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306 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
chanical properties and actions of the supracoracoideus in eight
acute in situ experiments for each species by direct nerve stimulation.
All experiments were performed following anesthesia
with ketamine (60 mg/kg) and xylazine (6 mg/kg); supplemental
ketamine was given as needed. We bisected the lattisimus
dorsi and rhomboideus muscles to expose the brachial plexus
and isolate the nerve to the supracoracoideus. We intubated the
birds unidirectionally via the trachea (80% oxygen, 20% nitrogen)
after opening the posterior air-sacs. We severed all components
of the brachial plexus except the nerve to the supracoracoideus
to prevent stimulation of adjacent muscles.
Following surgical preparation we clamped the sternum and
coracoid to a rigid frame and maintained body temperature at
40° C with warmed avian ringers and a heat lamp. We mounted
the supracoracoideus nerve on silver bipolar electrodes and established
a stimulation voltage (2x threshold) to elicit a twitch
or a tetanus. For four birds of each species, we measured maximal
tetanic tension by connecting the tendon of the supracoracoideus
directly to a force transducer.
ROTATION AND ELEVATION.—We made independent measurements
of the rotational force (torque) about the longitudinal
axis of the humems and of the force of elevation on the humems
during isometric contraction of the supracoracoideus for
two birds of each species. To measure torque, we threaded a
short piece of silver wire (0.38 mm diameter) through a small
hole drilled in the deltopectoral crest, attached the wire to the
force transducer, and measured isometric force at that point.
We placed a 23-gauge pin in the shaft of the humems to prevent
elevation while still permitting "free" rotation about the
bone's longitudinal axis. To measure elevational force, we secured
the humerus to the transducer with surgical silk. We
stimulated the supracoracoideus nerve tetanically with the humems
positioned at joint angles of elevation/depression and
protraction/retraction coincident with the downstroke-upstroke
transition and midupstroke of flight.
EXCURSION OF THE HUMERUS.—We measured the total in
situ elevation excursions of the humems during tetanus of the
supracoracoideus for two birds of each species. During these
measurements, the humems was not restricted in any way but
was allowed to move during stimulation. We stimulated the
nerve tetanically (60 hertz; 500 ms train duration) and measured
elevation of the humems with a protractor. We made all
elevational measurements relative to the dorsal border of the
scapula in lateral view. Subsequent to the elevation measurements,
we measured rotation by placing a 23-gauge pin guided
by a rack and pinion through a small hole drilled in the distal
end of the humems. We threaded the needle into the long axis
of the humeral shaft, which served as a pivot for rotation while
restricting the elevational component of movement. We placed
a 26-gauge pin perpendicular to the long axis of the humems,
which served as a dial with which to measure the degree of rotation.
We made measurements at the two wing positions noted
earlier; the downstroke-upstroke transition and midupstroke.
Discussion
The downstroke-upstroke transition in both species begins
with the humems depressed below the horizontal (10° for starling,
estimated 10° for pigeon). The angle formed by the long
axis of the humems and the vertebral column in dorsal view at
the downstroke-upstroke transition is about 55°-60° in both
species. Upstroke commences by retraction, rotation, and elevation
of the humems, flexion of the elbow, and flexion/supination
of the wrist. During upstroke, the right humems rotates
counterclockwise about its longitudinal axis and elevates about
40° above the horizontal (Figure 4). During muscle shortening
the potential for active force production decreases as the humems
is rotated and the wing is elevated. Nevertheless, at humeral
angles corresponding to the downstroke-upstroke transition,
we measured tetanic forces of 6.5 ±1.2 newtons (N) in the
starling (H=3) and 39.4 ±6.2 N in the pigeon {n=6); forces 8
times or more the body weight of each species. The supracoracoideus
imparted an average isometric force for rotation measured
at the deltopectoral crest for the starling of 4.9 N (downstroke-upstroke
transition) and for the pigeon of 32.1 N. The
forces at the midupstroke positions were about half of these
values. Although we measured in situ humeral rotations of up
to 80°, maximum elevations of the humems were only about
55° above the horizontal. From these data we conclude the primary
action of the supracoracoideus to be high-velocity rotation
of the humems about its longitudinal axis during wing upstroke;
active wing elevation may be of secondary importance.
Further support for this conclusion comes from an analysis of
the glenoid and the anatomical arrangement of the avian supracoracoideus.
The avian shoulder joint is structurally derived and
functionally complex. The glenoid, best described as a hemisellar
(half-saddle) joint, faces dorsolateral^ and articulates with a
bulbous humeral head. Jenkins (1993) reviewed the structural/
functional evolution of this joint and provided an interpretation
of its function based on a cineradiographic analysis of the wingbeat
cycle. His study illustrated the articulation of the humeral
head on a dorsally facing surface of the glenoid, the labmm
cavitatis glenoidalis, which allows for full abduction of the
wing into the parasagittal plane at the upstroke-downstroke
transition. We believe full abduction is not so much by elevation
of the humems but by rotation about its longitudinal axis. It
bears emphasis that during the wingbeat cycle of European
Starlings flying in a wind tunnel, where we have precise cineradiographic
data, the angle formed by the long axis of the humems
and the vertebral column is never greater than 55° (Jenkins
et al., 1988, fig. 1; Dial et al., 1991, fig. 4). We have made
in situ measurements of humeral protraction/retraction in anaesthetized,
intact starlings and pigeons. The humems cannot
be drawn forward to intersect the body axis at an angle greater
than 60°-65° unless forced; its forward angle beyond these angles
is constrained by the ligaments and muscles surrounding
the shoulder.
The mechanics of the musculoskeletal organization of the supracoracoideus
also supports our conclusion. The supracoracoideus
in both pigeons and starlings, as well as in all other species