Chapter 3.4 - Anterolateral Abdominal Wall PDF

# ANTEROLATERAL ABDOMINAL WALL ## OBJECTIVES At the completion of this section, the student should be able to: - Describe the muscles and fascial layers of the anterolateral abdominal wall. - Describe the formation of the rectus sheath. - Describe the innervation and vascular supply of the anterolat...

# ANTEROLATERAL ABDOMINAL WALL ## OBJECTIVES At the completion of this section, the student should be able to: - Describe the muscles and fascial layers of the anterolateral abdominal wall. - Describe the formation of the rectus sheath. - Describe the innervation and vascular supply of the anterolateral abdominal wall. - Define the inguinal ligament and associated structures. - Relate the boundaries of the inguinal canal to muscles and aponeuroses of the abdominal wall. - List the coverings of the spermatic cord and give their continuities in the abdomen. ## ABDOMINAL WALL LAYERS ### Skin The most prominent feature is the umbilicus, located midway between the xiphoid process and the pubic symphysis. In non-obese individuals, the umbilicus overlies the intervertebral disc between the 3rd and 4th lumbar vertebrae. ### Superficial Fascia This connective tissue serves as a major site for fat deposits. Inferior to the umbilicus, the fascia can usually be divided into a superficial fatty layer (commonly called Camper's fascia) and a deeper membranous layer (commonly called Scarpa's fascia). The membranous layer is continuous with the superficial perineal fascia (called Colles' fascia by clinicians) and adheres to the fascia lata of the thigh. In the midline over the pubic symphysis, the membranous fascia is thickened in the male and passes inferiorly on each side of the penis, forming the fundiform ligament of the penis. ### Muscles The combination of three flat muscles (external abdominal oblique, internal abdominal oblique, and transversus abdominis), their aponeuroses, and the strap-like rectus abdominis provide considerable protection and support to the abdominal viscera. The flat muscles are lateral flexors and rotators of the trunk that increase intra-abdominal pressure during defecation, urination, and parturition. The rectus abdominis is also a powerful flexor of the trunk. - **External oblique:** Largest and most superficial of the flat muscles, it originates from the lower eight ribs and inserts onto the iliac crest, pubic tubercle, and the linea alba (intertwining of aponeurotic fibers of the three flat muscles in the midline). Most muscle fibers are oriented medially and inferiorly. The inferior thickened border of the muscle aponeurosis forms the inguinal ligament (called Poupart's ligament by clinicians). - **Internal oblique:** Deep to the external oblique, it originates from the iliac crest and inguinal ligament. Superior and middle fibers insert onto the lower three ribs and the linea alba, respectively, and are perpendicular to fibers of the external oblique. Inferior aponeurotic fibers arch downward to insert with aponeurotic fibers of the transversus abdominis muscle onto the pubis. - **Transversus abdominis:** Deepest of the flat muscles, it originates from the lower six costal cartilages, thoracolumbar fascia, iliac crest, and inguinal ligament. Most of its fibers run transversely to insert into the linea alba, while inferior fibers combine with fibers of the internal oblique to form the sickle-shaped conjoint tendon, which inserts onto the pubis. - **Rectus abdominis:** Lying on either side of the linea alba, it arises from the superior ramus of the pubis and inserts onto the xiphoid process and the costal cartilages of ribs 5-7. This muscle is enclosed in a sheath derived from the aponeurotic fibers of the three flat muscles. ### Transversalis fascia (endo-abdominal fascia) This firm membranous sheet covers the deep surface of the transversus abdominis and its aponeurosis. ### Extraperitoneal Fat and Peritoneum The amount of extraperitoneal fat varies regionally; the peritoneum is the extensive serous membrane that lines the abdominal cavity walls (parietal peritoneum) and abdominal viscera (visceral peritoneum). # RECTUS SHEATH The fusion and separation of the aponeuroses and associated fasciae of the three flat muscles form the sheath of the rectus abdominis. The layers of the sheath vary at distinct levels: - At the level of the xiphoid process, only the aponeurosis of the external oblique is present and forms the superior portion of the anterior rectus sheath. - From just above the umbilicus to midway between that structure and the pubic symphysis, the sheath has a complex arrangement due to the splitting of the internal oblique aponeurosis into anterior and posterior layers. The anterior layer of the internal oblique and all of the external oblique aponeurosis pass anteriorly to the rectus muscle, while the posterior layer and all the transversus abdominis aponeurosis pass posteriorly. Consequently, the anterior and posterior layers of the sheath are equal in thickness. - Midway between the umbilicus and pubic symphysis, the aponeurosis of the internal oblique does not split, and all three aponeurotic layers pass anteriorly to the rectus muscle. This line of shift in the arrangement of the sheath is called the arcuate line. Inferior to the arcuate line, the posterior wall of the sheath consists of only transversalis fascia and peritoneum. - The rectus abdominis is attached to the anterior wall of the sheath at three to four tendinous intersections that interrupt the continuity of the muscle fibers. There is frequently an insignificant, small triangular muscle, the pyramidalis, which lies on the anterior surface of the inferior part of the rectus muscle. # INNERVATION AND VASCULATURE The skin and muscles of the abdominal wall are supplied almost entirely by the continuations of the lower intercostal nerves (T7-11) and subcostal nerves (T12). These nerves run between the internal oblique and transversus abdominis throughout most of their course. The iliohypogastric (L1) and ilioinguinal (L1) nerves, branches of the lumbar plexus, supply the inferior part of the abdominal wall. The dermatome around the umbilicus is innervated by T10; the xiphoid process is located at the T6 dermatome, while the pubic symphysis underlies the L₁ dermatome. The main arteries of the abdominal wall are the inferior epigastric and deep circumflex iliac arteries (branches of the external iliac artery) and the superior epigastric artery (a terminal branch of the internal thoracic artery). Other arteries include intercostal arteries, lumbar arteries, and superficial branches of the femoral artery (superficial epigastric, superficial circumflex iliac, and superficial external pudendal). The inferior epigastric artery runs superiorly (deep to the rectus abdominis) toward the umbilicus and enters the rectus sheath at the arcuate line; within the sheath, its branches anastomose with branches of the superior epigastric and and intercostal arteries. # INGUINAL REGION The inguinal ligament is the inferior free border of the external oblique aponeurosis. It extends from the anterior superior iliac spine to the pubic tubercle. Laterally the ligament provides attachments for the internal oblique and transversus abdominis muscles. Medially the fibers of the ligament flatten down and attach to the pectineal line of the pubis to form the lacunar ligament (called Gimbernat's ligament by clinicians). Superior to the pubic tubercle is a triangular cleft in the external oblique aponeurosis. This opening, which is sizeable in the male but small in the female, is the superficial inguinal ring. The superficial ring is bordered on either side by a strong band of fibers, the medial and lateral crura, reinforced by intercrural fibers along the ring's superolateral limit. The superficial ring is the exit for the inguinal canal, which transmits the spermatic cord in the male and the round ligament of the uterus in the female. This passageway is about two inches long and lies superior and parallel to the inguinal ligament. The entrance to the canal is the deep inguinal ring and located above the midpoint of the inguinal ligament and just lateral to the inferior epigastric vessels. Instead of a ring-like opening, the deep ring is a finger-like diverticulum of transversalis fascia. The boundaries of the inguinal canal reflect its oblique course through the abdominal wall: - Floor: inguinal and lacunar ligaments. - Roof: arching fibers of internal oblique and transversus abdominis. - Posterior wall: transversalis fascia and conjoint tendon. - Anterior wall: aponeurosis of external oblique. Since most of the posterior wall is formed only from transversalis fascia, this is a potential weak area of the abdominal wall. The inguinal ligament, lateral border of rectus abdominis, and inferior epigastric vessels bound this area; the triangle thus formed is called the inguinal triangle (called Hesselbach's triangle by clinicians). ## COVERINGS OF SPERMATIC CORD A ligament called the gubernaculum first indicates the site of the future inguinal canal. This ligament connects and guides the developing testis from the lumbar region to the scrotum, which is derived from skin and superficial fascia of the abdominal wall. As the testis descends, it is preceded by an evagination of peritoneum, the processus vaginalis, which carries extensions of the abdominal wall layers before it. When the testis migrates through the inguinal canal, it is covered by these extensions, which become the coverings of the spermatic cord. | Layers of Abdominal Wall | Corresponding Layers in Scrotum/Spermatic Cord | |---|---| | Skin | Skin in scrotum | | Superficial fascia | Perineal fascia & dartos muscle | | External oblique | External spermatic fascia | | Internal oblique | Cremaster muscle & fascia | | Transversus abdominis | Internal spermatic fascia | | Transversalis fascia | Areolar tissue in cord | | Extraperitoneal fat | | | Peritoneum | Tunica vaginalis | The internal spermatic fascia invests the components of the spermatic cord throughout the inguinal canal. They become covered by the cremaster muscle and fascia in the middle of the canal but receive the external spermatic fascia only as they emerge through the superficial ring. The cremaster muscle draws the testis to a more superior position in the scrotum. This is a reflex action mediated by the genitofemoral nerve (L1,2), a branch of the lumbar plexus. Besides the coverings, components of the cord include the *ductus deferens*, *deferential artery and vein*, *testicular artery*, *pampiniform plexus of veins* (network of veins that converge to form the left and right testicular veins), *lymphatics*, *autonomic nerves*, and the tiny *genital branch of the genitofemoral nerve*. The genitofemoral nerve and ilioinguinal nerve, which accompanies the cord within the inguinal canal and emerges through the superficial ring, provide anterior scrotal or labial cutaneous nerves. In females, the ovaries remain in the pelvis, but the gubernacula persist as the *round ligaments of the uterus*. The round ligaments traverse the inguinal canals in a fashion like the spermatic cords and terminate in the labia majora, which are homologous to the scrotum. # CLINICAL ANATOMY Rupture of the male urethra can lead to accumulation of urine between layers in the abdominal wall. This is due to the continuity of the superficial membranous fascia with the superficial fascia of the perineum. The *cremaster reflex* tests the integrity of the genitofemoral and ilioinguinal nerves (since both of the nerves share the L₁ dermatome of the superomedial thigh). This reflex is more active in young boys and tends to diminish with age. Accumulation of fluid in the tunica vaginalis or a patent processus vaginalis is called a *hydrocele*. *Varicocele* is a condition in which the pampiniform plexus of veins become dilated and tortuous. *Vasectomy* is the bilateral ligation of the ductus deferens. In this procedure, the ductus deferens is exposed by an incision in the superior wall of the scrotum. Protrusion of abdominal viscera through either inguinal ring constitutes an *inguinal hernia*. An indirect inguinal hernia follows the route normally taken by the testis before birth, i.e., it traverses the deep ring (lateral to the inferior epigastric vessels), the inguinal canal, and the superficial ring. Indirect inguinal hernias are often due to a patent processus vaginalis and most common in young male patients. A direct inguinal hernia protrudes directly through the abdominal wall in the inguinal triangle (medial to the inferior epigastric vessels) and passes through the superficial ring. This type of hernia occurs most commonly in adult males. The inguinal region in females has a demonstrably firmer construction since fewer and smaller structures pass through the abdominal wall. Consequently, indirect inguinal hernias are infrequent and direct inguinal hernias are rare. # SAMPLE QUESTIONS 1. Which of the following forms the anterior wall of the rectus sheath at the umbilicus? - A. Superficial membranous fascia and external oblique aponeurosis. - B. External oblique and internal oblique aponeuroses. - C. Internal oblique and transversus abdominis aponeuroses. - D. Transversus abdominis aponeurosis and transversalis fascia. - E. Transversalis fascia and peritoneum. 2. The cremaster muscle is derived from which of the following? - A. External oblique. - B. Internal oblique. - C. Superficial membranous fascia. - D. Transversus abdominis. - E. Transversalis fascia. # ANSWERS 1. The answer is **B**. In the umbilical region, the anterior layer of the internal oblique aponeurosis fuses with the external oblique aponeurosis to form the anterior wall of the rectus sheath. 2. The answer is **B**. The cremaster muscle is derived from the fibers of the internal oblique that arch over the spermatic cord. The external spermatic fascia is derived from the fascia of the external oblique, while the internal spermatic fascia is derived from transversalis fascia. The superficial membranous fascia contributes to the dartos muscle and fascia of the scrotum.