Abdomen

The abdomen is that part of the trunk which lies below the thorax. Like the thorax, the abdomen has a cavity which is enclosed by walls. The walls of the abdomen can be conveniently divided into three parts an upper part formed by the lower portions of the thoracic cage, an intermediate portion comprising the anterolateral and posterior abdominal walls, and a lower portion formed by the bony pelvis with its muscular and ligamentous attachments. The cavity of the abdomen is limited superiorly by the diaphragm and inferiorly by another sheet of muscle placed across the bony pelvis, the pelvic diaphragm. The inlet of the pelvis (pelvic brim) or linea terminalis is used as a landmark to divide the cavity of the abdomen into an upper abdominal cavity proper and a lower pelvic cavity. It must be remembered that these two cavities are functionally and developmentally one unit. The serous membrane lining the inner aspect of the abdominal wall is called the parietal peritoneum, while that covering the viscera is known as the visceral peritoneum. The space between these two layers is known as the peritoneal cavity which is a capillary interval containing a film of lubricating fluid. The peritoneal cavity acts as one enormous bursa which permits varying degrees of freedom of movement for the abdominal and pelvic organs. The disposition of the abdominal viscera can be more easily understood if one visualises the arrangement of these viscera in the early embryo. In the embryo, the most ventral structure filling the abdominal cavity is the liver, which lies ventral to the gut. Dorsolateral to the gut are those structures developed from the intermediate mesoderm such as the kidneys and suprarenals. Dorsal to the gut are the large vessels, i.e., aorta and inferior vena cava. In the adult, the arrangement is similar to that in the embryo except that the space occupied by the liver is smaller, with the result that the intestines have come ventrally to abut against the anterior abdominal wall. This disposition of the intestines is also facilitated by the presence of long folds of peritoneum, mesenteries, which allow them greater freedom of movement. Those structures which are not provided with a fold of peritoneum and which lie against the posterior abdominal wall behind the peritoneum are said to be retroperitoneal. Finally, it must be remembered that the mobile diaphragm causes the abdominal viscera to move with respiration. This is most marked in those organs lying directly under the diaphragm and is of clinical importance.

ANTERIOR ABDOMINAL WALL AND EXTERNAL GENITALIA

Surface Anatomy of the Anterior Abdominal Wall

1. With the body lying on its back and with a skeleton available for reference, begin the study of the abdomen by examining the surface of the abdominal wall on the cadaver. At a convenient moment, revise what you learn on your own body.

2. The regions of the abdomen The abdomen is divided into regions, with the point of your scalpel lightly draw a line transversely across the body through a point midway between the jugular notch of the sternum and the upper border of the pubic symphysis. This line represents the transpyloric plane, which passes through the lower border of the first lumbar vertebra. Draw a further horizontal line across the body at the level of the tubercles of the iliac crests. This is known as the transtubercular plane. Draw a vertical line upwards on each side, commencing from a point midway between the pubic symphysis and the anterior superior iliac spine (mid-inguinal point). In this way, the abdomen is divided into nine regions. These are, from above downwards: the epigastric, umbilical, pubic in the middle, flanked on either side by the hypochondriac, lateral and inguinal regions. The transpyloric plane marks the position of the pylorus of the stomach, which lies 1 to 2 cm to the right of the midline; the blind end or fundus of the gall bladder; the hila of the kidneys; and the lowest limit of the spinal cord. In a young healthy adult, the umbilicus lies at the level of the intervertebral disc between the third and fourth lumbar vertebrae. As age advances and fat is deposited, the umbilicus tends to lie below this level.

Superficial Anterior Abdominal Wall

3. Make the following skin incisions:

(a)a vertical incision along the midline from the xiphoid process to the upper margin of the pubic symphysis encircling the umbilicus;

(b) from the pubic symphysis laterally to the pubic tubercle and thence a curved incision along the fold of the groin to the anterior superior iliac spine (if not already made); and

(c) from the anterior superior iliac spine along the iliac crest as far as the level of the posterior axillary line. Reflect the skin flaps.

(a)Next make a vertical incision through the superficial fascia in the midline of the body extending from the xiphoid process to the symphysis pubis.

(b) Make a horizontal incision extending from the anterior superior iliac spine to the midline of the body. The superficial fascia below the horizontal incision has a superficial fatty and a deeper membranous layer.

(c) Insert a finger deep to the membranous layer and in front of the underlying aponeurotic portion of the external oblique muscle. Separate the membranous layer from the muscle inferiorly.

(d) Identify the spermatic cord (in the male) or the round ligament of the uterus (in the female) above and lateral to the pubic tubercle. Note that the finger can be passed medial to the cord or round ligament into the perineum, the subcutaneous perineal pouch. However the finger cannot enter the thigh laterally because of the fusion of the membranous layer to the deep fascia of the thigh, the fascia lata.

5. Reflect the fascia of the abdominal wall laterally. Note that the anterior cutaneous branches of the lower five intercostal, subcostal (T12) and iliohypogastric (L1) nerves become cutaneous about 3–4 cm from the midline. The seventh intercostal nerve supplies the skin over the xiphoid while the tenth intercostal nerve supplies the skin around the umbilicus.

6. Note the lateral cutaneous branches of the lower five intercostal nerves and the lateral cutaneous branches of the subcostal and iliohypogastric nerves along the midaxillary line. Branches of the latter two nerves cross the iliac crest to pass into the gluteal region.

Muscles of the Anterior Abdominal Wall

7. Remove the remains of the superficial fascia and clean the first muscle layer, which is the external oblique muscle. Note the absence of deep fascia over the muscle. Why is the deep fascia absent over the anterior abdominal wall?

8. The origin of the external oblique muscle is from the outer surfaces of the lower eight ribs, where its slips of origin interdigitate with those of the serratus anterior and latissimus dorsi muscles. Observe that the muscle fibres slope downwards, forwards and medially. Note the insertion of the posterior part of the muscle into the anterior half of the outer lip of the iliac crest. Trace the anterior aponeurotic portion to its insertion into the linea alba, pubic crest, pubic tubercle and anterior superior iliac spine. Note that the linea alba is a bloodless thickened fibrous band between the symphysis pubis and xiphoid, while the “rolled in” aponeurotic part of the muscle between the pubic tubercle and anterior superior iliac spine is the inguinal ligament.

9. Detach the external oblique muscle from the ribs and iliac crest. Make a horizontal incision through the muscle from the anterior superior iliac spine to the linea semilunaris. Reflect the upper part of the external oblique medially and examine the internal oblique muscle whose fibres run upwards, forwards and medially, i.e., in a direction opposite to that of the external oblique. The internal oblique originates from the lateral two thirds of the inguinal ligament, the anterior two thirds of the middle lip of the iliac crest and the thoracolumbar fascia. Note its aponeurotic insertion into the lower four costal cartilages, xiphoid process, linea alba, pubic crest and the pecten pubis. Observe that the part of the aponeurosis passing towards the upper two thirds of the linea alba splits to enclose the paramedian muscle called the rectus abdominis.

10. Taking care, make a vertical incision in the internal oblique muscle from the costal margin to the anterior superior iliac spine and from here, extend the incision horizontally to the linea semilunaris. As you reflect the muscle forwards, observe the main nerves and vessels of the abdominal wall lying on the horizontally running fibres of the transversus abdominis muscle. This is the neurovascular plane.

1. Now verify the origin of the transversus abdominis muscle from the lateral one third of the inguinal ligament, the anterior two thirds of the inner lip of the iliac crest, thoracolumbar fascia and the inner surfaces of the lower six costal cartilages. Note its insertion into the xiphoid process, linea alba, pubic crest and pecten pubis. The Rectus Sheath 1

2. Open the aponeurotic covering of the rectus muscle called the rectus sheath by a vertical paramedian incision from the lower rib margin to the pubic crest. Identify the rectus abdominis muscle, which has vertically disposed fibres and exhibits transverse tendinous intersections on its anterior surface. Verify the attachments of the rectus muscle to the pubic crest and symphysis pubis inferiorly and to the outer surface of the xiphoid and seventh, sixth and fifth costal cartilages superiorly. Near the lower end of the rectus, observe the slender pyramidalis muscle lying anteriorly and running from the pubic crest and symphysis to be inserted into the linea alba above. Note that all the muscles of the anterior abdominal wall are supplied by the lower six intercostal and L1 nerves, except rectus abdominis (lower six intercostal nerves) and pyramidalis (subcostal nerve).

3. Divide the rectus muscle transversely in the middle and reflect the two halves. Note the intercostal and subcostal nerves entering the sheath and piercing the rectus to become subcutaneous and the anastomosis between the superior and inferior epigastric arteries, as well as those between their accompanying veins on the deep surface of the muscle. What is the significance of these arterial and venous anastomoses?

4. Examine the anterior and posterior walls of the rectus sheath at the following levels:

(a)at the level of the xiphoid, the anterior wall of the sheath is formed by the external oblique aponeurosis while posteriorly the muscle rests directly on the costal cartilages. Note that the superior epigastric artery enters the deep surface of the rectus by passing between the sternal and costal origins of the diaphragm;

(b) from the xiphoid to a level midway between the umbilicus and symphysis pubis, the anterior wall is formed by the external oblique aponeurosis and the anterior lamella of the internal oblique, while the posterior wall is formed by the posterior lamella of the internal oblique and aponeurosis of the transversus abdominis; and

(c) below a level midway between the umbilicus and symphysis pubis, the aponeurosis of all the three muscles pass in front of the rectus muscle, while the posterior wall is deficient and thus the rectus lies directly on the fascia transversalis. Note that the posterior wall ends inferiorly in a sharp border called the arcuate line. The inferior epigastric artery enters the rectus sheath by ascending in front of the margin.

The Inguinal Canal

5. Now turn your attention to the inguinal region and the dissection of the inguinal canal. The inguinal canal is an oblique intermuscular passage, 5 cm long, situated above the medial half of the inguinal ligament. It transmits the spermatic cord in the male and the round ligament of the uterus in the female. The inguinal canal is directed medially, downwards and forwards, and extends from the deep inguinal ring in the fascia transversalis to the superficial inguinal ring in the external oblique muscle. The deep inguinal ring is situated 1.5 cm above the midinguinal point a point half way between the anterior superior iliac spine and the symphysis pubis. And the superficial inguinal ring is situated above the pubic tubercle. Note that the external oblique aponeurosis forms the anterior wall of the inguinal canal. Identify the superficial inguinal ring above the pubic tubercle and note that the spermatic cord or the round ligament of the uterus emerges through it.

6. From the lateral end of the horizontal incision already made in the external oblique aponeurosis, make a cut running downwards, forwards and medially, parallel to and a fingers breadth above the inguinal ligament, cut towards the pubic symphysis, passing above the superficial inguinal ring. Reflect the superior and inferior portions of the cut external oblique aponeurosis and carefully examine the concave upper surface of the inguinal ligament. Follow the inguinal ligament medially to the pubic tubercle. Note the extension of the ligament backwards to the pecten pubis. This is called the lacunar ligament. Observe that it is triangular with its sharp base facing laterally.

7. Detach the fibres of the intenal oblique arising from the inguinal ligament and turn this part of the muscle medially. Now identify the lower margin of the transversus abdominis muscle and follow its fibres as they arch medially where they will be found to join those of the internal oblique to form the conjoint tendon. Examine the attachment of this tendon to the linea alba, pubic crest and pecten pubis. The conjoint tendon is formed by those fibres of the internal oblique and transversus abdominis muscles that originate from the inguinal ligament. Note that the conjoint tendon lies immediately posterior to the superficial inguinal ring thus contributing to the posterior wall of the inguinal canal.

8. Note the triple relationship of the lower fibres of the internal oblique to the spermatic cord or round ligament of the uterus. The fibres of the internal oblique arising from the inguinal ligament first pass in front of the cord or round ligament, then above it and finally behind the cord or round ligament where the muscle contributes to a part of the conjoint tendon. Thus the fibres of this muscle contribute to the anterior wall, roof and to the posterior wall of the inguinal canal in that order. You may find the fibres of the cremaster muscle passing on to the spermatic cord from the lower edge of the internal oblique and transversus abdominis muscles.

9. Observe that the fascia transversalis forms the posterior wall of the inguinal canal in this region.

20. Exert traction on the spermatic cord or the round ligament of the uterus and identify the deep inguinal ring, a deficiency in the fascia transversalis 1.5 cm above the midinguinal point. Confirm that the deep ring lies lateral to the inferior epigastric vessels. Follow the spermatic cord or round ligament medially and note how it lies on the lacunar ligament and upper grooved surface of the inguinal ligament which therfore constitute the floor of the inguinal canal. Now review the boundaries of the inguinal canal: The anterior wall is formed by the external olique aponeurosis in its entirety and by the internal oblique in its lateral half behind the aponeurosis. The posterior wall is formed by the conjoint tendon in the medial half of the canal and behind it by the fascia transversalis in all the length of the canal. The roof is formed by the lower arching fibres of the internal oblique and transversus abdominis muscles. And the floor is formed by the inguinal ligament and most medially by the lacunar ligament.

1. Secure the ilioinguinal nerve which reaches the inguinal canal by passing deep to the internal oblique or by piercing it close to the deep inguinal ring.

2. Next, note the coverings of the spermatic cord. These coverings are generally fused and cannot be separated. They are, from superficial to deep: the external spermatic fascia (continuation of external oblique), cremaster muscle and its fascia (continuation of internal oblique and transversus abdominis) and internal spermatic fascia (continuation of fascia transversalis). Try to identify some of the contents of the spermatic cord: ductus deferens, testicular vessels, pampiniform plexus of veins, lymph vessels, autonomic nerve plexuses and genital branch of the genitofemoral nerve. All these structures enter the deep inguinal ring and traverse the whole length of the canal. The ilioinguinal nerve traverses only the medial part of the canal. Note the easy access to the ductus deferens by a small incision on the upper lateral scrotal skin.