Chapter 2. Prehension

210 THE PHASES OF PREHENSION
the periosteum (covering connective tissue) of the terminal phalanx
and to envelop fat cells in the finger pad, enabling shape adaptation to
the contacting object (Quilliam, 1978). Such fat pads are present in
the palmar surface of fingers, on the thenar and hypothenar
eminences, and distal to the simian crease (heartihead lines) on the
palm, but notably absent from the centre of the palm (Napier, 1980).
When applying forces in pad opposition, the fingerpads do not
displace but comply with the object's surface. When applying forces
in palm opposition, the fat free centre part of the palm facilitates
gripping. All these characteristics contribute to stable gripping, force
generating and sensing capabilities of the hand.
Mechanical properties of the skin can be considered in terms of
resting tension, nonlinear load deformation relationship, elastic, plastic
and viscoelastic properties (Moore, 1972; Wilkes et al., 1973).
Preexisting tension in the skin is reflected by retractiblity of skin in
incised wounds, and the contraction of excised skin after removal
from the body. There is nonlinear deformation; in the low load
ranges, great extensions are produced by the application of small
loads, but, as the load increases, the skin becomes progressively
stiffer (Wilkes et al., 1973). As discussed below, as load increases,
stiffness of skin increases, skin loses its ability to comply and the
coefficient of friction decreases.
Elastic fibers give skin its supple characteristics while collagen
provides strength, and resistance to stresses. Elastic fibers in the
dermis behave like an elastomer: they have a low coefficient of
elasticity. This means they can be deformed by a small force, and
then recover their original dimensions even after considerable
deformation. The collagen provides resistance to mechanical stress,
especially compression. At rest, there is a random directional
arrangement of the collagen fibers. However, when a load is applied,
the collagen fibers realign parallel to the direction of force. The
removal of epidermis and subcutaneous fat from excised skin has little
effect on the elastic modulus or strength, indicating that strength rests
in the dermis, particularly the collagen (Tregear, 1966: cited in
Montagna & Parakkal, 1974). How the dermal collagen network
regains its normal organization and returns to the resting state after
mechanical distortion is unknown, but may be a function of the elastin
fibers (Montagna & Parakkal, 1974).
Viscoelasticity is a term applied to materials such as skin that
behave like neither solids nor liquids, but have characteristics typical
of both. When a solid is subjected to load, stresses (forces acting to
deform) are produced in the solid which increase as the load is