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. One of these
also, and this not long like the rest but short and thick, is placed
laterally. For were it not so placed all prehension would be as
impossible, as were there no hand at all. For the pressure of this
digit is applied from below upwards, while the rest act from above
downwards; an arrangement which is essential, if the grasp is to be
firm and hold like a tight clamp. As for the shortness of this
digit, the object is to increase its strength, so that it may be able,
though but one, to counterbalance its more numerous opponents.
Moreover, were it long it would be of no use. This is the
explanation of its being sometimes called the great digit, in spite of
its small size; for without it all the rest would be practically
useless. The finger which stands at the other end of the row is small,
while the central one of all is long, like a centre oar in a ship.
This is rightly so; for it is mainly by the central part of the
encircling grasp that a tool must be held when put to use.
No less skilfully contrived are the nails. For, while in man these
serve simply as coverings to protect the tips of the fingers, in other
animals they are also used for active purposes; and their form in each
case is suited to their office.
The arms in man and the fore limbs in quadrupeds bend in contrary
directions, this difference having reference to the ingestion of
food and to the other offices which belong to these parts
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