134 THE ABDOMEN. 



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 trans- 

 versalis 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 in- 

 serted 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 vertebrae and the 

 adjacent iliac crest to the last rib and the upper four 

 lumbar vertebrae. 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 vertebrae 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 



