In 1783, he published in Edinburgh 'Observations on the Structure and Functions of the Nervous System,' dedicated to the Right Hon. Henry Dundas, and it is in consequence of the description in this book of the communication between the lateral ventricles of the brain that his name is known to every student of medicine at the present day. The opening now always spoken of as the 'foramen of Monro' is very small in the healthy brain, but when abnormal accumulation of CSF on the brain is present (known as hydrocephalus) may be as wide as 20 mm. It was this morbid condition that drew Monro's attention to the foramen, and he first described it in a paper read before the Philosophical Society of Edinburgh in 1764, but gives a fuller account in this work on the nervous system. A further important observation in this paper was that the healthy cranial cavity is rigid and of constant volume and, he argued, that since the brain 'is nearly incompressible, the quantity of blood within the head must remain the same.' This was taken up by his former student George Kellie who set out the hypothesis that the contents of the skull (blood, CSF, and brain tissue) are a state of volume equilibrium, so that any increase in volume of one of the cranial constituents must be compensated by a decrease in volume of another. This has since been validated and is known as the Monro–Kellie doctrine.