In the treatise Passions of the Soul (Les passions de l'âme), the last of Descartes' published work, completed in 1649 and dedicated to Princess Elizabeth of Bohemia, the author contributes to a long tradition of theorizing "the passions." The passions were experiences often equated with or labeled as precursors to what are commonly called "emotions" in the Modern period. However, significant differences exist between what a passion putatively was and what an emotion allegedly is. For example, the passions, as suggested by the etymology of the word, were passive in nature; that is to say the experience of a passion was always caused by an object external to the subject. An emotion, as it is commonly rendered in both contemporary psychological discourse as well as popular culture, is usually explained as an event internal to, or taking place within, a subject. Therefore, an emotion is produced by the subject while a passion is suffered by the subject.
In the Passions of the Soul, Descartes defines these phenomena as follows: "perceptions or sensations or excitations of the soul which are referred to it in particular and which are caused, maintained, and strengthened by some movement of the spirits." The "spirits" mentioned here are the "animal spirits" central to Descartes's account of physiology. They function similarly to how the medical establishment now understands the nervous system. Descartes explains that the animal spirits are produced by the blood and are responsible for stimulating the body's movement.
By affecting the muscles, for example, the animal spirits "move the body in all the different ways in which it can be moved." from Wikipedia
P.S En passant, I remember watching an interview with Artur Rubinstein by Jim Lehrer on Channel 13,many years ago,when Jim asks the pianist,how much he practices and his thoughts about contemporary musicians: " I hardly practice,when I play make many mistakes, but I play avec l'âme" was his response.