This thesis proposes a link between the natural history of the red deer, as seen in the writings of naturalists and huntsmen, with the folklore of Western Europe. It is a confrontation of tales that are supposed to be true, since based on observable facts, with purely fictional tales. We have altered the definition of the mytheme developed by Claude Lévi-Strauss, on the basis of narrative terms (T) and their opposite (T-1), and narrative functions (F) and their opposite (F-1). We define the mytheme as being the combination of four possible versions of a tale, the TF version and the contrapositive T-1F-1 version, as well as TF-1 or T-1F variations; like Lévi-Strauss, we consider the myth along with all of its variations. Since actual objects exist independently of the uses and representations that human populations may have of them, mythemes have been formulated concerning the red deer, its place in the environment and the right ways to use its heart, its hide or its antlers to represent the passage of time or man's destiny. We have isolated some twenty mythemes and classified them in increasing order of abstraction, considering that each of them enables a relationship to be forged between narrative terms and functions, or resolves a cognitive dissonance. However, no tale may claim to connect all these mythemes together. Most are only connected in an elliptical manner, since the narration does not provide all the details necessary to establish the plausibility or veracity of the tales. To resolve this issue, we prove the existence, in Breton contemporary religious rites, of a coherent system of cardinal and intermediary directions, based on sunrise and sunset during the solstices. Based on time and direction indications, then time and calendar indications, we restore the dates dedicated to human saints to the red deer, and the positioning of worship sites in relation to one another and the shape of procession routes to the summer solstice. By extrapolation to the upper levels of the scale of space measurements, we recreate three types of ritual itineraries; at the present time, the relationship between the rite and its territorial extent does not enable any presumption regarding the social function of routes on a national or European scale. By extrapolation to the lower levels of the time scale, we recreate the “units” of calendar calculation of less than one day, and in particular the idle heart rate (60 beats / minute in man). A link is established between the time metric, between its rapid rate (heart rate) and slow rate (axial precession), and the space metric, opposing the domestic microcosm and the macrocosm limited by the “sphere of fixed stars”. The social function of the themes related to the lower levels of the measurement scales can then be deduced intuitively: heart, household, etc. A separate interpretation of summer and winter rites is then proposed, based on the mythical tales of insular Celts.