H.S. Versnel, Ter Unus: Isis, Dionysos, Hermes. Three Studies in Henotheism
(Leiden: E.J. Brill, 1990 = Studies in Greek and Roman Religion
, 6), pp. 9-10:
The human femur is a strongly curved bone, Galen taught in the second
century AD, basing his thesis on the observation of animal thighs. In the
early sixteenth century the famous Paris anatomist Sylvius refused to be
convinced of the contrary. When his pupil Vesalius finally dissected a human thigh and demonstrated ad oculos that the femur was dead straight, Sylvius sought refuge in the assumption that the tight trousers which were the fashion in his day must have gradually straightened the human
24 ῥεῖα δέ τ' ἰθύνει σκολιόν! The story is only known to me through J. Boeke, Andreas
Vesalius als hervormer der ontleedkunde, Ned. Tijdschr. v. Geneeskunde 59 (1915) 31-45,
esp. 38. Despite all their efforts, Dr. F.G. Schlesinger and Dr. H.F.J. Horstmanshoff
(to whom I owe this information) have not been able to confirm it from authentic sources.
Update from Kenneth Haynes:
The story about Galen, Sylvius, Vesalius, tight clothes, and straight bones was recorded in vol. 3 of Kurt Sprengler, Versuch einer pragmatischen Geschichte der Arzneikunde (1794), p. 526:
Auch die grosse Krümmung, welche Galen dem Oberarm und Hüftknochen gegeben hatte, verwarf Vesalius ; und Sylvius vertheidigte den Galen aus dem Grunde, weil durch die engen Kleidungsstücke heut zu Tage die Knochen grader geworden sein .
 Lib. I. c. 23. p. 92
 L. c. p. 85. — Vergl. Eustach. p. 186.
("Vesalius also rejected the strong curvature which Galen had attributed to the upper arm and the hip bone; and Sylvius defended Galen on the grounds that nowadays bones had become straighter through tight articles of clothing.")
The citations seem to correspond to unspecified editions of Vesalius’ De humani corporis fabrica and Eustachius’ Examen ossium.