=Muscles.=--The _external oblique muscles_ form the outermost layer of the abdominal wall. They rise from the external surface of the eight lower ribs on either side and are inserted in the anterior half of the iliac crest as well as by aponeurosis in the linea alba, where each joins its fellow from the opposite side, the fibers running downward and inward like the fingers in the trouser’s pocket. Along the lower border of the aponeurosis is a broad fold, _Poupart’s ligament_. The _internal oblique_ rises on either side from the outer half of Poupart’s ligament and the anterior part of the crest of the ilium and is inserted into the crest of the os pubis, the cartilages of the lower ribs, and the linea alba. Its fibers run at right angles to those of the external oblique. These oblique muscles serve to compress the viscera, to flex the body, and also assist in expiration. The deepest of the abdominal muscles is the _transversalis_, which rises from the outer third of Poupart’s ligament and the adjoining part of the crest of the ilium, from the six lower costal cartilages, and by a broad aponeurosis, the _lumbar fascia_, from the lumbar vertebræ. It is inserted into the pubic crest and by aponeurosis into the linea alba. There is one of these muscles on either side. [Illustration: FIG. 51.--Muscles of the trunk from before (left side, superficial; and right side, deep): 1, Pectoralis major; 2, deltoid; 3, portion of latissimus dorsi; 4, serratus magnus; 5, subclavius; 6, the pectoralis, sternocostal portion; 7, serratus magnus; 12, rectus abdominis; 13, internal oblique; 14, external oblique; 15, abdominal aponeurosis and tendinous intersections of rectus abdominis; 16, over symphysis pubis; 17, linea semilunaris; 18, gluteus medius; 19, tensor vaginæ femoris; 20, rectus femoris; 21, sartorius; 22, femoral part of iliopsoas; 23, pectineus; 24, adductor longus; 25, gracilis. (Dorland’s Dictionary.)] The _rectus abdominis_ is also really two muscles and 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.