CONTROL OF THE HAND IN LOWER PRIMATES

The Primatc hand preceded thc Primate brain. Prosimians have fingernails, enlarged touch-pads, long, mobile fingers, and a widely divergent thumb. Such fossils as Notharctcis show these characters were wellestablished in the Eocene. But the relative brain size of the Primates only advanced beyond that of other mammals with the appearance of true monkeys during the Oligocene. Elliot Smith (1902, 1927) first elaborated on the importance of Prosimian hands and eyes in the development of true simian intelligence. Good vision shows final spatial detail. Hands allow the animal to manipulate an object precisely, while watching lvhat it is doing. Hands, and acute eyes, are of great advantage for locomotion on branches. But branches are small, three-dimensional objects, so the arboreal specializations of Eocene Prosimians fitted them to develop more understanding of objects. There is an obvious analogy here with the evolution of man from a tool-using hominoid. The evolution of intelligence is intimately linked to the instrument of intelligence, whether hand or tool. Therefore, it is of interest to trace the control of the hand in lower Primates, and to see what relation this has to the general behavior of modern Prosimians. Prehension is the act of holding to an object. A prehensiue grip is a position of the hand while it is holding on. I shall call the position of the hand while it is reaching to take hold the prehensioe pattern. The pattern or reach is determined only by the neuro-muscular control of the hand. The final prehensive grip is, of course, determined largely by the shape of the object held. J. R. Napier (1956) has shown that human beings have two kinds of prehensive grips, clearly diff erentiatecl. In the power grip, an object is held under the first phalanges of digits 2-5, and braced against thumb and hypothenar pad. In the precision grip, the object is held between thumb-tip and index-tip, with the thumb rotated to give maximum pulp-to-pulp contact. These grips vary according to the shape of the object, but the fundamental difference between precision and power grips is determined by active, voluntary control of the hand. Even while the human hand reaches in a prehensive pattern, it is apparent whether that hand will pick up a crowbar or a pencil, with power or with precision. Prosimians, on the contrary, have only whole-hand control. They use a single prehensive pattern in a n y situation, whether they are reaching for a raisin or a branch. Their final grips, however, are determined by the shape of the object, and are even more variable than human grips. Tupaia glis, when it must reach for an object with one hand, reaches straight forward, hooks the object under its claws, and scrapes it straight