Some medical treatises from medieval France and Italy demonstrate surprising rhetorical exuberance, especially in the paratexts where an authorial persona can emerge in the first person. This essay launches from the scatological metaphors and epithets in the exordia of works and commentaries by the physicians Archimattheus, Gilles of Corbeil, and Gentile da Foligno. In captationes benevolentiae styled after Ciceronian precepts, these authors attack their rivals before presenting their own superior science. Their scurrilous invectives—“Hoc salernitani cacantes sanitatem nominant!” (This is what the shitting Salernitans call health!); “discursores alienis fecibus imbuti” (vagrants steeped in other dreck)—tap into carnivalesque modes where excrement is organic, filthy, vituperative, and comic, in contrast to the sterility of the treatises’ technical, Scholastic discourse. A close reading of Gilles’s twelfth-century Carmina de urinarum iudiciis (Songs on Judging Urine) and Gentile’s fourteenth-century commentary on this verse treatise shows that both of these experts in uroscopy tie their excremental imagery into a nuanced poetics that extends from the paratexts into the heart of the work. Both writers demonstrate acute metaliterary sensibility, and respectable training in classical and medieval theories of rhetoric and poetry. Gilles defends his choice to write in verse through a constellation of metaphors pitting the synthetic clarity of both urine and poetry against the muddled confusion of feces and prose. He further ennobles his work by comparing the hermeneutics of uroscopy with allegorical interpretation. Gentile assumes the role of exegete, interpreting Gilles’s verses and unveiling their philosophical and theoretical underpinnings.