He then studied medicine at the University of Montpellier, and afterwards surgery at the Hôpital de la Charité in Paris.

Between 1693 and 1713 he was a military physician in the armies of Louis XIV, and after the Peace of Utrecht (1713), he returned to Paris as an eye specialist.

[1] Petit is remembered for his detailed anatomical studies of the eye, as well as physiological research of the sympathetic nervous system.

As a military physician, Petit noticed that there was a striking correlation between soldiers' head wounds and contralateral motor effects, which he documented in a 1710 treatise called Lettres d’un medecin des hopitaux du roi a un autre medecin de ses amis.

[3] He performed pioneer investigations on the internal structure of the spinal cord, and gave an early, detailed description of the decussation of the pyramids.