According to Lakoff et al., human beings draw upon "metaphorical expressions" to express their ideas. These are surface expressions of the "conceptual metaphors" that form the structure of a given field of knowledge. As such, the terminology and conceptual structure of one field are used to describe another. In fact, a number of studies have shown that scientific statements, particularly in the field of biomedicine, are often expressed using conceptual metaphors. Indeed, in translation, enunciation in the target language is precisely related to the question of coherence of conceptual metaphors in both source and target languages. Using examples found in English and French biomedical texts, the author analyses the mapping of metaphors in both languages. Particular attention is devoted to the analysis of some generalizations established by Lakoff and related to polysemy and the "invariance principle".