(Chapter 31) Footnote 19:

The marginal note cites Book 3 of De usu partium (where see 3.253.1-5: “the epiphyses of the thigh bone ... fit into concavities in the tibia in such a wonderful way that the contact of the bones is neither too loose nor too tight to move easily,” tr. May 1968, p. 196) and Galen’s Commentary on Hippocrates’ book “On Fractures,” where we find only the statement that the joint of the knee is secured by “several prominences and depressions” whose shallowness makes it more prone to dislocation than the ankle (18b.624, commenting on Hipp. De fract. 37.16)