extends from the symphysis pubis to the cartilages of the fifth, sixth, and seventh ribs. At first it passes back of the oblique and transversalis muscles, but about a fourth of the way up it passes in front of the transversalis and between two layers of the internal oblique, which thereafter forms its sheath. Its chief duty is to flex the chest on the pelvis, though it also compresses the abdominal viscera.
One other muscle, a small one, is found in front, the pyramidalis, which rises from the pubic crest and is inserted into the linea alba midway to the umbilicus.
At the back the open space over the kidneys, between the lower ribs and the os innominatum, is closed in on either side by the quadratus lumborum, which extends from the three or four lower lumbar vertebræ and the adjacent iliac crest to the last rib and the upper four lumbar vertebræ. It flexes the trunk laterally or forward according as one muscle or both are used, and may aid in either expiration or inspiration.
The nerves of the abdominal muscles are chiefly the internal intercostals.
The Peritoneum.—Lining the abdominal cavity is a serous membrane, the peritoneum, which is reflected back over the viscera within in such a way as to cover each one wholly or in part. Folds of peritoneum, the omenta, connect the stomach with the other viscera, the most important being the great omentum, which has one layer descending from the anterior and another from the posterior wall of the stomach. The mesenteries are double layers of peritoneum which hold the intestines to the vertebræ and posterior wall. Between their folds run the blood-vessels.
Abdominal Regions.—For convenience of description the abdominal cavity has been divided into nine regions by means of two transverse paralled lines, the one through the ninth costal cartilages and the other just over the iliac crests, and two perpendicular parallel lines through the cartilage of the eighth rib and the middle of Poupart's
