Abdomen

The abdomen in a human and an ant.

For the human abdomen, see human abdomen.

The term abdomen is used to describe the part of the body between the thorax (chest) and pelvis in mammals and other vertebrates. In arthropods it is used to describe the most posterior section of the body, which lies behind the thorax or cephalothorax. It is also known as the belly. The region enclosed by the abdomen is termed the abdominal cavity.

Vertebrates

In vertebrates the abdomen is a large cavity enclosed by the abdominal muscles on the ventral and lateral aspects and the spinal column on the dorsal aspect. The lower ribs may also form some of the ventral wall. The abdominal cavity is separated from the thoracic cavity by the diaphragm and is continuous with the pelvic cavity. Structures which pass into the abdominal cavity from the thoracic cavity must do so by passing through the diaphragm. This includes the aorta, vena cava and the esophagus. Both the abdominal cavity and pelvic cavity are lined by a serous membrane, known as the parietal peritoneum. This membrane is reflected onto the viscera of the abdominal cavity and becomes known as the visceral peritoneum In vertebrates, the abdomen contains the abdominal organs. These are typically the majority of the digestive tract and parts of the urinary system. Below is a list of viscera (internal organs) typically found within the abdominal cavity:

• Components of the Digestive tract: stomach, small intestine, large intestine, cecum and appendix.
• Accessory organs of the digestive tract: liver, gallbladder and pancreas.
• Organs of the Urinary system: kidneys, ureters and bladder.
• Other organs: Spleen

These organs can be highly specialized in some animals, for example the stomach of the suborder of mammals known as ruminants it is divided into four chambers, known as the rumen, reticulum, omasum, and abomasum, to aid with digestion.

Invertebrates

In the worker ant the abdomen (metasoma) is composed of the gaster and petiole.

The invertebrate abdomen is built up of a series of concave upper plates known as tergites and convex lower plates known as sternites, the whole being held together by a tough yet stretchable membrane.

The abdomen contains the insect's digestive tract and reproductive organs, it consists of eleven segments in most orders of insects though the eleventh segment is absent in the adult of most higher orders. The number of these segments does vary from species to species with the number of segments visible reduced to only seven in the common honeybee. In the Collembola (Springtails) the abdomen has only six segments.

Unlike other Arthropods, insects possess no legs on the abdomen in adult form, though the Protura do have rudimentary leg-like appendages on the first three abdominal segments, and Archaeognatha possess small, articulated "styli" which are sometimes considered to be rudimentary appendages. Many larval insects including the Lepidoptera and the Symphyta (Sawflies) have fleshy appendages called prolegs on their abdominal segments (as well as their more familiar thoracic legs), which allow them to grip onto the edges of plant leaves as they walk around.

See also

Human abdomen

Opisthosoma

Metasoma