The type of joint of the second thumb bone with the first, and its motions
The second bone of the thumb
[phalanx proximalis] (see here C in
fig. 1, which is inserted into D) is articulated with the first by that type of
Greeks call e)na/rqrwsij.
lower part of the first thumb bone ends in something like a round capitulum
[caput metacarpale] that enters the
[basis phalangis] of the second
bone. But because this head protrudes transversely, i.e., from the inner side
of the thumb more to the outside, and is more or less transversely elongated
and is depressed on the sides scarcely more than in the center, and because the
depression of the second bone is precisely fitted to this head, the second bone
of the thumb is flexed and extended more than it is bent to the sides. It could
be moved as well to the side if the capitulum of the first bone were perfectly
round, and if it were as compressed on the sides as it is on its anterior
surface. Because of the type of joint (since the capitulum is quite depressed
in its inner surface), the second bone should be capable of extreme flexion
into a very sharp angle; to prevent this, two ossicles
[ossa sesamoidea] (T in fig. 1, ch.
25) which are compared by the Greeks to a sesame seed stand in the way here.
The head of the first bone is the reason why the second bone of the thumb can
scarcely be extended beyond a straight line,