ABDOMEN, the Belly, from abdo, to hide, because it conceals or hides its contents. The last syllable is only a termination. The Lower Belly, Imus Venter, Alma, Gaster, &c. &c., are synonymous.
The human body is divided by anatomists into three portions—the head, the trunk, and the extremities. The head and trunk enclose cavities which contain the organs or the instruments by which the most important functions of the living body are performed. The trunk forms two cavities, the superior of which is termed the Thorax or Chest, and the lower constitutes the Abdomen.
In the artificial skeleton nothing is shown, because nothing remains except the mere framework of the body, or the bones ; but in the natural state, when the soft parts remain as well as the bones, there is a complete partition between the cavity of the chest and that of the abdomen (fig. I., 1, 2). This partition is effected by means of an organ which is termed the Diaphragm (fig. I., 1, 2), a name derived from a Greek word signifying to divide. The diaphragm is composed partly of membrane, but chiefly of muscle (Fig. II.) It is placed transversely across the trunk at about its middle portion, dividing it into two pretty nearly equal halves (Fig. I.) But the dia phragm is a moveable body ; it is, in fact, one of the main organs of respiration : its chief function consists in alternately increasing and diminishing the capacity of the thorax and abdomen, for the purpose of respiration. But since the very partition which separates these two cavities from each other is perpetually changing its rela tive position—n o w encroaching upon the one, and now upon the other—it is obvious that their natural capacity must be constantly varying.
The cavity of the abdomen is above by the diaphragm (Fig. I., 1,2), below by the bones of the Pelvis or basin (Fig. L, 3, 3), which may be considered as belonging to the bones of the lower ex tremities, before and at the sides by the abdominal muscles, behind partly by the muscles of the loins, and partly by the bone of the spine. The Spine is composed of a number of separate bones, each of which is termed a Vertebra. The vertebra; are firmly united together, and by their union form what is commonly called the back-bone, termed by anatomists the Spinal Column, or the Vertebral Column.
The cavity of the abdomen is lined throughout by a thin, but dense, firm, and strong membrane, termed the Peritoneum, from a Greek word signifying to extend around.
We have thus spoken of the abdomen as a cavity, but without explanation this mode of expression may occasion misconception.
During the state of life there is co cavity. The abdomen is always no mpl etely full. It has been stated that the diaphragm alternately enlarges and diminishes the space proper to the abdomen ; but the abdominal and lumbar muscles, which form so large a part of the boundaries of the abdomen in front, at the sides and behind, in like manner, alternately contract and relax. The consequence is, that a firm and uniform pressure is at all times maintained upon the whole contents of the abdo men, so that there is always the most exact adaptation of the con taining to the contained parts, and of the viscera one to the other, not the slightest space or cavity ever intervening, either between the walls of the abdomen and its viscera, or between one viscus and another. By the cavity of the abdomen, therefore, is not meant what the expression might at first view seem to denote, namely, a void or empty space ; but the term is merely employed to mark the extent of the boundary within which the abdominal viscera are enclosed.
The abdomen, for practical purposes, is artificially mapped out into the following regions.
Two imaginary lines are drawn across the abdomen, one of which is supposed to extend from about the seventh rib on one side to the same point on the opposite side (Fig. III., 1, 1). The second line is supposed to extend from the fore part of the large bone of the pelvis to the same projecting point on the other side (2, 2). These lines mark out three large and distinct spaces (3, 4, 5). The space above the upper line is termed the Epigastric Region (3). The space below the lower line is termed the Hypogastric Region (5). The space included between the two lines is termed the Umbilical Region (4).