Skin and Subcutaneous Layer
Within the skin, the term Langer lines describes the orientation of dermal fibers. In the anterior abdominal wall, they are arranged primarily transversely (Fig. 38-1). As a result, vertical skin incisions sustain more lateral tension and thus, in general, develop wider scars compared with transverse skin incisions.
Langer lines of skin tension.
The subcutaneous tissue of the anterior abdominal wall lies between the skin and the rectus fascia. This tissue is separated into two layers. The superficial, predominantly fatty layer contains less fibrous tissue and is appropriately called the fatty layer of the subcutaneous tissue (formerly Camper fascia). The deeper, more fibrous layer is found closer to the rectus fascia and is named the membranous layer of the subcutaneous tissue (formerly Scarpa fascia) (Fig. 38-2). The fatty and membranous layers are not discrete layers but represent a continuum. If traced caudally, the membranous layer of the anterior abdominal wall is continuous with the membranous layer of the perineum, also known as Colles fascia.
Transverse sections of the anterior abdominal wall above (A) and below (B) the arcuate line.
The external oblique, internal oblique, and transversus abdominis muscles (flank muscles) all contain a lateral muscular component and medial fibrous aponeurotic portion. All of their aponeuroses conjoin, and these united layers create the rectus sheath, which invests the vertical muscles of the abdomen (rectus abdominis and pyramidalis muscles) (see Fig. 38-2). In the midline, the aponeurotic layers fuse to create the linea alba. The muscle fibers of the external oblique become fibrous approximately at the midclavicular line. In the lower abdomen, the muscular to the fibrous transition gradually takes place more laterally and closer to the anterior superior iliac spine. For the internal oblique and transversus abdominis muscles this same transition is seen more medially.
The anatomy of the rectus sheath changes at the arcuate line, which typically lies one third of the distance from the umbilicus to the pubic crest (see Fig. 38-2). Cephalad to the arcuate line, the rectus sheath lies both anterior and posterior to the rectus abdominis muscle. Here, the anterior rectus sheath is formed by the aponeurosis of the external oblique muscle and the split aponeurosis of the internal oblique muscle. The posterior rectus sheath is formed by the split aponeurosis of the internal oblique muscle and aponeurosis of the transversus abdominis muscle. Caudad to the arcuate line, all aponeurotic layers pass anterior to the rectus abdominis muscle. Thus, in the lower abdomen, the posterior surface of the rectus abdominis muscle is in direct contact with the transversalis fascia, described next.