(Chapter 20) Footnote 4:

Os cordis, a calcified plate of cartilage found in the hearts of older ruminantia and equus caballus, and cartilago cordis, found in the hearts of carnivora, sus scrofa, and equus caballus (Nomina Histologica, 2nd edn., p. 25, in Nomina Anatomica Veterinaria, 1994).

Vesalius cites book 8 of Galen’s De anat. adm. in his margin. The passage he has in mind comes in book 7 (§618-620): “The bone in the heart, which people think is present only in large animals and not in all of them, is there in others too, yet sometimes not quite as a bone but rather as a cartilage. ... An elephant of the largest size was lately killed in Rome . ... When the heart was removed by Caesar’s cooks, I sent one of my colleagues, experienced in such things, to beg the cooks to allow him to extract the bone from it. This was done and I have it to this day.” (tr. Singer 1956, pp. 186-7). Aristotle Historia animalium 506a8 states that there is a species of cattle that has a bone in its heart, as does the horse. So also in De partibus animalium 666b18; De generatione animalium 787b18 makes the same statement about some bulls.