Text and photo: Sarah Winstanley and Kathy Coulombe
The brachium of the bat is highly specialized for flight. This method of locomotion is unique to bats, compared with all other mammals. The latissimus dorsi and pectoralis muscles pronate the forelimb, and the spinodeltoideus, supraspinatus and infraspinatus supinate the forelimb. These actions are necessary to maintain the stiffness of the plagiopatagium and propatagium, or wing membrane. Tadarida brasiliensis has five digits on each limb. The bones of the first digit are small (2-3mm each); however, the bones of the remaining four digits are grossly elongated and incorporated between the dorsal and ventral layers of the plagiopatagium. The proportions for the skeletal elements of the forelimb are:
Humerus = 2cm: Ulna+Radius = 4cm: Metacarpals = 4cm: Phalanges = 2cm
Note that the carpals remain small. This retention of the small and round carpal
joints allows the bat to rotate freely at these joints in order to maintain
and maximize the tight surface of the wing for flight.
The dissection was performed using a needle point to carefully reflect the
fascia from the muscles. This became increasingly difficult moving distally
along the humerus, as the distal bones are encased in the ventral and dorsal
surfaces of the wing membrane.
Muscle |
Origin |
Insertion |
Action |
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Elbow Extensors: |
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triceps
brachii |
lateral border
of the scapula, slightly posterior to the glenoid fossa |
proximal end
of the olecranon |
This muscle is probably the most essential to maintaining extension of the wing after the humerus is extended. |
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Remarks: |
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triceps
brachii |
posterior surface
of the humerus: runs from the greater tuberosity to the pectoral ridge,
and from the medial ridge and the adjacent posterior surface of the humerus |
proximal end of the olecranon |
|
|
triceps
brachii p. 158 [1] |
posterior, distal surface of the humerus |
deep to other
tendons of the triceps group, on the proximal portion of the olecranon |
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|
Elbow Flexors: |
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|
brachialis p. 167 [1] |
near the middle of the anterior surface of the humerus | the radius, into the flexor fossa |
|
|
biceps
brachii p. 166-7 [1] |
coracoid head
of biceps brachii originates along the distal portion of the coracoid process;
the glenoid head originates at the base of the coracoid process |
flexor fossa of the radius |
The biceps and triceps muscle
groups act to stabilize the wing when it is in a fully extended position,
in order to maximize thrust and lift during the downstroke while flying.
However, the biceps muscles are positioned such that they have
a greater mechanical advantage over than the triceps muscles, and therefore
have greater control over the flexion of the forearm.
The biceps muscles are probably also involved in maintaining rotational
stability of the forearm during the wing-beat cycle.
|
|
coracobrachialis p. 166 [1] |
tip of the
coracoid process of the scapula |
medial surface of the humerus, opposite the pectoral ridge |
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Remarks: |
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Wing
Muscles:(Muscles unique to bats)
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Occipito-pollicalis |
lambdoidal crest, adjacent to the midline | distal portion of the second metacarpal |
Specifically,
it acts to pull the propatagium craniad and ventrad, which increases and
the surface area of the proximal segment of the wing. |
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Remarks: |
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coraco-cutaneous |
distal portion
of the medial ridge of the humerus |
axillary portion of the plagiopatagium |
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|
humeropatagialis |
medial surface of the epicondyle of the humerus, and the posterior surface of the base of the ulna |
axillary portion
of the plagiopatagium Humeropatagialis inserts more
distally on the plagiopatagium than coraco-cutaneous. |
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The biceps brachii on T. brasiliensis is divided into two heads, analogous to that of humans. Once the pectoralis muscles are reflected, the coracoid head demonstrates a large protuberance of muscle from the origin of the coracoid process, whereas the glenoid head, lying cranially to the coracoid head, is more tapered. Both heads of the biceps brachii become aponeurotic by the mid-humerus and remain so to insertion on the proximal radius.
The brachialis and coracobrachialis muscles are deep tiny slivers of muscle running just superficial to the humerus. These muscles are difficult to detect even under a microscope due to their small dimensions. These two muscles aid in flexion of the forearm at the humero-ulnar joint, however given their size, it is assumed that this action is performed in large part by using the larger biceps brachii.
On the dorsal side of the specimen, the triceps brachii can be observed. The orientation of the long, medial, and lateral heads is analogous to that of an anatomically correct human. To find the triceps brachii on the specimen, it is easiest to look for the common tendon formed by the three heads in the most superficial orientation of the dorsal muscles after the fascia is reflected. The long head is the longest section of muscle mass originating from the lateral border of the scapula. The lateral head is indeed more lateral and superficial to the long head. In the position of the bat, the medial head is most medial and deep so that the best way to observe it is the medial side of the humerus from a ventral view.
There are three muscles in the brachium which are unique to bats. The cranial most of these muscles is the occipitopollicalis and acts to stretch the propatagium, which is the wing membrane formed cranially to the humerus and ulna. This muscle is very thin and is similar to the dimensions of a sliver of dental floss.
The coracocutaneous and humeropatagialis are the two muscles imbedded in the
plagiopatagium. Coracocutaneous originates from the mid-humerus, whereas the
humeropatagialis originates from the distal humerus. Both muscles run distally
and caudally into the wing membrane, thus contraction of either of these muscles
will alter the flight surface of the wing for manoeuvring in the air. We were
able to reflect the plagiopatagium to expose these wing muscles; however, they
are also observable from the surface anatomy, as the wing membrane is slightly
raised where the muscles are.
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.