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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.
the human hand
Pounding with hand-held hammer
stones has possibly been the tool-using and tool-making activity with the
greatest frequency and antiquity in hominid evolution.
It is an activity
which directs large, repetitive forces toward the central region of the palm.
Production of forceful and accurate blows by a hand-held stone requires
control of the hammer stone by firm precision grips which assure both retention
of the stone in the hand and fine adjustments in its orientation by the thumb
and fingers.
When a
mammal is motivated to learn, the brain responds
plastically.
Repetitious hand movements actually change brain
structure.
As brain maps grow larger
individual neurons get more efficient in two stages.
Initially
individual neurons within the
map grow and became more efficient.
Progressively 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 firm
precision grips 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 the stones and
to hold them firmly, exploiting the leverage of the fingers, or bracing the
stones 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 the thumb and
fingers for grasping and orienting the stones.
Grips that were found
through experimentation to accommodate and control the 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 about 500
grams, comparable in size 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, which employs all the fingers and active 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 of
these pads. |
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