Chapter 2. Prehension

216 THE PHASES OF PREHENSION
thermoregulatory. However at environmental temperatures below
visible sweating, the sweat glands periodically secrete invisible sweat
beads (though these are microscopically visible, as seen in Figure
6.3). This 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). The periodicity of
insensible sweat gland secretion, studied most on the hand and
forearm, appears to be more regular on the palmar aspects of the
fingers than the dorsal aspects (Kuno, 1934, cited in Rothman, 1954).
The frequency of such bursts increases with temperature, and seems to
vary from 2 - 7 bursts/min. Figure 6.4 shows this periodicity in the
palm of the hand. Note that relative inactivity is followed by fairly
uniform discharges. Note also the marked increase in sweat discharge
when the hand is gripping a dynamometer'o, compared to the palm at
rest. The ability of an individual to apply forces using palm opposition
appears to be correlated with this sweating.
On moderate thermal stimulation, the eccrine glands of the palms
often do not respond at all. On more intense heating, palms and soles
are the last regions to show sweating. In contrast, on mental and
sensory stimulation, palmar and plantar sweat glands respond first and
immediately. Further, palms and soles sweat most abundantly in
pressure areas. Mental and sensory sweating are distinct from
thermoregulatory sweating. These types of sweating include sweat
production during mental efforts like arithmetic, as well as that elicited
by response to visual, auditory, or cutaneous stimulation. The
distinction, in terms of these regional responses of the glands and the
time scale of response, suggests different parallel pathways are
involved. Mental sweating occurs as fast as nerve conduction velocity
permits, whereas thennoregulatory sweating has a latent period,
perhaps due to the time required to raise the temperature of the skin
and blood to a critical level (Kuno, 1934, cited in Rothman, 1954).
The composition of eccrine sweat and aqueous surface film is
99% water and 1% solids (Rothman, 1954). Of the solids, one half
are inorganic salts, e.g., sodium choloride and one half are organic,
e.g., urea. Potten (1985) indicated that the secretions were rich in
lactic acid. Lobitz and Mason (1948, cited in Rothman, 1954) found a
higher concentration of most salts and organic materials (e.g.,
chlorides, urea, uric acid and ammonia) in the intermittently produced
sweat of the palm than in profusely secreted sweat. The sweat
composition in the eccrine glands of the hand should be compared
l0A dynamometer is an instrument for measuring force or energy.