Alexander Monro secundus, the third and youngest son of Isabella Macdonald of Sleat, and Alexander Monro primus was born at Edinburgh on 20 May 1733. He was sent with his brothers to Mr Mundell's school, where he learned the rudiments of Latin and Greek, and showed early evidences of great ability. Among his school-fellows were Ilay Campbell who was afterwards Lord President of the Court of Session and William Ramsay of Barnton, the banker. Monro's father decided to make him his successor and sent him to the University of Edinburgh when he was 12 years old, to attend the ordinary course of philosophy before beginning his professional training. He studied mathematics under Colin Maclaurin and ethics under Sir John Pringle. He was also a favourite of Matthew Stewart, Professor of Experimental Philosophy. He showed an interest for anatomy and after entering on the medical course, aged 18, he became a useful assistant to his father in the dissecting room. He attended the lectures of Drs Rutherford, Andrew Plummer, Alston and Sinclair. He possessed an insatiable thirst for medical knowledge, an uncommon share of perseverance, and a good memory. In the session of 1753–54, his father found his class too large for the lecture room and had to divide the class, repeating his lecture in the evening. This he found difficult, and he experimented with his son in Greyfriars Kirkyard in central Edinburgh. The grave lies west of the church and north of the Adam mausoleum.
Publications
In 1771, he wrote a paper on the effect of drugs on the nervous system. He published two controversial 'observations' on the lymphatics in 1758, maintaining that he, in a short essay printed at Berlin in 1758, and reprinted in 1761 and 1770, 'De Venis Lymphaticis Valvulosis,' and not William Hunter, had first correctly described the general communications of the lymphatic system. Frederick Hoffman had, however, preceded both Monro and Hunter in the description. 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 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 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. He had always paid much attention to comparative anatomy, and published in 1785 'The Structure and Physiology of Fishes explained and compared with those of Man and other Animals.' In 1788, he published an account of seventy pairs of bursae under the title, 'Description of all the Bursae Mucosse of the Human Body, their Structure, Accidents, and Diseases, and Operations for their Cure,' which is stated by several anatomical writers to be the first full description of the bursae. In 1793, he published 'Experiments on the Nervous System with Opium and Metalline Substances, to determine the Nature and Effects of Animal Electricity.' These experiments led him to the conclusion that nerve force was not identical with electricity. His last book, 'Three Treatises on the Brain, the Eye, and the Ear,' was published at Edinburgh in 1797. Manuscript copies of notes of his lectures on anatomy delivered in 1774 and 1775 are preserved in the library of the Royal Medical and Chirurgical Society of London,' and some 'Essays and Heads of Lectures on Anatomy, Physiology, Pathology, Surgery,' very imperfectly arranged, were printed by his son Alexander in 1840.
