Bat antebrachium
Text and photos: Sarah Winstanley and Kathy Coulombe
Both the propatagium and the plagiopatagium sections of the wing membrane were reflected on the dorsal and ventral sides of the humerus, radius, and ulna to expose the muscles of the antebrachium. The dissection was performed using a microscope and needle point to separate the thin and delicate wing membrane from the deep muscles.
Points of Note:
All of the extensor muscles originate from the dorsal side of the skeletal forelimb, whereas the flexor muscles originate from the ventral side. Many muscles of the antebrachium become tendinous towards the point of insertion. This allows the animal to decrease the weight of the limb to aid with less energy expended for movement, while still maintaining the power of the muscle movement from the origin.
The extensor carpi radialis longus and brevis are the most important flight muscles in the forearm because their action results in the extension of all the digits, thereby spreading the chiropatagium. In order to achieve maximum thrust necessary for propulsion during flight, the chiropatagium must be kept fully extended during the downstroke.[1]
The extensor carpi radialis longus is partially covered by a fibrous sheath. This muscle can be observed from both the ventral and dorsal views of the antebrachium. The dorsal view reveals the tendinous origin of the muscle from the lateral epicondyle of the humerus. This tendinous section displays a yellow-green sheen. The ventral view shows extensor carpi radialis longus muscle becoming tendinous at the distal third of the humerus.
The extensor carpi radialis brevis is proximally aponeurotic from the origin of the lateral epicondyle of the humerus, similar to the extensor carpi radialis longus which is slightly superficial to brevis. However, the aponeurosis on extensor carpi radialis brevis radiates a blue-purple sheen. The difference in color indicates a difference in the microscopic structures of these tendinous fibers. This difference of fiber structures is further substantiated by the observation that just proximal to the point of insertion, the tendon of this muscle has a sesamoid bone, which forms within the tendon.[1]
At the origin of the supinator muscle, the tendon of this muscle has a sesamoid bone. The functionality of the sesamoid bone allows the muscle to have an increased pivot, so that the power of the in-lever leads to a higher out-lever force.
The extensor carpi ulnaris is a superficial, dorsal muscle of the antebrachium. This muscle is important for terrestrial locomotion in genera that rely on the forelimbs to locomote terrestrially. It counteracts the action of the radial extensors by maintaining closure of the chiropatagium.
The extensor digitorum communis is a relatively small dorsal muscle that has three tendinous points of insertion. Due to the small size of this muscle relative to the importance of its function during flight, it is thought that air currents aid in keeping the chiropatagium taut during flight.[1]
Bats allow for high maneuverability during flight by changing the angle of the plagiopatagium. This is primarily achieved by flexion or extension of the fifth digit, which forms the distal edge of the plagiopatagium. The ability to quickly change and maintain this angle is of great aerodynamic importance, and therefore bats have developed many muscular and structural specializations in order to brace this joint, or prevent movement other than flexion or extension. One such muscle is the abductor pollicis longus, which acts to stabilize the fifth carpometacarpal and prevent over-articulation.
Flexor digitorum profundus is deep to the other antebrachium flexor muscles, and splits to form three tendons to insert on digits one, three, and four.
The flexor carpi ulnaris has a proximally aponeurotic fascia, and lies deep to pronator teres. This muscle has a tendinous core, which reduces its elasticity. This muscle is important in flight because flexion and extension of the fifth digit affects the tautness of the plagiopatagium as well as the degree of extension of the chiropatagium. As in humans, the fifth digit can not extend farther than to roughly right angles with the radius.
Note that Tadarida braziliensis does not have a brachioradialis muscle or a palmaris longus muscle.
| Muscle | Origin | Insertion | Action | Remarks |
| Forearm Extensors: | ||||
| extensor carpi ulnaris p. 163-4 [1] | proximal part of the ulna, and distal portion of the radius | lateral base of the third metacarpal | closure of the distal part of the wing (chiropatagium) by flexion of the third digit | In Macrotus and Hipposideros, the extensor carpi ulnaris inserts on the lateral base of the fifth metacarpal. |
| extensor digitorium communis p. 162 [1] | lateral epicondyle of the humerus | lateral surfaces of second phalanges of digits three, four, and five | spreading of the chiropatagium by extension of digits three, four, and five | This is the only bat genus in which this muscle remains connected to the first phalanx of the second digit. |
| extensor indicus p. 163 [1] | posterior surface of the radius, and lateral surface of the ulna | lateral surface of the second metacarpal | spreading of the chiropatagium by extension of the second digit. Extensor indicus acts in much the same way as the radial extensors. | |
| extensor digiti quinti proprius p. 163 [1] | lateral base of the ulna, and the adjacent surface of extensor digitorium communis | lateral base of the fifth metacarpal (second phalanx of digit five) | extension of the fifth digit | This muscle is not present in Eumops or Hipposideros. |
| extensor carpi radialis longus p. 160-1 [1] | from lateral epicondyle of the humerus, to the distal end of the shaft | lateral bases of the first and second metacarpals | extension of all digits: direct extension of the first and second digits, and indirect extension of digits three, four, and five | |
| extensor carpi radialis brevis p. 160-1 [1] | lateral epicondyle of the humerus | lateral surface of the third metacarpal | extension of all digits | |
| supinator p. 161 [1] | lateral epicondyle of the humerus | proximolateral surface of the radius | stabilization of humero-brachial articulation | |
| extensor pollicus brevis p. 161 [1] | lateral surface of the ulna, and the fascia on abductor pollicus longus and extensor indicus | metacarpophalangeal joint, and the second phalanx of the first digit | extension of digits one, two, and three | |
| abductor pollicus longus p. 161-2 [1] | posterior surface of the radius | scaphoid bone of the carpus | stabilization of the fifth carpometacarpal articulation | |
| Forearm Flexors | ||||
| pronator teres p. 169 [1] | base of spinous process of the medial epicondyle of the humerus | proximomedial surface of the radius | stabilization of the elbow joint | |
| flexor carpi radialis p. 169 [1] | distal portion of pronator teres | fascia at base of first metacarpal | probably functionless | In several genera, this muscle is considered vestigial and is probably functionless. |
| flexor digitorium profundus p. 169-70 [1] | spinous process of medial epicondyle of the humerus | second phalanges of digits one and three, and third phalanx of digit four | flexion of the digits of the hand | |
| flexor carpi ulnaris p. 167-8 [1] | tip of spinous process of the medial epicondyle of the humerus | proximal portion of the pisiform (bound to the fifth metacarpal by ligaments) | flexion of the fifth digit |
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.
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