(Chapter 19) Footnote 59:

In De usu partium Galen says “The number of the ribs articulating with the sternum tells why it is composed of seven bones; for there is one bone of the sternum for each rib.” (Tr. May 1968, p. 378). In early life the body of the human sternum consists of four segments or sternebrae; the entire sternum would thus have a total of six bones when the manubrium and the xiphoid process are included. Galen’s total of seven is based on canine and simian anatomy, as Vesalius charges. For Sylvius’ attempt to defend Galen on the number of bones in the sternum, see Sylvius 1555, pp. 83-84.