The skin of the abdomen is thin, smooth, and movable. In certain abdominal affections, such as ascites, or in pregnancy, the skin becomes stretched and glazed, and presents dark horizontal marks from stretching of the connective tissue bundles. On removal of the distending cause, these present a pale cicatricial appearance, and are known as lineae albicantes.
The subcutaneous tissues consist of two layers, a superficial continuous with the general subcutaneous fatty layer or panniculus adiposus, which varies greatly in amount in different individuals, particularly in the parts below the umbilicus, and a deeper, less fatty layer, which constitutes the deep layer of superficial fascia, and which is attached below to the crest of the ilium and Poupart's ligament, but is continued over the penis and scrotum to the perinaeum, where it forms Colles' fascia. Thus, extravasated urine may find its way up on to the abdomen, being limited to one side, however, by the median attachment, while it is kept from the thigh by the attachment to Poupart's ligament.
In very stout persons two deep transverse furrows run across the abdomen, one at the level of the umbilicus, which is thereby concealed, and the other just above the pubic fat. The point of intersection of this latter line with the linea alba indicates the position for the introduction of the trocar in tapping the bladder above the pubes.
The anterior abdominal wall is composed of the two recti muscles in front, and laterally of three muscular planes. The linea alba forms a vertical median furrow, marking the interval between the recti muscles, which are generally slightly separated above the umbilicus, but close together below it. Hence the line only extends from the ensiform to the umbilicus, or slightly beyond it. Along this line the abdominal wall is thin, aponeurotic, and free from bloodvessels, and accordingly it is a favourite site for abdominal incisions. Lying under it from above downwards are the left lobe of the liver, the stomach when distended, the transverse colon (generally above the umbilicus, but very variable), the great omentum covering the small intestines, and the bladder when distended. Spaces sometimes exist in the linea alba, through which small masses of subperitoneal fat may project and simulate irreducible herniae, and ventral hernia do occasionally protrude through it. The umbilicus is nearer the pubes than the xiphoid, and corresponds to the interval between the third and fourth lumbar vertebrae. Normally it is above the central point of the whole body. The parts are supplied with blood by small branches from the internal mammary and lower intercostals above, and by three branches of the femoral (superficial epigastric, circumflex iliac, and external pudic) below, together with twigs from the lumbar arteries and perforating branches of the deep epigastric.
Some of the superficial veins are of importance, as connecting links between the systemic and portal systems. Thus, the superficial epigastric vein is connected to the portal vein by its communication with the deep epigastric vein at the umbilicus, and so with the para-umbilical veins which run along the round ligament to the liver, where they join the portal. A small vein sometimes runs vertically from the umbilicus to the ensiform, connecting the para-umbilical with the internal mammary vein. In cases of portal obstruction the flow of blood through these veins may be greatly increased, and in consequence they may become distended and somewhat varicosed, the condition being known as Caput Medusa. The long superficial thoracico-epigastric vein communicates below with the femoral or superficial epigastric, and above with the long thoracic, and is sometimes greatly dilated, especially when there is obstruction of the vena cava. Its valves are so arranged as to direct the blood from its upper part to the axilla, and from its lower part to the thigh.
The superficial abdominal lymphatics drain the portions under the umbilicus to the inguinal glands, and those above the umbilicus to the axillary glands.
The linea semilunaris marks the outer boundary of each rectus muscle, and corresponds to the splitting of the internal oblique aponeurosis to enclose the rectus. It extends from the tip of the ninth costal cartilage in a slight curve, with the convexity outwards, toward the spine of the pubes, passing about 3 inches outside the umbilicus. It disappears some distance above the pubic spine. Here, as in the case of the linea alba, the abdominal wall is thin and comparatively bloodless, and accordingly the incision for certain operations on the gallbladder, stomach, and kidney is made through it. The recti muscles extend from the xiphoid, and fifth, sixth, and seventh costal cartilages to the crest of the pubes and symphysis. Each presents two or more lineae transversa?, or transverse tendinous intersections, the first being situated about the lowest level of the tenth rib, or about 4 inches above the second, which is situated about the level of the umbilicus. A spasmodic contraction of one of these sections of the rectus muscle, which occurs most frequently in hysterical subjects, has been mistaken for a projecting abdominal tumour. The sheath of the rectus muscle over three-fourths of the abdominal wall is derived from the aponeurosis of the internal oblique, which splits at the linea semilunaris to enclose it, the anterior division being joined by the aponeurosis of the external oblique, while the posterior division is joined by that of the transversalis. Below the level of a point midway between the umbilicus and pubic crest, however, the aponeurosis of the internal oblique does not split, but, accompanied by that of the transversalis, passes entirely in front of the rectus muscle, to form the conjoined tendon. The sheath, therefore, is deficient posteriorly from this point downwards, the lowest limit of the posterior layer forming a crescentic margin, called the fold of Douglas. Further, the aponeurosis of the external oblique also separates, and becomes distinct from the othei layers about this level. The portion of the rectus which lies on the chest-wall is only covered anteriorly by a layer of aponeurosis derived from the external oblique. The sheath of the rectus contains the deep epigastric and superior epigastric arteries, and the terminations of the six lower intercostal and last dorsal nerves.