115. The second division of the trunk, the _abdomen_, is bounded above by the diaphragm (fig. LXI. 2), below by the pelvis (fig. LXI. 3), behind and at the sides by the vertebræ and muscles of the loins (fig. LXIII.), and before by the abdominal muscles (fig. LXIII. 9). 116. The organ which forms the superior boundary of the abdomen, the diaphragm (midriff), is a circular muscle, placed transversely across the trunk, nearly at its centre (fig. LXI. 2). It forms a vaulted partition between the thorax and the abdomen (fig. LXI. 2). All around its border it is fleshy (fig. LXI. 2); towards its centre it is tendinous (fig. LXI. 2); the surface towards the abdomen is concave (fig. LXI. 2); that towards the thorax convex (fig. LXI. 2); while its middle tendinous portion ascends into the thorax as high as the fourth rib (fig. LXI. 2). [Illustration: Fig. LXI. View of the diaphragm. 1. Cavity of the thorax; 2. diaphragm separating the cavity of the thorax from that of the abdomen; 3. cavity of the pelvis.] 117. The diaphragm is perforated by several apertures, for the transmission of tubes and vessels, which pass reciprocally between the thorax and abdomen (fig. LXII.). 1. A separate aperture is formed to afford an exit from the thorax of the aorta, the common source of the arteries (fig. LXII. 2), and an entrance into the thorax of the thoracic duct, the tube that bears the digested aliment to the heart. 2. A little to the left of the former, there is another aperture, through which passes the esophagus or gullet (fig. LXII. 3), the tube that conveys the food from the mouth to the stomach. 3. On the right side, in the tendinous portion of the diaphragm, very carefully constructed, is a third aperture for the passage of the vena cava (fig. LXII. 4), the great vessel that returns the blood to the heart from the lower parts of the body. [Illustration: Fig. LXII. View of the diaphragm with the tubes that pass through it. 1. Arch of the diaphragm; 2. the trunk of the aorta passing from the chest into the abdomen; 3. the esophagus passing from the chest through the diaphragm to the stomach; 4. the vena cava, the great vein that returns the blood to the heart from the lower parts of the body, passing from the abdomen, into the chest, in its way to the right side of the heart; 5. 6. muscles that arise in the interior of the trunk and that act upon the thigh; 5. the muscle called psoas; 6. the muscle called iliacus.] 118. The partition formed by the diaphragm between the thorax and abdomen, though complete, is moveable; for as the diaphragm descends in inspiration and ascends in expiration, it proportionally enlarges or diminishes the cavities between which it is placed; consequently, the actual magnitude of these cavities varies every moment, and the size of the one is always in the inverse ratio of that of the other.