In 1639, doctor Edward May published a 40-page text about a serpent he found in the heart of a 21-year-old man during a post-mortem examination. We don’t know exactly what it was, but there are plenty of theories.
Research:

Bush, Sargent Jr. “Bosom Serpents before Hawthorne: The Origins of a Symbol.” American Literature , May, 1971, Vol. 43, No. 2 (May, 1971). Via JSTOR. https://www.jstor.org/stable/2924236
Church, William Selby. “The Rise of Physiology in England: The Harveian Oration Delivered Before the Royal College of Physicians, October 18th, 1895.” Adlard. 1896.
Denham, D.A. “A most certaine and true relation of a strange monster or serpent found in the left ventricle of the heart of John Pennant, gentleman, of the age of 21 years.” Transactions of The Royal Society of Tropical Medicine and Hygiene, Volume 71, Issue 5, 1977, Page 455, https://doi.org/10.1016/0035-9203(77)90066-9
Eades, Bentley Gerald. “The Jacobean and Caroline Stage Vol-ii.” The Clarendon Press, Oxford, 1941.
Healy, Simon. “KYNASTON, Sir Francis (1587-c.1649), of Oteley, Ellesmere, Salop; later of St. Martin-in-the-Fields, Westminster and Covent Garden, Mdx.” he History of Parliament: the House of Commons 1604-1629, ed. Andrew Thrush and John P. Ferris, 2010. https://www.historyofparliamentonline.org/volume/1604-1629/member/kynaston-sir-francis-1587-1649
May, Edward. “A most certaine and true relation of a strange monster or serpent found in the left ventricle of the heart of John Pennant, Gentleman, of the age of 21 yeares.” London : George Miller. 1639. https://archive.org/details/b3033973x
Morris, Thomas. “The man with a snake in his heart.” http://www.thomas-morris.uk/man-snake-heart/
Pender, Stephen. “Examples and Experience: On the Uncertainty of Medicine.” The British Journal for the History of Science , Mar., 2006, Vol. 39, No. 1. Via JSTOR. https://www.jstor.org/stable/4028546
Pennant, Thomas. “The History of the Parishes of Whiteford and Holywell.” B. and J. White. 1796.
Perella, Chrissie. “Teratology: ‘Monster’ as a medical term.” Historical Medical Library of the College of Physicians of Philadelphia. https://histmed.collegeofphysicians.org/for-students/teratology-monster-as-a-medical-term/
Richardson, Ruth. “Pennant's serpent.” The Lancet. Vol. 357, Issue 9260. 3/24/2001. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(05)71674-3/fulltext
Ross, Alexander. “Arcana microcosmi.” London : T. Newcomb. 1652. https://archive.org/details/b30329140/
Slights, William W.E. “The Narrative Heart of the Renaissance.” Renaissance and Reformation / Renaissance et Réforme. Winter/Hiver 2002. https://www.jstor.org/stable/43445471
The Public Domain Review. “A Monster in the Heart: Edward May’s A Most Certaine and True Relation (1639).” https://publicdomainreview.org/collection/heart-serpent
Woolley, Benjamin. “The herbalist : Nicholas Culpeper and the Fight for Medical Freedom.” HarperCollins, 2004.
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