[4] He died unmarried on 16 July 1719 of a cancer of the mouth, and was buried in St Giles' Church, Northampton, where a monument, with a Latin inscription, was erected to his memory by his brother John.
He discussed by mathematical methods, combined with experiment, several physiological problems, such as secretion, the amount of blood in the body, muscular motion, and the force of the heart.
On the latter point he corrected an exaggerated estimate of Giovanni Alfonso Borelli; but his own results were not satisfactory, and were criticised by James Jurin in the Philosophical Transactions.
He also made a series of physiological observations on himself, after the manner of Santorio Sanctorius, published as ‘Medicina statica Britannica,’ in the third edition of his essays.
Keill's major work appeared first as ‘An Account of Animal Secretion, the Quantity of Blood in the Humane Body, and Muscular Motion,’ London, 1708; 2nd edit.