Chapter 2. Prehension

334 CONSTRAINTS AND PHASES
The definition of prehension stresses the fact that the human hand
is both an input and output device. As an output device, it can apply
forces both to match the anticipated forces in the task and to impart
motion to the object as necessary. As an input device, it can gather
sensory information about the state of the interaction with the object
during the task in order to ensure grasping and manipulative stability.
Like the eye, the sensory and motor functions of the hand are
intertwined and indivisible -- as the hand is moving and feeling to
grasp, it is grasping to feel and move objects. Covariance of
cutaneous and articular motion provides information for haptic form
perception (Gibson, 1962).
Sensorimotor features of the hand, as described in Chapter 6,
include glabrous skin structure, eccrine glands, cutaneous
mechanoreceptors, proprioceptors, muscles, tendons, and joints. In
terms of the glabrous skin structure, properties include papillary
ridges; ability to comply, hold and resist pressure and shearing forces;
dermis and epidermis interface; fat pads; increase in stiffness to loads;
elasticity; viscoelastic properties; and response to friction. (See
Bowden 8z Tabor, 1967; Comaish 8z Bottoms, 1971; Eckert, 1989;
Montagna 8z Parakkal, 1974; Moore, 1972; Quilliam, 1978; Wilkes,
Brown 8z Wildnauer, 1973). In terms of the eccrine glands, properties
include spacing, large number in palms, periodicity, discharge during
gripping, mental responses, composition of sweat, efferent pathways,
effect on coefficient of friction, and the mechanical effect on skin in
terms of boundary lubrication creating adhesion. (See Gabella, 1976;
Jones, 1989; Kuno, 1934; Moberg, 1962; Naylor, 1955; Rothman,
1954). In terms of the cutaneous mechanoreceptors, properties
include quantity (they are numerous in the hand), morphology and
location of receptors, organization of afferent terminals in spinal cord,
receptive field size, receptor innvervation density, and adaptation
characteristics. (See Johansson, 1978; Johansson 8z Vallbo, 1983;
Vallbo 8z Johansson, 1984). In terms of proprioceptors, properties
include morphology and location of receptors (in joints, muscles, and
tendons), number, and response characteristics. (See Babu 8z
Devanandan, 1991; Devanandan, Ghosh 8z John, 1983; Devanandan,
Ghosh 8z Simoes, 1980; Sathian 8z Devanandan 1983). There is a lack
of tendon organs in the hand. With a large number and density of
muscle spindles in the hand, they seem to be implicated in perception
of direction, distinct from the perception of movement. Digital joint
mechanoreceptors are specialized particularly for detection of dynamic
mechanical changes, rather than static joint position. The traditional
view is that joint receptors respond to motion, while cutaneous