It is the stronger of the two, and is attached by one extremity to the tip of the transverse process of the first lumbar vertebra, and by the other to the body of the second lumbar vertebra and the tendinous part of the crus. Fleshy fibres arise from both of these arcuate bands; those from the internal ligament are more numerous and better marked than those which take origin from the external ligament, and they are continuous with the fleshy fibres of the crus. Very frequently a gap or interval exists between the fibres which spring from the ligamentum arcuatum externum and those which arise directly from the last rib. The anterior, lateral and posterior attachments of the diaphragm are therefore in this manner marked off from each other. The crura of the diaphragm are two thick fleshy processes which descend upon the bodies of the upper lumbar vertebræ, tapering as they proceed downwards, and finally ending in pointed tendinous extremities. The right crus is the larger and longer of the two. It takes origin from the bodies of the upper three lumbar vertebræ, and the intervertebral discs which intervene between them. The left crus springs from the left side of the bodies of the first two lumbar vertebræ. It is much smaller, and lies upon a posterior plane to the right crus. Follow the crura upwards; opposite the last dorsal vertebra they will be observed to be connected across the middle line by a strong fibrous band which arches over the aorta. From the upper border of this fibrous arch fleshy fibres arise which join both crura, and on this account we give it the name of ligamentum arcuatum medium.1 Above this level the fleshy fibres of the crura diverge and ascend to join the posterior border of the central 1 It may be regarded as a universal law that wherever an artery pierces the origin or insertion of a muscle, and comes to lie between bone and muscular fibres, it is protected by a fibrous arch. Of this nature is the arch in question, and also the fibrous arch thrown over the superior profunda artery on the back of the humerus, and the fibrous arches in the adductor magnus for the passage of the perforating arteries, and the femoral artery itself. tendon. The innermost fibres of each crus, however, decussate so as to separate the aortic from the œsophageal openings. The decussating fasciculus of the right crus is always the larger of the two, and moreover it usually passes in front of the decussating fasciculus of the left crus. Central Tendon. The central tendon is exceedingly strong. It is composed of strong tendinous bundles running in different directions, and closely woven together so as to give it a plaited appearance. This is well seen by an inspection of its abdominal surface. In shape the central tendon resembles a trefoil leaf, of which the right lobe is the largest, and the left lobe the smallest. Upon all sides it is surrounded by muscular fibres. Those which spring from the sternum are much the shortest. Foramina of the Diaphragm.-The continuity of the diaphragm is broken by three large openings, and by some smaller apertures or fissures for the passage of the splanchnic nerves, and the vena azygos minor. The three main openings receive the names of the most important objects which they transmit. They are 1. The aortic. 2. The vena caval. The aortic opening is in the mesial plane in front of the first lumbar vertebra, and between the crura of the diaphragm. It is bounded in front by the fibrous band which arches across the middle line and connects the tendinous portions of the two crura. Behind, a fibrous expansion is prolonged inwards on each side, so as to limit the aperture posteriorly, and in this manner an almost complete fibrous ring may be formed around the aorta. The structures which pass through the aortic opening are (1) the aorta, (2) the thoracic duct, and (3) the vena azygos major—in this order from left to right. The vena caval opening is at a higher level, being situated opposite the disc between the eighth and the ninth dorsal vertebræ, in front and slightly to the right of the aortic opening. Note that it is placed in the back part of the central tendon at the junction between its middle and right lobes. Its form is somewhat quadrangular, and its margins are prolonged upon the walls of the vena cava as it passes through it. The contraction of the muscular fibres of the diaphragm will therefore tend to increase the size of this opening and the calibre of the vein which it holds. In addition to the vena cava, one or two minute twigs from the right phrenic nerve may be transmitted through the vena caval opening. The esophageal opening is an oval or elliptical foramen in the muscular part of the diaphragm. It lies in front and slightly to the left of the aortic aperture, and also at a higher level, being placed opposite the tenth dorsal vertebra. In some cases its anterior border is tendinous, and formed by the posterior margin of the central tendon. Posteriorly it is separated from the aortic opening by the decussation of the internal fibres of the crura. The oesophageal opening transmits the gullet and the two pneumogastric nerves. The three large openings of the diaphragm, therefore, present very different features. The aortic opening is a fibrous ring behind the diaphragm, and it can in no way be affected by the contraction of the muscular fibres. The vena caval opening is in the central tendon, and its margins are attached to the wall of the vessel which it transmits ; contraction of the diaphragm must therefore have a tendency to widen this aperture. The esophageal opening is placed in the muscular part, and consequently it is highly probable that the fibres which surround it may be capable of exercising a constricting influence upon the œsophagus, and in this way help to prevent regurgitation of food during the descent of the diaphragm. Little need be said regarding the smaller foramina. Each crus is pierced by the three splanchnic nerves, and the left crus is likewise perforated by the vena azygos minor. The superior epigastric artery descends in the interval between the sternal and costal attachments of the diaVOL. I.-34 phragm, and the musculo-phrenic artery pierces the costal attachment opposite the eighth or ninth rib. VESSELS ON THE POSTERIOR WALL OF THE ABDOMEN. Dissection. The abdominal aorta and its branches and the vena cava must now be cleaned. In doing this, care should be taken of the gangliated cord of the sympathetic which extends downwards on the vertebral column along the anterior border of the psoas. It is necessary to bear in mind that the lumbar branches of the aorta, as they proceed outwards, pass behind this. Separate the right crus of the diaphragm from the aorta, and dissect in the interval between them. Here the receptaculum chyli and the vena azygos major will be found. A chain of lymphatic glands, termed the lumbar glands, will be noticed in relation to the aorta. The only branches of the aorta which are liable to injury are the spermatic arteries. These are two slender arteries which spring from the front of the vessel a short distance below the renal arteries. They are so small that they are apt to be overlooked. The Abdominal Aorta (aorta abdominalis).-The abdominal aorta is the direct continuation of the thoracic aorta, and enters the abdomen through the aortic opening of the diaphragm. It begins in the mesial plane in front of the last dorsal vertebra, and it ends upon the left side of the body of the fourth lumbar vertebra by dividing into the two common iliacs. It therefore pursues an oblique course-slightly inclining to the left as it proceeds downwards. A line drawn between the highest points of the iliac crests would indicate the level of the bifurcation of the abdominal aorta; it takes place opposite the left side of the umbilicus. Most of the structures which lie in front of the abdominal aorta have been removed. In immediate relation to it from above downwards are :-(1) The solar plexus and the layer of peritoneum which forms the posterior wall of the lesser bag. (2) The pancreas and splenic vein. (3) The third part of the duodenum and the left renal vein. The peritoneum and the aortic plexus of nerves. Superficial to these it is covered by the liver, the gastro-hepatic omentum and the stomach, the transverse colon and its (4) mesentery, and by the great omentum and the coils of the small intestine. Behind, the abdominal aorta rests upon the bodies of the lumbar vertebræ and the intervertebral discs, separated from them, however, by the anterior common ligament and the left lumbar veins. On each side it is related in its upper part to the crus of the diaphragm. On the right side the inferior vena cava lies close to the aorta as high as the second lumbar vertebra, but above this it is separated from it by the fleshy part of the right crus. In the interval between the right crus of the diaphragm and the vessel, the student has already noted the receptaculum chyli and the vena azygos major. On the left side, the left gangliated cord of the sympathetic is in relation to the artery below the level of the left crus. Branches of the Abdominal Aorta.-The branches of the abdominal aorta may be described under two heads, viz. (1) Those which come off in pairs. (2) Those which arise singly. PAIRED BRANCHES. 1. Inferior phrenic arteries. 2. Middle capsular arteries. 3. Renal arteries. 4. Spermatic or ovarian arteries. 5. Lumbar arteries. SINGLE BRANCHES. I. The coeliac axis. 2. The superior mesenteric. It The single branches have already been described, with the exception of the middle sacral, which arises from the extremity of the aorta between the two common iliacs. will be studied in connection with the pelvis. The paired branches may now be examined. The Inferior Phrenic Arteries (arteriæ phrenicæ inferiores) have already been noticed upon the under surface of the diaphragm. They are two in number, and are the first branches which spring from the abdominal aorta. Diverging from each other, the artery of the right side passes behind the inferior vena cava, whilst the artery of the left side goes behind the oesophagus. Near the posterior border of the central tendon of the diaphragm |