Chapter 2. Prehension

Chapter 6 - During Contact 219
contralateral side in response to ‘emotional stimuli’ such as a loud
whistle or tapping on the nose (Schwarz, 1937, cited in Rothman).
Without sweat, Moberg (1962) suggested friction on the surface of the
hand would become greatly reduced. We consider below in more
detail the effects of sweat on friction.
Second, denervated areas become dry, and patients often report
that tools held in palm opposition, like hammers or axes are likely to
fly out of their hands (Moberg, 1962). Denervated areas of palmar
skin do not leave prints, and do not wrinkle when soaked in warm
water (Jones, 1989; O’Riain, 1973). It would be of interest to
measure these phenomena directly and relate them to grasping deficits
in patients with peripheral neuropathies, more central lesions, and
other injury or disease.
The final point with respect to neural control of sweating concerns
the notion that nonthermal skin stimuli could be contributing to neural
control of eccrine secretions. Once tactile contact with an object is
made, sensory afferents could trigger increased sweating through
sudomotor and sudosensory integration, though we are unaware of
evidence for this. These possibilities should also be investigated.
While sweating is primarily viewed as resulting fiom innervation
of the eccrine sweat glands, local sweating activity can occur in
response to direct application of heat (in the range of skin surface
temperatures of 39-43 degrees C), even in denervated areas, in
anesthetized areas and areas where circulation has been arrested.
Randall (1947, cited in Rothman, 1954) noted that: the latent period
following heat application was usually about 1-2 minutes, there were
larger droplets and increased output from individual glands, continued
sweating during cooling, and, unlike nervous sweating, there was no
conditioning, acclimation, or difference in critical temperature to
induce nonnervous sweating in summer and winter.
How is sweating related to friction and adhesion in grasping
objects? In general, friction is reduced by lubrication between two
surfaces. A thin lubricant film of molecular proportions is referred to
as boundary lubrication, in contrast to thicker lubricant films, called
hydrodynamic or fluid lubrication (Naylor, 1955). With boundary
lubrication, friction is influenced by the nature of the underlying
surfaces as well as by the lubricant, whereas with fluid lubrication,
frictional properties will depend entirely on characteristics of the
interposed fluid layer. Invisible palmar sweat is providing boundary
lubrication. Naylor (1955) showed how the coefficient of friction
varies with surface conditions of the skin induced by environmental
conditions. Subjects were forced to sweat by sitting under blankets,