Between 1968 and 1974, the German artist Rebecca Horn produced a series of sculptures to be worn by a performer. They are not props, nor are they costumes intended primarily to create visual effects. Instead, they intervene in the corporeal perceptions of the performer, distorting his or her sense of bodily proportions and motions, and they use presentation methods that invite viewers into this experience. I refer to these works as Extensions. Because they occupy a place between felt-as-body and seen-as-object, they point to this same ambiguity for the body itself, and raise compelling questions about the body-environment boundary in terms of how it is produced in human perception. This thesis investigates how the flexibility of the spatial experience of the body, highlighted by Horn’s Extensions, is closer to the material condition of the flesh and its environment than the usual, common-sense, ending-at-the-skin perception.