n :  a flat muscle with transverse fibers that forms the innermost layer of the anterolateral wall of the abdomen and ends in a broad aponeurosis which joins that of the opposite side at the linea alba with its upper three fourths passing behind the rectus abdominis muscle and the lower fourth in front of it and that acts to constrict the abdominal viscera and assist in expulsion of the contents of various abdominal organs (as in urination, defecation, vomiting, and parturition) .
 
 
 
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