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the primate hand
Versatility is the hallmark of the primate hand.
With
minor variations between species,
thirty-five joints accommodate the palm and fingers to branches and objects of
all sizes, shapes, and orientations.
Six layers of muscles produce
movements that propel the animals and effect gripping patterns used in
maintaining feeding and resting positions, securing an infant's hold on its
mother, removing parasites from the fur, catching insects, plucking fruits,
extracting foods from their source, and
positioning objects for tactile,
olfactory, and visual scrutiny.
Multiple structural constraints on mobility stabilize joints in regions
that are habitually exposed to stresses during these positional and
manipulatory activities.
The locations and configurations of these
constraints vary considerably among species, as do the relative proportions of
hand segments, reflecting the diversity of their locomotor and feeding
patterns.
The key to the
versatility of primate hands is to be found in the nature of the thumb and the
fingertips.
The thumb is structurally and functionally differentiated
from the rest of the fingers.
The tips of all five digits are
relatively broad, with moist, ridged, sensitive palmar pads that are supported
by nails.
The advantages of a
grasping hand are most apparent in the levels of the
forest where vines, bushes, and
the slender upper and outer branches of the canopy offer the least purchase to
a paw with claws on the fingertips.
Differentiation of the thumb
provides the ability for prehension of objects by
one hand.
The variety
and skill of prehensile activities depend upon the
details of joint structure, the
relative length of the thumb and fingers, the sensory nerve supply to the
distal digital pads, and the motor control of
hand movements by the
brain.
Pounding with
hand-held hammer stones has been the tool using and making activity with the
greatest frequency and antiquity in hominid evolution.
This activity
directs large, repetitive force to the central region of the palm.
Production of forceful accurate blows with a hand-held stone requires
control of the stone with a firm precision grip assuring both retention of the
stone in the hand while still allowing for fine adjustments in
orientation.
When a
mammal is motivated to learn, the brain responds
plastically.
Repetitious hand movements actually change brain
structure.
As brain maps grow individual
neurons get more efficient in two stages.
Initially
individual neurons within the
map grow and became more efficient.
Fewer neurons are required to perform the task as
the brain responds.
Stabilization of objects that are held in the
other hand and pounded by hammer stones in the production of tools also
requires a firm precision grip and the ability to vary the orientation of the
stone.
The central region of the modern human palm is stabilized,
buttressed, and protected against intrinsic and extrinsic forces associated
with the grasp and manipulation of stones in pounding by robust bones and a
fat-pad.
A secure grasp and controlled maneuvering of stones by the
thumb, fingers, and palm are facilitated by a unique pattern of hand
proportions and joint-and-muscle configurations that permit cupping of the hand
and the formation of a wide variety of grips.
The proportionately long
thumb and short fingers with broad fingertip pads are able to maneuver a stone
and to hold it firmly, exploiting the leverage of the fingers while bracing the
stone against the palm.
The unique arrangement of intrinsic musculature
and orientation of joints along the second, third, and fifth rays, favoring
rotation of the fingers, allow optimal positioning of thumb and fingers for
grasping and throwing stones.
Grips found through experimentation
to control stones most comfortably and effectively involved primarily the
thumb, index, and third fingers.
These included the pad-to-side and
three-jaw-chuck thumb/finger grips and extensions of these grips that
incorporate the palm as a passive buttress.
The three-jaw-chuck
thumb/finger grip is most effective both for wielding hammer stones and for
throwing stones.
Stones of ~ 500 grams, comparable to tennis balls, are
held by the thumb, index, and third fingers, frequently against the side of the
flexed fourth finger which in turn is buttressed by the flexed fifth finger as
a support.
The tip of the thumb and index and third fingertips control
the orientation of the stone and keep it away from the palm, so that the
leverage of these rays is exploited in propelling the stone.
The
pressure and leverage of these rays are important factors in controlling the
rotation and speed of an object thrown by the hand.
The modern human
hand structure of the joints along the fifth ray probably contributes to the
effectiveness of the finger/active-palm squeeze grip.
The
finger/active-palm squeeze grip employs all the fingers in convergence of the
palm around a cylindrical tool, such as an antler hammer, to secure it, so that
the tool functions
as an extension of the hand and/or arm.
The use of small modern
tools such as needles and pencils involves the rotation and
translation of objects by
the pads of the fingertips opposed to the tip of the thumb pad, exploiting
a unique human
compartmentalization.
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