(Chapter 23) Footnote 15:

Beginning at this point, a passage extending to the point marked by note 22 was rewritten as follows in the 1555 version: “This is because of the head of the humerus to which you will hear that the radius is articulated. This capitulum, being situated at the outer side of the trochlea, without a very bulging projection of itself, together with the radius elegantly prevents the ulna from being dislocated outwardly. In the upper part of this trochlea, Nature has carved two large depressions: one in the anterior portion and the other in the posterior, much wider, deeper, and in every way larger than the anterior. These depressions, separated only by a scale-like layer, receive the processes that are seen in the upper end of the ulna. When the forearm is flexed, the anterior depression admits the anterior of these processes, and when the forearm is extended, the posterior depression receives the posterior process of the ulna. These depressions are the limits of extreme flexion and extension, made with such foresight that they allow the forearm to be flexed to a very sharp angle and scarcely let it extend beyond a straight line — for clearly we have no need at all of extension into an angle as we very much do of flexion, particularly since the humerus can be rotated quite loosely on the scapula and moved significantly to the posterior.”