This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The rounded form of the abdomen is influenced by its bony support, by the muscles and fascias attached to these bones, and by the organs within. In the upper portion of the abdomen the tip of the ensiform cartilage can be felt - it is opposite the eleventh dorsal vertebra. Immediately above the ensiform cartilage is its junction with the second piece of the sternum, which is opposite the tenth dorsal vertebra, - the sixth and seventh costal cartilages meet at this point, - the seventh, eighth, ninth, and tenth cartilages can be followed down to the lower border of the chest; just below this, one free rib, the eleventh, can be distinguished and sometimes in thin people the twelfth; but the twelfth is often not palpable because it is buried beneath the erector spinae muscles. The most certain way of identifying any particular rib is to count from the sternal (Ludwig's) angle, opposite the second rib.
Below, the crest of the ilium can be followed back to the posterior superior spine of the ilium and in front to the anterior superior spine. The spines of the pubes can be recognized, as well as the upper edge of the pubic bones. The depressions for the linea alba, lineae semilunares, and lineae transversae are all more marked above the umbilicus. The umbilicus lies on the disk between the third and fourth lumbar vertebrae, about 2.5 cm. (1 in.) above a line joining the highest points of the crests of the ilia. It is just below the midpoint between the symphysis and ensiform cartilage.
For clinical purposes the abdomen has been divided into regions, so that the location of tumors, signs, etc., can be readily indicated. The most convenient division is into nine regions by two transverse and two longitudinal lines. The upper transverse line passes from the tip of the tenth rib - which corresponds to the lower end of the thorax - on one side to that of the other. The lower transverse line passes from the anterior superior spine of the ilium on one side to that of the opposite; it is on a level with the second sacral vertebra. The two longitudinal lines pass directly up on each side from the middle of Poupart's ligament. They strike the cartilages of the eighth ribs, but at too indefinite a point to serve as a guide.
The middle regions are the epigastric, the umbilical, and the hypogastric, or pubic. The lateral regions are the right and left hypochondriac, the right and left lumbar, and the right and left iliac.
The abdomen is sometimes divided into four quadrants by a longitudinal median line and a transverse line through the umbilicus. This mode of division is used more by physicians than by surgeons.
The lower transverse line is drawn by Quain and Cunningham from the top of the crest of one ilium to that of the other, but as the umbilicus is often lower than usual this line may pass above it. Anderson (Morris's "Anatomy") suggests using the lineae semilunares instead of the usual longitudinal lines, but as yet this modification has not been generally accepted.