13-4. Hand Grip

The skill in using fingers (digital dexterity) varies over seven ranks along the evolution of mammals to primates (examples in brackets): (1) Fused retrained fingers [horse]; (2) Nonconvergent digits [cat]; (3) Nonprehensile digits [rat]; (4) Nonopposable thumb [marmoset]; (5) Pseudo-opposable thumb [bushbaby]; (6) Power grip [chimp]; (7) Precision grip [man] (Napier and Napier, 1967). Attempts have been made to correlate this rank of digital dexterity to the size of the corticospinal tract, but in vain (Heffner and Masterson, 1975). Digital dexterity might well be related to the development of the structures involved in the cerebrocerebellar communication loop. The following studies have provided knowledge about neuronal mechanisms for the highest ranks (6 and 7) of digital dexterity.