Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 27 On the Digits of the Hand

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 joint 37 which the Greeks call e)na/rqrwsij. 38 The lower part of the first thumb bone ends in something like a round capitulum [caput metacarpale] that enters the round cavity [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,


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it being so slightly depressed there, and protruding here (as also on the sides) nearly as much as in its center. Nothing more readily teaches us how ingeniously, and for what reasons, Nature fashioned these things than the functions of the thumb, which after the construction of the bones is known suggest far more things to the minds of its beholders at first glance than anyone filling many pages with wordy explanations can tell.



Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 27 On the Digits of the Hand