The vertebral or smaller muscle of the dia phragm is placed almost perpendicularly. The fibres pass off from the concave margin of the tendon which is turned to the spine. They run downwards and a little backwards at first, then along the lumbar vertebrae, into which they are principally inserted. The shortest and most external of them go to the internal lifT,amentuin arcuatum ; but the greater number form two large and long fasciculi, the crura, or pillars, or appendices of the diaphragm.
The right crus is longer and thicker than the left, and is nearer to the middle line. It is attached by tendinous slips to the bodies of the three (often of the four) superior lumbar ver tebrre and to the intervertebral substances. The left is attached in a similar way, but never de scends so low. Both become smaller as they pass down, the more external fibres being soonest inserted. The muscular bundles, on quitting the cordiform tendon, separate imme diately from each other, to permit the esopha gus to pass into the abdomen, and unite again behind that tube. here a crossing or inter lacing of the fibres takes place, a considerable bundle descending from the left side of the (esophagus to the right crus, and a smaller one from the right side to the left ems. In general the latter is placed anteriorly ; and occasionally two bundles descend from each side alternat ng with their opposites. The fleshy fibres again separate on a level with the lower edge of the last dorsal vertebra to allow the aorta to pass, and they continue afterwards distinct.
The foramina or openings which present themselves in this septum require to be noticed. Three large ones have been already mentioned ; but as the organs which they transmit are of great importance, they deserve more minute at tention. The first is situated in the tendon of the diaphragm, toward its posterior part, a little to the right of the centre (fig.1, c). It corre sponds to the line of division between the middle and right lobes. Its shape is quadrangular, (foramen quadratum ,) having an anterior, a pos terior, a right and a left edge. The right is the longest, the anterior the shortest, and these two often appear to form but one. The inferior vena cava passes through this opening and im mediately empties itself into the right auricle of the heart. The vein is firmly connected to the
foramen by means of thin aponeuroses sent off' from the tendinous margins; the posterior margin sending fibres upwards, the lateral downwards, and the anterior in both directions. This is the highest opening in the diaphragm, being on a level with the lower edge of the ninth dorsal vertebra and fifth rib. As the boundaries of it are entirely tendinous they cannot act on the vein themselves, and the ac tion of the muscular fibres only serves to keep it dilated. Some branches of the phrenic nerve accompany this vein.
A little to the left of the median line, and close behind the central tendon, we find an opening of an elliptical form through which the (esophagus and pneumogastric nerves pass (fig. 1, e). Its major axis, two inches in length, is di rected obliquely downwards and backwards. The borders arc entirely muscular, at least very ge nerally, for it sometimes happens that the ante rior extremity is bounded by the cordiform ten don. It results from a separation of the fibres which are descending to constitute the crura, and may be said to lie between the crura. The crossing or interlacing of the fibres which takes place just behind it must enable them to shut up this opening completely when they act strongly. This foramen is on a level with the tenth dorsal vertebra, its upper and lower an gles corresponding to the planes of the upper and lower surfaces of that bone.
About two inches below the inferior point of the (esophageal opening the aorta may be seen, coming out of the thorax, opposite the lower edge of the last dorsal vertebra (fig. 1,a.) This great vessel enters the abdomen by a canal which is formed posteriorly by hone, anteriorly by the decussating fibres, and on either side by the crura of the diaphragm. These crura, after passing along the sides of the artery, almost meet behind it by their tendinous expansions lower down. The margin of the aortic opening is bordered with tendon, and the fleshy fibres are so connected with it that their action does not at all diminish the size of the passage Along with the aorta and to its right side we see the vena azygos and thoracic duct passing into the thorax.