Cum corporis humani pars fluida a corpore externo determinatur ut in aliam mollem sæpe impingat, ejus planum mutat et veluti quædam corporis externi impellentis vestigia eidem imprimit.
English (Elwes 1883)
When the fluid part of the human body is determined by an external body to impinge often on another soft part, it changes the surface of the latter, and, as it were, leaves the impression thereupon of the external body which impels it.
Modern English
When a fluid part of the human body is determined by an external body to strike repeatedly against another soft part, it alters that soft part's surface and, as it were, impresses traces of the external body upon it.