quite difficult because at the proximal end of the first phalanx, each tendon divides and sends a tendon on either side of the common tendon of flexor digitorum fibularis and plantaris. | |||
| tibialis posterior [1, p. 186 ] | distal portion of the medial surface of the fibula, and the anterior surface of flexor digitorum fibularis (essentially it originates on the distal third of the shank) | ventral surface of the tarsus | extension of the foot |
| popliteus [1, p. 186 ] | medial portion of the head of the fibula | the tibia, just distal to the head | attachment and binding of the fibula to the tibia |
| Remarks: In Myotis and probably Tadarida, popliteus is a vestigial muscle. The popliteus muscle is absent altogether in some genera, including Hipposideros. | |||
1. Vaughan, T.A. 1970. "The Muscular System", p. 139-194 in Wimsatt, W.A. (ed.) Biology of Bats: Vol. 1. Academic Press, New York.