DIAPHRAGMA, in anatomy, a part popularly called the midriff, and by anatomists septum transversum. It is a nervous muscle, separating the breast or thorax from the abdomen or lower ventre, and serving as a partition between the natural and the vital parts, as they are called. See ANATOMY, no 115.

It was Plato, as Galen informs us, that first called it diaphragm, from the verb διασπάσθαι, to separate or be between two. Till his time it had been called στερεύς, from a notion that an inflammation of this part produced phreny; which is not at all warranted by experience, any more than that other tradition, that a tranverse section of the diaphragm with a sword causes the patient to die laughing.