Text and photos: Kathy Coulombe and Sarah Winstanley
Preserved Mexican Free-Tailed bat (Tadarida brasiliensis), ventral view
Some interesting observations of the general surface anatomy of Tadarida
braziliensis include:
Bats have wings, and flight is their primary method of locomotion. The wing
membrane is organized around the limb bones into three main areas. The membrane
that stretches from the scapula to the carpals cranially is the propatagium.
The plagiopatagium is the membrane that forms a webbing between the metacarpal
and phalanges caudally to the abdomen and leg. The uropatagium membrane is between
the femur, tibia and tarsals, and the proximal third of the tail. The distal
tail is not attached to the uropatagium of the wing, which is why the common
name for this species is the "free-tailed bat". The species has a
wrinkled, hairy lip. The eyes have palpebrae, and are very small- an indication
that sight is not the primary sense of T. braziliensis. The ulna,
radius, metacarpals and phalanges became elongated with the evolution of the
wing.
Preliminary Note:
Given the small size of the specimen, it is necessary to use a dissecting microscope and extra care to dissect successfuly, and to identify anatomical structures..
Reflect the fascia from the level of the first cervical vertebra to the level of the upper lumbar vertebrae, and from the midline to the elbow to observe the muscles of the shoulder.
Ventral view:
The most superficial muscle is pectoralis superficialis
pectoralis superficialis
Origin: clavicle (anterior section); sternum, extending from manubrium to
xiphoid process (posterior section)
Insertion: pectoral crest of the humerus
Action: adduction and pronation of the humerus at the gleno-humeral joint
This muscle is enlarged to cover the entire ventral thorax because it is involved
in producing the powerful downstroke during flight.
Directly deep to pectoralis superficialis is pectoralis profundus. It is difficult to separate these two muscles because they are so closely connected.
pectoralis profundus
Origin: medial clavicle, manubrium and body of sternum
Insertion: pectoral crest of the humerus
Action: adduction and pronation of the humerus at the gleno-humeral joint
It is interesting to note that bats have a clavicle, which is not present in all mammals. This bone plays a large role in the actions of the shoulder, as several of the shoulder muscles attach to the clavicle. Since flight is the bat's primary mode of locomotion, it is important to have powerful muscles in the shoulder region for both the downward and upward strokes. The clavicle may play a role in stability of these muscles and the gleno-humeral joint in general.
Subclavius is a very small muscle and can be observed upon reflection of the pectoralis muscles.
subclavius
Origin: ventral surface of the first costal cartilage
Insertion: proximal end of the dorsal surface of the clavicle
Action: pulls the clavicle caudally and ventrally
This steadies the clavicle against the opposite pull of the dorsal
musculature. It can be seen in both ventral and dorsal views; however, given
its small size, it is easier to locate ventrally as it is directly deep to the
cranial portion of the pectoralis superficialis and pectoralis profundus muscles.
Also deep to the pectoralis superficialis and pectoralis profundus muscles is pectoralis abdominis, and directly deep to this muscle lies serratus anterior, which can be seen as a sheet of muscle superficial to the ribs.
pectoralis abdominis
Origin: deep fascia of the abdomen (aponeurosis)
Insertion: ventral base of the pectoral crest (tendon- distal third of muscle)
Action: flexion, pronation (weak) of the humerus at the gleno-humeral joint
Serratus anterior
Origin: along the mid-axillary line and first four ribs
Insertion: cranioventral edge of the vertebral border of the scapula
Action: moves the craniomedial portion of the scapula ventrally
Dorsal view:
With the fascia reflected, the most superficial muscles are the trapezius group, deltoideus group, and latissimus dorsi.
Trapezius group
clavotrapezius
Origin: dorsal midline of dorsal spines of vertebrae between C7 and T6
Insertion: lateral clavicle, dorsal surface
Action: moves clavicle dorsally and medially
acromiotrapezius
Origin: dorsal midline of dorsal spines of vertebrae between C7 and T6
Insertion: acromion process of the scapula
Action: moves acromion process dorsally and medially
The clavotrapezius and acromiotrapezius are indistinguishable at origin, and can only be differentiated given their different points of insertion.
spinotrapezius
Origin: dorsal midline between T10 and L2
Insertion: vertebral border of the scapula (thin aponeurosis)
Action: moves vertebral border of scapula caudally and medially
Deltoid group
acromiodeltoideus
Origin: acromion process and scapular spine
Insertion: pectoral crest and proximal humeral surface
Action: abduction, flexion, and supination of the humerus at the gleno-
humeral joint
spinodeltoideus
Origin: transverse scapular ligament and vertebral border of the scapula
Insertion: dorsal humerus distal to pectoral crest
Action: abduction, flexion, and supination of the humerus at the gleno-
humeral joint
latissimus dorsi
Origin: dorsal spines of caudal thoracic and cranial lumbar vertebrae
Insertion: ventral ridge of humerus
Action: abduction, flexion, and pronation of the humerus at gleno-humeral
joint
Another superficial muscle present in the Molossidae family of bats is omocervicalis.
omocervicalis
Origin: ventral arch of the atlas vertebra
Insertion: midpoint of the dorsal surface of the clavicle
Action: moves the clavicle craniodorsally
The occipitopollicalis crosses the shoulder, elbow and carpal joints.
occipitopollicalis
Origin: lambdoidal crest of the medial occiput of cranium
Insertion: distal half of second metacarpal, anterior surface
Action: adjusts area and stiffness of propatagium, to maintain flight surface.
After reflecting the trapezius group, latissimus dorsi and the deltoid group, one can see the spinatus muscles, serratus anterior, teres major, rhomboideus, and levator scapulae. As the scapula of Tadarida braziliensis is relatively large, the infraspinatus and serratus anterior muscles are primarily visible.
rhomboideus
Origin: dorsal spines of vertebrae C7-T9
Insertion: vertebral border of the scapula
Action: moves the scapula craniomedially
The fibers are difficult to differentiate given their small size, and it is easy to mistake wrinkles in the muscle (which run perpendicularly to the fibers) for the fibers themselves. Care must be taken during observation under the microscope to determine muscles and directions.
teres major
Origin: medial dorsal edge of the axillary border of the scapula
Insertion: ventral ridge of the humerus
Action: flexion and pronation of the humerus at the gleno-humeral joint
subscapularis
Origin: subscapular fossa
Insertion: lesser tubercle of humerus
Action: adduction and extension of the humerus
levator scapulae
Origin: transverse processes of vertebrae C3-C7
Insertion: ventral edge of vertebral border of the scapula
Action: moves the cranial portion of the vertebral border of the scapula
craniomedially and ventrally
supraspinatus
Origin: floor and boundaries of supraspinous fossa
Insertion: greater tubercle of humerus
Action: abduction, extension, and supination of the humerus at the gleno-
humeral joint
infraspinatus
Origin: floor of infraspinous fossa
Insertion: dorsal and caudodorsal surfaces of the trochiter of the humerus
Action: abduction, supination, and flexor (weak) of the humerus at the gleno-
humeral joint
Muscle origin, insertion, and action information is based on the detailed findings of the following sources, and was confirmed by our own dissection.
Strickler, T.L. 1978. "Functional Osteology and Myology of the Shoulder
in the
Chiroptera." in M.K. Hecht & F.S. Szalay (eds.) Contributions to Vertebrate
Evolution, Volume 4. Switzerland: Tanner & Bosshardt AG.
Vaughan, T.A. 1970. The Muscular System. in W.A. Wimsatt (ed.)Biology of the
Bats, Vol. I, New York: Academic Press.