The Abdominal Aorta

(Fig. XV, 24.) The thoracic aorta enters the abdominal cavity by passing beneath the middle arcuate ligament of the diaphragm at the level of the twelfth dorsal vertebra. The vessel then changes its name, and the abdominal aorta passes vertically downwards as far as the left side of the body of the fourth lumbar vertebra, at which level it bifurcates into the two common iliac arteries. The course of the vessel may be mapped out on the surface by taking a point about two fingers' breadth above the transpyloric plane and slightly to the left of the middle line, and by drawing a line vertically downwards to a second point situated 1/2 inch below and to the left of the umbilicus.

The first large vessel which arises from the abdominal aorta is the coeliac axis. (Fig. XV, 25.)

This trunk is given off at the level of the twelfth dorsal vertebra, and divides, after a course of about 1/2 inch, into three main trunks—the hepatic, splenic, and coronary or gastric arteries.

The superior mesenteric (level of disc between the twelfth dorsal and the first lumbar vertebrae) follows next, (Fig. XV, 26.) springing from the anterior aspect of the aorta immediately above the transpyloric plane.

The renals (level of the first lumbar vertebra) pass outwards from the lateral aspect of the aorta immediately below the level of the transpyloric plane. (Fig. XV, 27.)

The inferior mesenteric (level of the third lumbar vertebra) arises from the left side of the main trunk at about the level of the subcostal plane. (Fig. XV, 28.)

The common iliac artery corresponds to the upper third of a line drawn from a point 1/2 inch below, (Fig. XV, 29.) and to the left of the umbilicus to a second point situated half-way between the anterior superior iliac spine and the symphysis pubis. (Fig. XV, 31.)

The external iliac artery corresponds in direction to the lower two-thirds of this line.

The deep epigastric artery is given off from the external iliac just as that vessel passes under Poupart's ligament half-way between the anterior superior iliac spine and the symphysis pubis. (Fig. XX, 20.)

The epigastric artery then passes upwards and inwards along the inner side of the internal abdominal ring towards a point situated 1/2 inch to 1 inch outside the umbilicus, entering the rectus sheath at the level of the semilunar fold of Douglas.

This vessel forms the outer boundary of Hesselbach's triangle, (Fig. XX, 16.) the inner boundary of this space being formed by the linea semilunaris of the same side, and the base by Poupart's ligament. Each triangle is vertically subdivided into two parts by the obliterated hypogastric artery, on either side of which herniae may protrude.

The inferior vena cava is formed by the junction of the two common iliac veins on the right side of the body of the fifth lumbar vertebra, about 1 inch below and 1/2 inch to the right of the umbilicus. The vein passes upwards to pierce the quadrate opening of the diaphragm at the level of the eighth dorsal vertebra,(Fig. XV, 23.) entering the right auricle of the heart opposite the fifth right interspace and the adjoining part of the sternum.