DIAPHRAGM (Diaphragma), in Anatomy, the midriff, called by anatomists septum transversum. It is a strong muscular substance, separating the breast or thorax from the abdomen or lower venter, and serving as a partition between the abdominal and the thoracic viscera. (See ANATOMY, vol. iii., p. 39.) Plato, as Galen informs us, first called it diaphragm, from the verb διαφράσσειν, I separate. Till his time it had been called φρένας, from a notion that an inflammation of this part produced frenzy, which is not more warranted by experience than another tradition, that a transverse section of the diaphragm with a sword causes the patient to die laughing. The term is used analogously to denote something that divides or separates; as, for instance, the plate which divides the cavity of certain shells into two parts, and so forth.