Chapter 2. Prehension

206 THE PHASES OF PREHENSION
hands (Hale, 1949, 1952, cited in Montagna & Parakkal, 1974).
Inked reproductions, a unique identifying characteristic or “signature”,
are called finger and palm prints. Most of us are familiar with
medical, legal and forensic applications of fingerprints obtained
through inking, otherwise called the ninhydrin reaction (Wen and von
Hofsten, 1954). Yet these papillary ridges are likely more important
for grasping than we have recognized previously. Interestingly, as
well as human hands and feet, prehensile-tailed South American
monkeys have papillary ridges on the ventral surface of the tail, and
chimpanzees and gorillas have them on the weight-bearing, knuckle
pads of the hands (Montagna & Parakkal, 1974). The function of
such patterning is similar to the ridges on an automobile tire, to
increase grip and facilitate weight bearing. The increased surface area
and pattern of asperities on soles and palms provides for a better grip
in walking and in grasping. The concentric arrangement of the ridges
makes some asperities always perpendicular to shearing forces.
In addition to papillary ridges, Napier (1980) discussed the more
pronounced flexure lines and tension lines (Langer’s lines) on the
hand. Permanent creases, “like skin hinges”, flexure lines define the
primary axes of movements about the underlying joints of the hand
(refer to Appendix A). For example, the distal palmar crease is a
composite of two lines. Used by palmists, the heart and head lines of
humans are merged into the single simian line in nonhuman primates,
reflecting the concerted motion of all four fingers perpendicular to the
palm, in palm opposition. In humans, the increased independent
mobility of the index finger effects a separation of the heart and head
lines. Tension lines (or wrinkles, comparable to Crow’s feet around
the eyes), reflect loss of elasticity in the skin and form horizontal lines
on the back of the hand, obliquely on the thenar eminence, and
vertically on the phalanges.
All skin has three layers, the surface epithelial layer, the
epidermis, the connective tissue layer, the dermis, and the deepest,
adipose tissue3 layer, the hyodermis (Eckert, 1989). Quilliam (1978)
and Eckert (1989) argued that the superficial fascia4 or hypodermis,
attached to the under surface of the dermis and the outer aspect of the
muscles and bones, should be considered part of the skin along with
the dermis and epidermis.
The epidermis or epithelial layer, about 1 mm thick in human
3Adipse tissue is an aggregation of minute cells which draw fat from the blood.
4Fascia is the thin layer of connective tissue covering, supporting, or connecting
the muscles.