Chapter 25: Abdominal walls
The xiphisternal joint is at the apex of the infrasternal angle, which is formed by costal cartilage 7 on each side. The xiphoid process extends into the angle, and the slight depression of the anterior abdominal wall in front of it is the epigastric fossa (or "pit of the stomach"). The costal margin is formed by costal cartilages 7 to 10.
The whole of the iliac crest is usually palpable. Its most superior part is situated somewhat posteriorly. The anterior superior iliac spine is frequently visible, and the posterior superior iliac spine is usually marked by a dimple. The pubic tubercle is about 2 to 3 cm lateral to the median plane.
The linea alba (a median furrow) and the linea semilunaris (lateral border of the rectus muscle) are usually evident in lean, muscular individuals on contraction of the abdominal wall. The umbilicus is usually between the L3 and L5 vertebral level, but its position is highly variable.
The inguinal ligament is in the groin and extends from the anterior superior iliac spine to the pubic tubercle. The skin crease at the junction of the abdomen and thigh lies immediately inferior and parallel to the inguinal ligament. The deep inguinal ring is immediately superior to the midinguinal point, and the superficial inguinal ring is about 1 cm superior and lateral to the pubic tubercle.
Planes and regions (fig. 25-1)
For descriptive purposes the abdomen can be divided into quadrants (left and right, upper and lower) by using the median plane and the umbilicus. It is customary to relate pains, swellings, or the positions of organs to one of these regions. Other nomenclature for abdominal planes and regions has been used in the past (see fig. 25-1) and may still be encountered from time-to-time.
Anteriorly, the abdominal wall includes the rectus abdominis muscles, and the aponeuroses of the three muscles (the external and internal oblique and the transversus) that form much of the side of the abdomen. Posteriorly, the lumbar vertebrae and intervertebral discs project anteriorward (fig. 25-2). Most of the abdominal wall is layered in the following manner: (1) skin, (2) subcutaneous tissue, (3) muscles and fasciae (or bone), (4) extraperitoneal tissue, and (5) peritoneum. The superficial part of the subcutaneous tissue is fatty; the deep part is more membranous (i.e., collagenous) and is anchored to the inguinal ligament and fascia lata, thereby limiting the spread of urine into the thigh after rupture of the urethra below the urogenital diaphragm.
The anterior part of the abdominal wall contains the rectus abdominis and pyramidalis muscles. The external and internal oblique muscles and the transversus abdominis muscle are lateral (table 25-1).
The fibers of the rectus run vertically. In general, those of the external oblique muscle (cf. the external intercostal muscles) run inferior and anterior (as in inserting a hand in a pocket), those of the internal oblique muscle (cf. the internal intercostal muscles) go mostly superior and anterior, and those of the transversus pass transversely.*
The aponeurosis of the external oblique muscle passes anterior to the rectus abdominis. Its inferior edge extends from the anterior superior iliac spine to the pubic tubercle and is known as the inguinal ligament. A part that continues posterior (toward the superior pubic ramus) is termed the lacunar ligament. A further extension laterally (along the pectin pubis of the superior pubic ramus) is called the pectineal ligament (fig. 25-7).#
Lateral to the pubic tubercle, the aponeurosis of the external oblique muscle divides into medial and lateral crura, which diverge to form the superficial inguinal ring. The superficial inguinal ring may be found by pushing the scrotal skin upward along the spermatic cord to a point immediately above the pubic tubercle, and then passing the finger backward. The ring normally admits the tip of the little finger.
The aponeurosis of the internal oblique muscle divides into anterior and posterior layers, which pass, respectively, in front of and behind the rectus muscle to reach the linea alba. The linea alba is the median, fibrous intersection of the aponeuroses, extending vertically from the xiphoid process to the pubic symphysis. The division into anterior and posterior layers is absent inferiorly, where the aponeuroses of all three muscles pass anterior to the rectus muscles to reach the linea alba. Inferiorly, the medial portion of the fused internal oblique and transversus aponeuroses is termed the conjoined tendon.
The fascia on the internal surface of the transversus abdominis serves as epimysium and is known as the transversalis fascia. It passes posterior to the rectus sheath and crosses the median plane. It is continuous with the general fascia of the abdomen to the extent that it is regarded by some as a part of the extraperitoneal connective tissue.
The rectus sheath (fig. 25-6) is described as consisting of anterior and posterior layers (lamella) formed by the aponeuroses of the external and internal oblique and transversus abdominus muscles. These aponeuroses meet at the lateral edge of the rectus along a curved line termed the linea semilunaris, which extends from the 9th costal cartilage to the pubic tubercle and is often visible in thin, muscular people. Through most of the abdomen, the aponeuroses divide to pass around either side of the rectus muscle creating an anterior and posterior sheath. The anterior sheath is comprised of the aponeurosis of the external oblique and an anterior layer of the aponeurosis of internal oblique; the posterior sheath of the posterior layer of the internal oblique and the aponeurosis of the transversus abdominis muscle. Inferior to the plane that is located approximately halfway between the umbilicus and the symphysis pubis, all three aponeuroses pass anterior to the rectus muscle. This anterior displacement of the aponeuroses creates a crescentic line of demarcation in the posterior lamella of the rectus sheath called the arcuate line, below which only the transversalis fascia separates the rectus abdominis muscle from the parietal peritoneum.
The muscles of the abdominal wall protect the viscera and help to maintain or to increase intra-abdominal pressure. They also move the trunk and help to maintain posture. The recti flex the trunk against resistance, and they can be tested by having a supine subject flex the trunk without using the arms. The obliqui and transversi increase intra-abdominal pressure and hence are important in respiration, defecation, micturition, parturition, and vomiting. The obliqui also aid in movements of the trunk.
The inguinal canal is an oblique passage through the abdominal wall. It is occupied by the spermatic cord or by the round ligament of the uterus, and it contains the ilioinguinal nerve. It is about 3 to 5 cm long (figs. 25-7, 25-8, 25-9 and 25-10 and table 25-2). The canal is potentially a weak area through which an inguinal hernia may occur. Many divergent viewpoints exist concerning the canal and its surgery, largely because of variations and a confusing terminology.
The ductus (vas) deferens curves around the lateral side of the inferior epigastric artery and is joined by nerves and vessels embedded in extraperitoneal connective tissue to form the spermatic cord. Superior to the midinguinal point, the cord traverses the deep inguinal ring. The deep inguinal ring is a slit-like opening in the transversalis fascia. The cord then runs medially and inferiorly in the inguinal canal and emerges through the superficial inguinal ring. As the spermatic cord traverses the muscular part of the internal oblique muscle, it acquires a covering of cremaster muscle and cremasteric fascia. The superficial inguinal ring is a triangular opening in the aponeurosis of the external oblique muscle (fig. 25-10A). In its course through the canal, the spermatic cord acquires sheaths from each of the layers of the abdominal wall through which it passes. The posterior wall of the inguinal canal is formed by the transversalis fascia and the aponeurosis of the transversus abdominis muscle. The anterior wall is formed by the aponeurosis of the external oblique muscle and, laterally, by internal oblique muscular fibers. Superior to the canal are the arching fibers of the internal oblique muscle and the transversus abdominis muscle, which come together to attach to the conjoined tendon (falx inguinales). The floor (inferior aspect) of the inguinal canal is formed by the inguinal and lacunar ligaments. The inferior epigastric vessels lie posterior to the canal, immediately medial to the deep ring. The inguinal triangle is formed by the inferior epigastric vessels (superolaterally), the lateral border of the rectus (medially), and the inguinal ligament (inferolaterally) (fig. 25-9).
The chief protection of the inguinal canal is muscular. The muscles that increase intra-abdominal pressure and tend to force abdominal contents into the canal at the same time tend to narrow the canal and close the rings. For example, the deep ring moves laterally and upward, closing like a shutter and making the canal longer and more oblique.
A hernia is an abnormal protrusion of a structure through tissues which normally contain it. Herniae occur most commonly in the inguinal, femoral, and umbilical regions.
Inguinal herniae are either indirect or direct. In an indirect, or oblique, inguinal hernia, abdominal contents enter the inguinal canal through the deep inguinal ring. Congenital factors, especially a partly or wholly patent processus vaginalis, are believed to be important. The layers are basically those of the spermatic cord. In a direct inguinal hernia, abdominal contents enter the inguinal canal through its posterior wall, medial to the inferior epigastric artery, i.e., within the inguinal triangle. The hernia protrudes forward toward (but rarely through) the superficial inguinal ring. Direct herniae are acquired and are due to weakness of the posterior wall. The sac is formed by peritoneum.
Femoral herniae (see fig. 25-8) are discussed with the thigh (Chapter 15).
Umbilical herniae are usually congenital and result from an incomplete closure of the abdominal wall. Ventral herniae may also occur through defects or weaknesses in the linea alba or along the linea semilunaris.
The cutaneous veins and lymphatic vessels drain in two directions from approximately the level of the umbilicus: (1) upward to the thoraco-epigastric and lateral thoracic veins (thereby providing collateral circulation in caval obstruction) and to the axillary nodes, respectively, and (2) downward to the great saphenous vein and superficial inguinal nodes, respectively. Subcutaneous veins near the umbilicus anastomose with the portal vein by way of branches along the ligamentum teres of the liver.
Apart from branches (the superficial epigastric and superficial circumflex iliac) of the femoral artery, the chief arteries of the abdominal wall are two above (the superior epigastric and musculophrenic) from the internal thoracic artery and two below (the inferior epigastric and deep circumflex iliac) from the external iliac artery. The superior epigastric artery enters the rectus sheath and descends behind that muscle. The musculophrenic artery courses along the costal margin. The inferior epigastric artery (see fig. 25-9), arising near the midinguinal point, ascends past the medial margin of the deep ring, where the ductus deferens hooks around its lateral side. As it proceeds toward the lateral edge of the rectus abdominis, it forms the superolateral boundary of the inguinal triangle. Finally the artery ascends behind the rectus in a compartment of the rectus sheath. The anastomoses between the superior and inferior epigastric arteries provide collateral circulation between the subclavian and external iliac arteries. The deep circumflex iliac artery (see fig. 32-1) proceeds laterally between the transversus and the internal oblique muscles and reaches the anterior superior iliac spine.
The abdominal wall is supplied by intercostal nerves 7 to 11 (the thoraco-abdominal nerves) and by the subcostal, iliohypogastric, and ilio-inguinal nerves (fig. 25-12). A band of skin is supplied by the lateral and anterior cutaneous branches of each of these nerves (except the ilio-inguinal, which is a branch of the first lumbar nerve). The overlap in distribution is such that section of a single nerve results in only diminished sensation in its area of supply. The lower intercostal nerves travel basically between the internal oblique muscle and the transversus abdominis. (Similarly in the thorax, the intercostal nerves run between the middle [internal intercostal] and deep [innermost intercostal] layers.)
The umbilicus (omphalos in Greek), or navel, is a median depression some distance above the pubis. It indicates the site of attachment of the umbilical cord before birth, and, even in the adult, some constituents of the cord are recognizable on the inner aspect of the abdominal wall. All layers of the abdominal wall are fused at the umbilicus, and subcutaneous fat that accumulates around the margins causes the umbilicus to appear depressed.
Congenital anomalies may be (1) alimentary, e.g., persistence of the vitello-intestinal (vitelline) duct; (2) urachal, e.g., partial or complete patency; (3) vascular, e.g., a persistent omphalomesenteric vein; or (4) somatic, e.g., faulty development of the abdominal wall, including ventral herniae, such as omphalocele (or exomphalos), which involves a protrusion of intestine through a large defect at the umbilicus.
The posterior abdominal wall is formed by the bodies and intervertebral discs of the lumbar vertebrae and by the iliopsoas muscle, quadratus lumborum muscle, iliac bone, and diaphragm (fig. 25-13). Against this background lie the aorta, inferior vena cava, kidneys, suprarenal glands, and ascending and descending colon.
The iliopsoas is the main flexor of the thigh and trunk. The iliacus arises from the iliac fossa, and the psoas major arises from the lumbar vertebrae; the combined muscle (iliopsoas) is inserted into the lesser trochanter of the femur. The psoas may be assisted by an anterior slip, the psoas minor, which arises from vertebral bodies (T12 and L1) and is inserted chiefly into the arcuate line of the ilium. Fascial continuity occurs between the transversalis and thoracolumbar fasciae, psoas sheath, and fascia iliaca. The psoas sheath, attached to the lumbar transverse processes and bodies, allows the spread of infection (e.g., a tuberculous abscess from a vertebral body) into the thigh (psoas abscess). The quadratus lumborum ascends from the iliac crest to the last rib and is anchored medially to the lumbar transverse processes. It probably flexes the trunk laterally. The iliacus is supplied by the femoral nerve, and the psoas and quadratus lumborum muscles are supplied by the lumbar plexus.
25-1 At which vertebral level is the umbilicus?
25-2 What is the direction of the fibers of the external oblique muscle?
25-3 What is the inguinal ligament?
25-4 What and where is the superficial inguinal ring?
25-5 What and where is the deep inguinal ring?
25-6 What provides the chief protection of the inguinal canal?
25-7 What are the boundaries of the inguinal triangle?
25-8 In which dermatome is the umbilicus?
25-9 What is exomphalos?
Figure 25-1 Chief planes and classic regions of the abdomen. The right and left lateral planes cross the costal margins. T, transpyloric plane.
Figure 25-2 A, Survey, or "scout," film of the abdomen, with emphasis on the lumbar vertebrae. Note the twelfth ribs, the bodies and transverse and spinous processes of vertebrae (L4 spinous process is on the supracristal plane), and gas in the large intestine, particularly in the descending colon. B, Survey film with emphasis on soft tissues. Note the kidneys, psoas major muscles, and urinary bladder. The twelfth ribs are much shorter than in A. Note the transverse processes of the lumbar vertebrae and the sacroiliac joints, sacrum, and coccyx.
Figure 25-3 External and internal oblique muscles. The fibers of one external oblique muscle are approximately parallel to those of the opposite internal oblique muscle. The lower drawing shows the line of pull of the left external oblique and the right internal oblique muscles. These muscles, acting together, flex and rotate the trunk.
Figure 25-4 Internal and external oblique muscles. The lower drawings show the external and internal oblique muscles of one side acting together in bending the trunk toward that side.
Figure 25-5 The transversus abdominis.
Figure 25-6 The rectus and its sheath. A, The rectus abdominis. Arrows indicate levels of drawings B to D, which show a common arrangement of the sheath as seen in horizontal section. The fascia transversalis is not shown separately from the transversus aponeurosis.
Figure 25-7 Inguinal ligament (inset), inguinal canal, and layers of scrotum. The main diagram combines two planes: that of the inguinal ligament and a sagittal plane through the scrotum. The separation of the layers is exaggerated. The fascia transversalis forms most of the posterior wall of the inguinal canal and is prolonged along the ductus deferens as the internal spermatic fascia.
Figure 25-8 Structure of the inguinal and femoral regions in relation to hernia. The broad arrow represents the course of the spermatic cord. The small arrow indicates the median plane. (After Maisonnet and Coudane. The sagittal sections are based largely on Lytle.)
Figure 25-9 The posterior aspect of the anterior abdominal wall. The peritoneum has been largely removed from the right side. A direct inguinal hernia would enter the inguinal canal through its posterior wall, medial to the inferior epigastric artery.
Figure 25-10 Anterior aspect of the inguinal region, progressing from outside inward, layer by layer. A, External oblique muscle and superficial inguinal ring. B, Internal oblique muscle extending medially to reach pubis. C, Transversus abdominis and fascia transversalis. D, Cut edges of iliopsoas and pectineus.
Figure 25-11 Congenital indirect inguinal herniae. A, Normal. B, Patent processus vaginalis. C, Hernia into the process. D, Funicular process, E, Hernia into funicular process.
Figure 25-12 The cutaneous distribution of the thoraco-abdominal nerves.
Figure 25-13 Muscles of the posterior abdominal wall. A, Quadratus lumborum and iliopsoas. In B, the diaphragm has been added and the arcuate ligaments have been indicated. The outline of the right suprarenal, kidney and ureter are shown. Defects in the diaphragm (diaphragmatic herniae), many of which are congenital, are most frequent around the esophageal opening (hiatal herniae). Other defects are commoner on the left than on the right side. In B, a congenital defect (vertebrocostal trigone) on the left side would result in close contact between the left kidney and pleura.
* A more complicated arrangement of the anterior abdominal wall has been proposed (N. N. Rizk, J. Anat., 131:373-385, 1980). Each abdominal aponeurosis is said to be bilaminar, and the six layers of each side are oblique and cross the median plane in a common area of decussation, namely the linea alba.
# The traditional account of inguinal anatomy has been questioned. The inguinal "ligament," which shows no ligamentous thickening, is complicated (J. F. Doyle, J. Anat., 108:297-304, 1971). W. J. Lytle maintains that the curved edge of the lacunar ligament is fixed around the medial wall of the femoral sheath and that the femoral canal is not bounded medially by the curved edge, is not a vertical funnel, and does not enter the thigh (J. Anat., 128:581-594, 1979)