ABDOMEN, in Anatomy, the lower Belly, or that Part of the Body between the Hips and the Diaphragm. See BODY.

Anatomists divide the Body into three Regions, or Venters ; the Head ; the Thorax, or Breast ; and the Abdomen, or Belly, absolutely so call'd ; being the lowest Part of the Trunk. 'Tis separated from the Thorax by the Diaphragm, and reaches to the Ossa pubis. See VENTER.

It is called Abdomen, from the Latin abdo, I hide ; by reason that in its Cavity are wrapp'd up and conceal'd many of the principal Fiscera, viz. the Stomach, Omentum, Interims, Liver, Spleen, Bladder, &c. See STOMACH, OMENTUM, INTESTINES, &c.

The Abdomen is lined internally with a thin, soft Membrane ; which inverting all the Viscera above mention'd, contains and keeps 'em in their Place, call'd the Peritoneum : upon a Rupture or Dilatation whereof, they are apt to sal),. and form those Tumors call'd Hernias. See PERITONÆUM, and HERNIA.

It is cover'd and desended with five Pair of Muscles ; which not only defend the Viscera, but by their alternate Relaxations, and Contractions in Respiration, promote the Action of Digestion, and the Extrusion of the Fæces and Urine. By their Contraction, the Cavity of the Abdomen is straighten'd, and promotes the descent of the Contents of the Viscera thro' the Intestines. They are the proper Antagonists to the Sphincters or the Anus and Bladder, and forcibly expel the Excrements of thole Parts, as also the Fætus in Parturition. See RESPIRATION, DIGESTION, EXCRETION, DELIVERY, &c.

These Muscles are the Obliqui Descendentes, and Ascendentes, Linea alba, Recti, and Pyramidales ; see each under its proper Article, OBLIQUUS, RECTUS, PYRAMIDADIS, &c.

The Abdomen is subdivided into three lesser Regions, or Cavities : the uppermost, call'd the Epigastric, commences from the Diaphragm and Cartilago Ensiformis, and terminates two Fingers breadth above the Navel : The second, call'd the Umbilical, begins where the former ends, and terminates two Fingers breadth below the Navel : The third, call'd the Hypogastric, desends as low as the Os Pubis. See EPIGASTRIC, UMBILICAL, and HYPOGASTRIC.

Each of these Subdivisions, the more accurate Writers divide further into three Parts ; a middle, and two lateral ones, the Hypochondriums. The middle part of the Umbilical, is call'd the Umbilicus, or Navel; and its lateral Parts the Lumbi, Loins : The Middle of the Hypogastric, is call'd the Hypogastrium ; and its Sides the Ilia, or Flanks. See each under its proper Place, EPIGASTRIUM, HVPOCHONDRIUM, UMBILICUS, LUMBI, &c.