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 muscles of the abdomen are arranged in two distinct groups: a longitudinal group embracing the recti and pyramidales and a transverse group embracing the external and internal oblique and the transversalis of each side.
The pyramidalis is small, often undeveloped, and sometimes absent; as its direction is not markedly different from that of the rectus it may be considered from a surgical point of view as being a part of it.
Fig. 387. - The external oblique, rectus abdominis, and pyramidalis muscles.
The rectus muscle arises from the crest and symphysis of the pubis and inserts into the cartilages of the fifth, sixth, and seventh ribs and sometimes the ensiform cartilage (Fig. 387).
The rectus muscle is enclosed in a fibrous sheath formed by the external and internal oblique and transversalis muscles. The anterior layer is attached to the surface of the muscle by the lineae transversae already described (p. 372). The edge of the sheath on one side blends in the median line with that of the other side to form the linea alba. Above the umbilicus, an incision in the median line passes through fibrous tissues only and the muscles on each side are not exposed, but, as they rapidly approximate each other below, an incision usually passes either through the edge of one muscle or, if it passes exactly between them, may expose the edges of both.
The lateral edge of the sheath is formed primarily by the splitting of the tendon of the internal oblique muscle, one part going in front and the other behind the muscle. The tendon of the external oblique blends with the anterior layer of the tendon of the internal oblique a little to the medial side of the edge of the rectus, and as the pubes is approached the external oblique has its attachment nearer and nearer to the linea alba, so that close to the pubes the external oblique is separated from the internal oblique and goes to form the internal pillar of the external ring and has the conjoined tendon behind it (Fig. 388).
The tendon of the transversalis blends with the posterior layer of the internal oblique tendon until the lower fourth of the rectus is reached, when they both pass in front of the rectus to form the conjoined tendon. The medial portion of the sheath of the rectus is attached to the symphysis and crest of the pubis; its lateral portion, forming the conjoined tendon, is attached from the spine of the pubis along the iliopectineal line for the distance of 4 cm. (1 1/2 in.). It lies behind the external abdominal ring.
Fig. 388. - Sneath of the rectus abdominis muscle.
The lower edge of the posterior portion of the sheath of the rectus is called the semilunar fold of Douglas; the deep epigastric artery ascends beneath this fold about its middle, or a little to its outer side. From this arrangement it will be seen that an incision over or near the lateral edge of the rectus below the umbilicus will pass through two aponeurotic layers, viz., the external oblique and the blended tendons of the internal oblique and transversalis (Fig. 388).
If it is desired to examine the rectus muscle, its sheath can be opened at its edge and the muscle lifted up from the posterior layer, but it cannot be detached from the anterior layer above the umbilicus unless dissected loose from the lineae transversae.
The external oblique arises from the eight lower ribs. Its posterior portion passes almost directly downward to insert into the anterior half of the crest of the ilium. It is crossed obliquely by the anterior margin of the latissimus dorsi muscle a short distance above the crest, thus leaving a triangular space between them called Petit's triangle (trigonum lumbale) (see page 394). As the external oblique approaches the linea semilunaris and anterior superior spine it becomes tendinous, its fibres being nearly but not quite parallel with Poupart's ligament. Its lower edge forms Poupart's ligament (ligamentum inguinale) and continues down on the thigh as the fascia lata. Its inner portion, above and external to the spine of the pubis, divides to form the external abdominal ring for the passage of the spermatic cord. The outer side of the opening is called the external pillar or column; it is continuous with Poupart's ligament, inserts into the spine of the pubis, and is prolonged along the iliopectineal line for a short distance (2 cm.) to form Gimbernat' s ligament.
Fig. 389. - Internal oblique muscle.
Sometimes it is continuous upward and inward to the median line on the sheath of the rectus, forming what has been called the triangular fascia (Colles). The inner side is called the internal pillar or column. It inserts into the crest of the pubis. The transverse fibres passing from one pillar or column to the other are called ntercolumnar fibres. The internal oblique (Fig. 389) arises from the lumbar aponeurosis, the anterior two-thirds of the crest of the ilium, and the outer half of Poupart's ligament. It inserts into the lower three ribs and, through the sheath of the rectus and conjoined tendon, into the linea alba, the crest and spine of the pubis, and iliopectineal line for about 4 cm. The fibres arising from the lumbar aponeurosis and the posterior portion of the iliac crest pass upward and inward. Those from the region of the anterior superior iliac spine radiate like a fan; the lower ones, together with the fibres arising from the outer half of Poupart's ligament, arch over the cord and end in the conjoined tendon. Some fibres are continued down over the cord, forming the cremaster muscle. The cremaster muscle usually arises from Poupart's ligament, beneath the spermatic cord, from the lower edge of the internal oblique to near the spine of the pubes, thus obliterating the space usually shown to the under side of the cord, between it and Poupart's ligament. The fibres of the cremaster hang in loops on the cord, and are attached by their distal extremity to the pubic bone in the neighborhood of the spine. The transversalis muscle arises from the six lower ribs, through the lumbar fascia from the transverse processes of the five lumbar vertebrae, and from the anterior two-thirds of the iliac crest and outer third of Poupart's ligament. It inserts through the sheath of the rectus in the linea alba and crest of the pubis, and through the conjoined tendon into the spine of the pubis and iliopectineal line for about 4 cm. (1 1/2 in.). The transversalis does not come down so low as the internal oblique, because it arises from the outer third of Poupart's ligament instead of the outer half, as does the internal oblique. As already stated, the blended tendons of the external and internal oblique and transversalis muscles all pass in front of the rectus in its lower fourth. As the umbilicus is below the middle of the linea alba, this point, where the fold of Douglas is formed, is nearer to the umbilicus than it is to the symphysis (Fig. 390).
Fig. 390. - Transversalis muscle.