Applying himself to the study of anatomy, “Leonardo composed a book annotated in pen and ink in which he did meticulous drawings in red chalk of bodies he had dissected himself. He showed all the bone structure, adding in order all the nerves and covering them with muscles: the first attached to the skeleton, the second that hold it firm, and the third that moved it. In the various sections he wrote his observations in puzzling characters (written in reverse with the left hand) which cannot be deciphered by anyone who does not know the trick of reading them in a mirror . . . Reading Leonardo’s writings one is astonished at the brilliant way in which this inspired artist discussed so thoroughly art and anatomy (the muscles, nerves, and veins) and indeed every kind of subject.”
— Giorgio Vasari, Lives of the Artists

Leonardo also made other contributions to anatomy, especially of the nervous system, describing the frontal sinus and meningeal vessels, injecting hot wax into the brain of an ox to make a cast of the ventricles, and carrying out studies to understand how the brain processed visual and other sensory impulses.