Boyer diss 2009 1046..

the bone presently being described: the long axis of the proximal facet represents the
dorsoventral axis of the articulated hand in certain trapezium postures (see above). In a
metacarpal with no torsion, the dorsopalmar axis of the shaft and distal end is aligned
with the dorsoventral axis of the proximal facet. However, the dorsoventral axis of MC I
is laterally rotated some 45° from the dorsoventral axis. The bone is oriented according
to the distal end anatomical planes in Figure 4.14A and 4.15. Thus the existence of this
torsion can be appreciated by asymmetrical positioning of the proximal end facet in
dorsal view of the bone in Figure 4.14A.
Pollical metacarpal function.—Beard (1989) estimated MC I of N. intermedius to
diverge from the second metacarpal by 73°. This is in the range of euprimates with
specialized grasping capacities. He also suggested that there was limited mobility at the
carpo-metacarpal joint due to what he interpreted as a facet between MC I and MC II. He
considered these features and torsion of the metacarpal shaft to have enabled advanced
“pseudo-opposability” of MC I (Napier, 1961). As mentioned (see trapezium and
trapezid sections above), I disagree with this assessment based on my interpretation of the
relationship between the trapezium and trapezoid, and my interpretation of the mobility at
trapezium-trapezoid-scaphoid joints of P. cookei. While I agree that MC I was divergent,
I estimate the divergence to have been only ~24°, which indicates little about the degree
of grasping specialization.
The large radial tuberosity of the proximal end of P. cookei’s MC I seems likely
to have received the tendon of the abductor pollicis longus muscle, a ventroflexor of the
wrist and abductor of the wrist and pollex.
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