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.
There are certain lines on the abdomen, called linea alba, lineae semilunares, lineae transversae, and sometimes there are present lineae albicantes.
The linea alba passes in the median line from the ensiform cartilage to the symphysis pubis. It is formed by the fusion of the sheaths of the recti muscles. A little over half way down is the umbilicus.
The linea alba is broad and distinct above the umbilicus, separating the recti muscles a half centimetre ( 1/5 in.) or more; below, it diminishes and almost or quite disappears, leaving the recti muscles almost in contact with each other. Its fibres run longitudinally, obliquely, and transversely. The transverse fibres are the strongest and not infrequently have gaps between them which allow the subperitoneal fat to protrude and form a hernia in the median line which can be felt under the skin as a small, firm, rounded body. When these hernias are operated on they are found to be masses of subperitoneal fat with a somewhat constricted pedicle which emerges from a transverse slit in the linea alba. The peritoneum is not protruded. Some of the fibres of the linea alba are prolonged into the subcutaneous tissue and skin, thus binding it down and forming a groove distinctly visible above the umbilicus but disappearing below it. It does not long prevent extravasated urine from passing from one side to the other (Fig. 385).
Fig. 385. - Surface anatomy of the abdomen; lines and regions.
The umbilicus lies over the disk between the third and fourth lumbar vertebrae, and 2.5 to 4 cm. (1 to 1 1/2 in.) above a line joining the tops of the crests of the ilia. In the foetus it transmits the umbilical vein, the two umbilical arteries, and the remains of the vitelline duct and stalk of the allantois. The umbilical vein becomes the round ligament of the liver and is the only structure passing into the upper half of the umbilicus. The umbilical arteries within the body form the obliterated hypogastric arteries, being continuous with the superior vesicals. The vitelline duct in fetal life passes from the umbilical vesicle to the small intestine.
Normally it entirely disappears. If its proximal extremity persists it forms a Meekel's diverticulum, a projection 3 to 7 cm. long from the small intestine 1 to 3 feet above the ileocaecal valve. It may persist up to the umbilicus and cause a fistula through which feces may discharge, or form a fibrous cord which may cause a fatal strangulation of the intestine. The stalk of the allantois ends as a fibrous cord, called the urachus, running down to the fundus of the bladder. If the urachus remains patulous urine may be discharged through the umbilicus.
There are two lineae semilunares, which pass from the spines of the pubes in a curve upward and outward along the outer edges of the recti muscles to strike the chest at the ninth costal cartilage. In thin people with little subcutaneous fat their position can be seen, but in fat people, especially females, their location is not readily recognized. Ordinarily they are 6.25 to 7.5 cm. (2 1/2 to 3 in.) to the outer side of the umbilicus and midway between the anterior superior spine of the ilium and the median line. The fibrous tendon of the external oblique muscle passes on to the surface of the rectus muscle to blend with its sheath a short distance internal to its lateral border, while the internal oblique blends with the trans-versalis in the linea semilunaris; so that an incision through the latter would traverse two fibrous layers - one the expansion of the external oblique and the other the blended internal oblique and transversalis. The upper end of the right linea semilunaris indicates the position of the gall-bladder. The point where a line from the umbilicus to the right anterior superior iliac spine is crossed by the linea semilunaris is 2.5 cm. above the root of the appendix and just inside of McBurney's point, or the usual site of greatest tenderness in appendicitis.
In thin muscular people when the rectus muscle contracts grooves are seen on its surface which indicate the position of the fibrous lines called the lineae transversae. One is opposite the umbilicus, a second opposite the tip of the ensiform cartilage, a third midway between these two, and sometimes a fourth below the umbilicus. The one opposite the umbilicus is the most marked. They are adherent to the sheath of the rectus anteriorly, but pass only part way through the muscle, so that the rectus muscle can be lifted off of the posterior but not off of the anterior portion of its sheath. This fact is to be remembered in operating.
Lineae albicantes are the faint, white, atrophic lines left in the skin of the abdomen after it has been hyperdistended, usually by pregnancy or tumors.