The diaphragm is the muscular and tendinous partition which divides the chest from the abdomen (fig. 377).

The Diaphragm and Sub lumbar Muscles, seen from below.

Fig. 377.-The Diaphragm and Sub-lumbar Muscles, seen from below.

1, Fleshy periphery of Diaphragm. 2, Tendinous centre of Diaphragm. 3, Right Pillar. 4, Left Pillar. 5, Psoas Magnus. 6, Psoas Parvus. 7, Iliacus Internus. 8, Tensor Fasciae Latae. 9, 9, Rectus Femoris. 10, Adductor Longus. 11, Sartorius (cut to show underlying muscle). 12, Gracilis. 13, Obturator Externus. 14, Quadratus Femoris. 15, Crureus. 16, Quadratus Lumborum. 17, Hiatus Aorticus and posterior aorta. 18, Foramen Sinistrum and Œsophagus. 19, Foramen Dextrum and Vena Cava. 20, Ensiform Cartilage.

It passes obliquely downward and forward from the spine above to the sternum below. It is convex in front and concave behind. The muscular portion is situated around the circumference, and encircles a broad, flat, glistening tendon. Connected with the latter are two fleshy bundles situated in the centre towards the spine; these are known as the pillars of the diaphragm, and are united with the tendinous portion in the centre. Three openings are observed in this partition. 1. The Foramen sinistrum, through which the oesophagus or gullet passes from the chest into the abdomen. This is situated above, a little to the left of the middle line. 2. The Foramen dextrum, by which the posterior vena cava reaches the chest. It passes through the middle of the central tendon. 3. The Hiatus Aorticus gives passage to the posterior aorta, the vena azygos, and the thoracic duct. It is situated immediately beneath the spine, and is formed by the separation of the two muscular pillars.

Action. - When the diaphragm contracts it passes in a backward direction, and in so doing enlarges the chest so that the lungs may be filled with air. It is, therefore, a muscle of inspiration.