During a laryngeal anatomy lecture, a graduate student is reviewing the microstructure of the vocal folds and their role in phonation. The instructor explains that the vocal folds are composed of multiple layered components, each with distinct biomechanical properties that contribute to voice production. These layers range from a pliable surface covering to deeper muscular tissue that helps control tension and vibration. One of the listed options, however, is not considered a histological layer of the true vocal folds. 

Which of the following is not a layer of the vocal folds?