The innermost intercostal muscle is a layer of intercostal muscles. It may also be called the intima of the internal intercostal muscles. It is the deepest muscular layer of the thorax, with muscle fibres running vertically (in parallel with the internal intercostal muscles).
The most interior layer of intercostal muscles is sometimes called the innermost intercostal muscle or the internal intercostal muscle's lining. Located within the thoracic cavity, this muscular layer consists of vertical muscle fibers that align with those of the internal intercostal muscles.