From his first book published in 1637, the Discourse on Method, to his last book issued in November 1649, the Passions of the Soul, Descartes showed a constant interest in questions of anatomy and physiology. In Discourse V, he explains the difficult question of the movement of the heart and refers favorably to Harvey's recent discovery of the circulation of blood (1628). The Dioptrics discusses the eye, focusing on the optic nerve and explaining the structure and use of the nerves (discourses III and IV). The first part of the Passions contains a summary of Descartes’ mechanical physiology as detailed in posthumously published texts: the Description of the Human Body (AT XI 223–86, extracts CSM I 313–24), an updated version of the Treatise on Man (AT XI 119–202, extracts CSM I 99–108). In the Description of the Human Body, Descartes mentions the lacteal veins and the demonstration of their existence by Asellius (Aselli) (AT XI 267) in his study of the complex question of the formation of the animal, where he insists on the formation of the heart and on the importance of the blood and the animal spirits. The Meditations and the Principles also tackle medical issues. The First Meditation alludes to melancholia, an important pathology in the seventeenth century, as evidenced by the correspondence with Princess Elisabeth. The Sixth Meditation introduces innovations in the analysis of pain and of the phantom limb syndrome (see sensation), which in no way affects the unity of the soul. The Principles IV.196, echoing a letter to Plempius (AT I 420, CSMK 64), gives an illustration that expands on the analysis of sensation given in the Dioptrics.

After the publication of the Discourse, Descartes discussed medical issues with qualified physicians, as letters to Plempius, Regius, Meyssonnier, and Vorstius illustrate. Harvey himself acknowledged the Cartesian analysis of the motion of the heart and, invoking accurate observations, elegantly rejected it (Second Reply to Jean Riolan [the Younger], 1649). This is remarkable because Descartes did not study medicine in a university. But when, at the end of 1629, he decided to study anatomy to write the Treatise, a part of The World, he read several books on the subject and performed experiments on various animals.