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. [Illustration: FIG. 52.--Diagram showing the nine regions of the abdominal cavity: 1, Right hypochondriac; 2, epigastric; 3, left hypochondriac; 4, right lumbar; 5, umbilical; 6, left lumbar; 7, right iliac; 8, hypogastric; 9, left iliac. (Ashton.)] =Abdominal Regions.=--For convenience of description the abdominal cavity has been divided into nine regions by means of two transverse parallel 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 ligament on either side. These nine regions have been named as follows: The right and left hypochondriac regions up under the ribs with the epigastrium between, the right and left lumbar regions next below with the umbilical between, and the right and left inguinal with the hypogastric between. Others divide it into quadrants by one line drawn across and another down through the umbilicus. The contents of the abdomen in full are the stomach, intestines, liver, gall-bladder, spleen, pancreas, kidneys, suprarenal capsules, and the great vessels, that is, the organs of digestion and excretion. When distended the bladder extends up into the abdominal cavity, as does the uterus also when enlarged.