The model of the speech circuit proposed by Ferdinand de Saussure in "Course in General Linguistics" (1966 [1916]). A conversation is an exchange of vocalized mental signs. The drawing uses the oak clipart by rg1024 http://www.openclipart.org/detail/colored-oak-tree-by-rg1024
The model of the linguistic sign according to Ronald Langacker, as described in "Foundations of Cognitive Grammar" (1989). The sign is a pairing of a mental image of the concept and the mental image of the sound denoting this concept. The drawing uses the dog clipart by molumen http://www.openclipart.org/detail/13883