Abdomen

The abdomen is a part of the body. In humans, and in many other vertebrates, it is the region between the thorax and the pelvis (Separating the thorasic cavity and the abdominal cavity is the diaphragm). In fully developed insects, the abdomen is the third (or posterior) segment, after the head and thorax.

Vertebrates

In vertebrates, the abdomen contains several organs:

Part of the digestive system (stomach, liver, pancreas, and intestines),
The entire urinary system (kidneys, bladder).
The internal reproductive organs (ovaries, fallopian tubes and uterus in women, testis, Vas deferens and prostate in men)

The abdomen also contains some of the largest and most easily accessible blood vessels in many animals, and is often used in medicine and experimentation for catheterisation.

For various reasons, the abdomen is often coloured differently from the rest of the body. In animals with furry or hairy bodies, the abdomen is frequently hairless, or nearly so.

The abdomen is oval in shape and is the largest cavity in the body. It can be broken down into the lower and upper extremity. The lower extremity covers the inner surface of the bony pelvis. The Levator ani and Coccygeus are located on either side. The diaphragm forms the upper extremity and acts as a dome over the abdomen extending to the upper border of the fifth rib.

Invertebrates

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

Unlike other Arthropods the Insect possess no legs on the abdomen in adult form, though the 'Protura' do have rudimentary leg-like appendages on the first three abdominal segments. Many larval insects including the 'Lepidoptera' and the 'Symphyta' (Sawflies) have appendages called 'pseudo' or prolegs on their posterior 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.

Abdomen | Animal anatomy

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Vertebrates

Invertebrates