abdomen
(redirected from Abdominal segment)Also found in: Dictionary, Thesaurus, Medical.
abdomen,
in humans and other vertebrates, portion of the trunk between the diaphragm and lower pelvis. In humans the wall of the abdomen is a muscular structure covered by fascia, fat, and skin. The abdominal cavity is lined with a thin membrane, the peritoneum, which encloses the stomach, intestines, liver, and gall bladder; the pancreas, kidneys, and urinary bladder are located behind the peritoneum. The abdomen of the female also contains the ovaries, fallopian tubes, and uterus. The navel, or umbilicus, an exterior scar on the front of the abdomen, marks the point of attachment of the fetus to the maternal organism before birth. In insects, crustacea, and some other arthropods, the term abdomen refers to the entire rear portion of the body.Abdomen
the part of the body of arthropods behind the thorax. The abdomen is clearly articulated in most arthropods; in spiders and ticks the abdominal rings coalesce. The exoskeleton of each distinct segment of the abdomen usually consists of two half-rings: the dorsal plate (tergite) and abdominal half-ring (sternite). The abdomen is connected to the thorax by either the wide (sessile abdomen) or narrow (pedicellate abdomen) anterior part. Only in higher crustaceans does the abdomen have normally developed extremities. In lower crustaceans, arachnids, and some insects, the abdomen completely lacks extremities. In lower insects (bristle-tails), the extremities of the abdomen are either rudimentary or modified into sexual appendages (at the eighth and ninth segments) or cerci (tail fila; at the 11th segment). Higher insects have only sexual appendages and cerci.
Abdomen
the portion of the trunk in man that contains the abdominal cavity and its walls.
The walls of the abdomen are formed by skin, muscles, aponeuroses, and fasciae. The posterior wall includes the spinal column (from the 12th thoracic vertebra to the fifth lumbar vertebra); anteriorly and from the sides at the upper portions of the abdomen, the walls are reinforced by the ribs (from the seventh to the 12th) and, in part, by the sternum.
The linea alba abdominis projects along the middle of the anterior wall. The linea alba, a band 2–3 mm wide (sometimes wider), extending from the xiphoid process of the sternum to the pubic symphysis, is formed by the union of the aponeuroses of the abdominal muscles. The line is broadest in the umbilical region. Above, on the anterior wall, the substernal angle, where the costal arches converge, can be distinguished; below, the iliac crests and inguinal ligaments, which separate the abdomen from the hips, stand out in relief. Two transverse lines, the upper connecting the lower points of the tenth ribs and the lower connecting the anterosuperior spines of the pelvic bones (the most prominent bony processes), are considered to divide the anterior wall of the abdomen into the epigastric, celiac, and hypogastric regions. Two longitudinal lines, coinciding with the outer margins of the rectus abdominis muscles, divide the epigastrium into the right and left subcostal regions and the epigastric region proper, the celiac region into the right and left lateral and umbilical regions, and the hypogastrium into the right and left inguinal and pubic regions. In the inguinal region, the inguinal canals (through which pass the spermatic cords of males and the round ligaments of the uterus in females) extend parallel to the inguinal ligaments on the right and left. The internal and external openings of the inguinal canal, the umbilical region, and lumbar triangle are the weakest places in the abdominal wall. Under unfavorable conditions these regions become the sites of abdominal hernias. The shape of the abdomen varies with physique, age, fat deposits, size of the viscera, and the degree of development and condition of the muscles.
V. V. KUPRIANOV