Chapter 2. Prehension

4 WHAT IS PREHENSION?
The hand itself consists of five digits made up of a collection of
bones, muscles, ligaments, tendons, fascia, and vascular structures
encapsulated by skin. Thousands of sensors in the skin, muscles, and
joints let the brain know its current state. Yet, what are the functions
supported by that form? The hand of Homo supiens supiens repre-
sents millions of years of evolutionary pressures and changes. These
changes both allowed and necessitated the use of tools: the remod-
elling of the hand (e.g., shorter fingers, nails instead of claws) en-
abled the hand to grasp stones, bones, and wood, and to modify them
into functionally effective tools; but, the remodelling of the hand into a
general purpose prehensile device also created a need for those tools.
Tools became an extension of the hand. Without tools, the hand is
limited in strength and precision; with tools, the hand can either be en-
hanced in its power (as with a wrench) or in its precision (as with a
fine screwdriver or needle). If it is the brain that is envisioning the
process, then the hand is an extension of the mind.
We use our hands, as general purpose devices, to pick up objects,
to point, to climb, to play musical instruments, to draw and sculpt, to
communicate, to touch and feel, and to explore the world. One need
only examine a number of skills to discover many of the functional
postures that the hand can adopt. When a person reaches out to grasp
an object, the hand opens into some suitable shape for grasping and
manipulation - suitable, in the sense that the person’s understanding of
the task influences the shape of the hand. For example, when grasp-
ing a coffee mug by the handle, the index finger (and maybe other fin-
gers) extends to grasp the handle; the other fingers curl up against the
palm. The finger pads are used to make contact with the handle, and
then the fingers move along the surface, establishing a stable grasp.
Though invisible, complex adjustments of the fingers are made in or-
der to maintain a stable grasp, even while holding the mug. If one is
picking up the mug to drink, the size of the handle constrains the
grasp. One finger fits a teacup handle, four a beer stein. As well, the
hand posture adapted for drinking is dramatically different from that
used to throw the mug at someone else.
Some skills require that an object is grasped in order to act upon
another object. We use a screwdriver to tighten a screw into the wall
because using our hands directly would not be as effective. Of
course, under the pressure of finishing a job when the proper tools are
not available, we can use our hands (see Figure 1.1). First, grasping
the screw between our index finger and thumb tips, we insert the
screw into the hole and rotate it as we would if we were using a
screwdriver. Then, when the screw head gets too close to the wall to