Monro's most important work was his Observations on the structure and function of the nervous system, a study of the interior and exterior anatomy of the brain. It contains Monro's description of the intraventricular foramen between the lateral and third ventricle that was later given his name ('the foramen of Monro'). The Observations are illustrated with fifty plates (on forty-one leaves), drawn by Thomas Donaldson, Alexander Battoni, and A. Fyfe, engraved by Donaldson, Battoni, and G. Cameron.