Chapter 2. Prehension

Chapter 6 - During Contact 209
dermis forms a negative image of the undersurface of the epidermis.
Focussing on the epidermis of the hand in Figure 6.1, we see that
below a pauillaq ridge (also called a sweat ridge or primary ridge) on
the epidermis is found an intermediate ridge on the lower surface (free
floating in connective tissue). Thus, the visible ridges on the upper
surface are accompanied by downwardly projecting ridges on the
lower surface of the epidermis. There are also downwardly projecting
ridges, called limiting: ridges (firmly fixed) below the papillary
grooves of the epidermis. Looking at the dermis, there are primary
dermal padlae (distinct from the visible papillary ridges),
interdigitating with the intermediate and limiting ridges of the
epidermis. Superimposed on these primary papillae are smaller,
secondarv dermal papillae, often found in or near the bases of the
intermediate ridges, that house Meissner corpuscles, fast adapting
cutaneous receptors. A basal lamina, or basement membrane follows
the contours of the basal cells of the epidermis, facing the dermis.
There are anchor fibers in the membrane at regular intervals,
extending into the dermis. The function of these fibers is unknown,
but, along with the interdigitations, they likely serve to resist shearing
forces. The glabrous skin characteristics described above are likely to
bond/strengthen the interface between the epidermis and the dermis, to
prevent sliding of epidermis on the dermis, to resist shearing forces
and to increase tensile strength during grasping.
On friction surfaces like the finger pads, palms of the hand and
soles of the feet, skin demonstrates unique characteristics important
for friction, force generation, compliance6 and other transducing and
mechanical properties. These include: the cells of the horny, outer
layer of the epidermis are firmly cemented together for stability; the
epidermis is characterized by unique papillary ridges; there is a thick,
hyalin7 layer (stratum lucidum) of the epidermis for strength; the most
complex interdigitations are seen at the demo-epidermal junction, for
resisting shearing forces (Montagna & Parakkal, 1974); the dermis is
characterized by specialized mechanoreceptors for transducing skin
deformation; and there are fibrous septa (a wall of fibers), passing
through the subcutaneous tissue which serve to anchor the dermis to
6Compliance and stiffness are analytic expressions describing the relationship
between forces and displacements. They will be used informally through this
chapter. Compliance, the opposite of stiffness, determines what displacements will
occur in response to a force. For a more detailed view of these concepts, we refer
the interested reader to (Mussa-Ivaldi, 1986; Salisbury 1985)
7Hyalin is a translucent protein.