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 system of articulation of the third bone of the thumb with the second

The second bone of the thumb [phalanx proximalis] is joined with the third [phalanx distalis] (insert G and H in fig. 2 into I and K, and M into L) differently than the first with the carpus, or the the second with the first. The lower end of the second bone is divided into two more or less round capitula separated by an elongated depression which like the capitula themselves is coated with cartilage [cart. articularis]. The upper [proximal] end [basis phalangis] of the third bone has two depressions separated by an elongated projection. These depressions receive the two capitula [caput phalangis] of the second bone, while the projection of the third bone enters the depression carved between the capitula of the second bone. In this manner the second bone enters into the third and in turn receives the third bone. Both bones are so cleverly attached [articulatio interphalangealis] by mutual entry that the third bone can only be flexed and extended but not even in the least degree moved laterally. 39 The reason why the third bone is flexed to an angle but is not extended beyond a straight line and can only be straightened from a flexed position, lies in the capitula and depressions of the joint. For the capitula, which exactly match the depressions, are moved to the inside more than the outside, and are pressed down. So obscurely are they extended to the outside and positioned there that they scarcely present the appearance of a head, and so prevent the third bone from being bent backward: as we learn is advantageous for this bone when we consider its use in flexion. At the same time we infer in what ways this joint would be weakened if it could be as easily extended and moved to the sides as it is now flexed by the great providence of Nature.



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