Andreas Vesalius, De humani corporis fabrica libri septem, Basel: ex. off. J. Oporini, 1543, p. 184, woodcut, leaf height 43 cm, N*.1.2(A).

Vesalius sometimes deliberately added a muscle from an animal onto the human muscle figure. For example, the muscle in the neck labeled X, Vesalius explains, is from a dog (musculus scalenus ventralis) and is not present in humans. Furthermore r, towards the top end of the abdominal muscle (musculus rectus abdominis), should be where it ends in humans. The portion above it, continuously depicted between r and s, is the unfleshed portion of a monkey’s muscle, and t above it denotes a fleshy area inserted into the first and second ribs. Modern anatomists have identified this muscle as that of the dog and the baboon.

There was a point to including these muscles from animals, since these were the ones that Galen had described as human. Vesalius corrects Galen by showing that these muscles occur in animals, but not in humans.