Chapter 128

The fan-shaped muscle of the diaphragm arises from the internal circumference of the thorax, with attachments to the sternum, the lower six or seven ribs, and the lumbar vertebral bodies. The muscle fibers also attach posteriorly to the aponeurotic arch of the ligamentum arcuatum externum, which overrides the psoas and quadratus lumborum muscles. Laterally, the fibers of the diaphragm interdigitate with slips from the transversalis muscle of the abdomen to originate from the ribs.1 The right crus is larger and longer than the left and arises from the bodies of the upper three or four lumbar vertebrae. The left crus arises from the upper two lumbar vertebral bodies.

There are three natural openings within the diaphragm (Fig. 128-1). The aortic opening is the most posterior of the three and is formed from fibers comprising the right and left diaphragmatic crura.1 This tunnel is actually behind the diaphragm, not within it, and contains the aorta, azygos vein, and thoracic duct. The esophageal hiatus is slightly more ventral in relation to the aortic hiatus and consists of fibers passing between the aorta and the esophagus toward the right crus, as well as fibers converging on the pericardial tendon. The opening of the inferior vena cava lies within the confluence of the tendons of the right hemithorax and the tendon beneath the pericardium.

###### Figure 128-1.

Abdominal surface of the diaphragm with three natural openings.

During inspiration, the first rib is elevated and fixed by the scalene muscles of the neck (Fig. 128-2). The external intercostal muscles raise, in turn, each of the lower ribs. Raising these ribs, like a bucket handle that is attached to the sternum and vertebral column, enlarges the thorax and creates the negative pressure that ventilates the lung1 (Fig. 128-3).

###### Figure 128-2.

Relationship of the diaphragm to rib cage.

###### Figure 128-3.

“Bucket handle” movement of ribs on inspiration increases the thoracic space and negative thoracic pressure.

The diaphragm is the major muscle of inspiration.2 In the resting state, the central tendon is displaced cephalad into the thorax by the positive intraabdominal pressure. During contraction, the radial muscle fibers pull the tendon down toward the abdominal cavity like a drumhead. This further augments the negative intrathoracic pressure and increases the positive pressure in the abdomen. The diaphragmatic crus contributes to the magnitude of displacement of the central tendon (Fig. 128-4). In fact, if there were only circumferential attachments to the rib cage, the diaphragm would be limited in its ability to displace the lower ribs and enlarge the thoracic cavity. The thicker fascicles of the crus, which lie ...

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