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the posterior wall of the rectus sheath. In the lower one-third the fibres of transversus abdominis pass in front of the rectus abdominis and contribute to the anterior wall of the rectus sheath.) The muscle fibres of the transversus abdominis lie at an angle to the intermediate fibres of both the external and inter-nal obliques but are parallel to those of the exterinter-nal oblique superiorly and to the internal oblique infe-riorly. This arrangement gives maximum strength to the abdominal wall and holds the abdominal con-tents in place when the intra-abdominal pressure is raised by contraction of these muscles.
Nerve supply: these muscles are supplied by the ventral rami of the lower five or six intercostal nerves and the subcostal nerve. In addition, the in-ternal oblique and transversus abdominis are sup-plied by the iliohypogastric nerve. The cremaster muscle is supplied by the genital branch of the genitofemoral nerve.
Actions of the external oblique,
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DISSECTION 8.4 Muscles, vessels, and nerves of the anterior abdominal wall Objectives
I. To study the muscles of the anterior abdominal wall, the inguinal ligament, and the rectus sheath. II. To identify and trace the course of the abdominal nerves and the superior and inferior epigastric arteries. III. To study the formation of the rectus sheath. IV. To identify the contents of the rectus sheath.
Instructions
1. Remove any fascia from the surface of the external oblique muscle and its aponeurosis. Take special care superiorly where the aponeurosis is thin and easily destroyed, and also antero-inferiorly where the superficial inguinal ring forms a triangular deficiency immediately superolateral to the pubic tubercle.
2. In the male, identify the spermatic cord emerging from the superficial inguinal ring. Note the exten-sion of the fascia from the margins of the ring over the spermatic cord. This is the external spermat-ic fascia. In the female, the fatty, fibrous structure emerging from the superficial inguinal ring is the round ligament of the uterus. In both sexes, de-fine the margins of the ring by blunt dissection.
3. Identify the origin of the external oblique mus-cle from the lower eight ribs. Here it interdigitates with the serratus anterior and latissimus dorsi. Sep-arate the upper six digitations from the ribs. Cut vertically through the muscle down to the iliac crest, posterior to the sixth digitation. Separate the exter-nal oblique from the iliac crest in front of this, but avoid injury to the lateral cutaneous branches of the nerves which pierce it close to the crest.
4. Turn the superior part of the external oblique for-wards and expose the internal oblique and its aponeurosis. Follow the internal oblique medially to the line of fusion with the aponeurosis of the external oblique, anterior to the rectus abdominis.
5. Divide the external oblique aponeurosis vertically, lateral to this line of fusion, and turn the muscle and aponeurosis inferiorly as you do so. The cut should pass to the pubis medial to the superficial inguinal ring [Fig. 8.6]. This exposes the remainder of the internal oblique.
6. Note the inrolled margin of the external oblique aponeurosis between its attachments to the ante-rior supeante-rior iliac spine and the pubic tubercle. This
is the inguinal ligament. Note that this ligament gives origin to the internal oblique muscle from its lateral part and has the spermatic cord or the round ligament of the uterus lying on its superior surface medially.
7. Lift the cord or round ligament of the uterus and identify the deep fibres of the inguinal ligament passing posteriorly to the pecten pubis. This is the lacunar ligament on which these structures also lie.
8. Follow the lateral margin (lateral crus) of the superficial inguinal ring to the pubic tubercle and note the relationship of this crus and the tuber-cle to the spermatic cord. The medial crus may be followed to the pubic crest.
9. Remove the fascia from the surface of the internal oblique and its aponeurosis. Identify the lower fi-bres of the internal oblique which pass around the spermatic cord. These are the cremaster muscles which loop down on the cord and turn upwards to be attached to the pubic tubercle.
10. Lift the internal oblique and cut carefully through its attachments to the inguinal ligament, iliac crest, and costal margin. Do not cut deeply or the nerves of the anterior abdominal wall which lie deep to the inter-nal oblique will be divided. Cut vertically through the internal oblique from the twelfth costal cartilage to the iliac crest. Attempt to strip the muscle forwards from the transversus abdominis and the nerves. The separation of the internal oblique and the transver-sus abdominis is difficult superiorly because of the dense fascia between the muscles. The separation of the internal oblique and the transversus is impos-sible inferiorly where the aponeuroses of the two muscles fuse in the conjoint tendon [Fig. 8.6].
11. Superior to a horizontal line, midway between the umbilicus and the symphysis pubis, the aponeurosis of the internal oblique splits at the lateral edge of the rectus abdominis to enclose that muscle. The part of the internal oblique aponeurosis which pass-es anterior to the rectus abdominis fuspass-es with the aponeurosis of the external oblique. The part of the internal oblique aponeurosis which passes posterior to the rectus fuses with the underlying aponeurosis of the transversus abdominis. The three aponeuro-ses enclose the rectus abdominis to form the rectus sheath [Fig. 8.9].
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Muscles of the anterior abdominal wall
to a point midway between the umbilicus and pubic symphysis—the anterior wall of the rectus sheath is formed by the aponeurosis of the external oblique and the anterior part of the aponeurosis of internal oblique. The posterior wall is formed by the posterior part of internal oblique aponeurosis and the aponeu-rosis of the transversus abdominis. In the lower part—below the line midway between the umbilicus and pubis symphysis—the anterior wall is formed by the aponeurosis of all three muscles, and the poste-rior wall is deficient. The rectus lies directly on the transversalis fascia. The lower border of the posterior wall of the rectus sheath is called the arcuate line. On the anterior surfaces of the costal cartilages, the pos-terior layer of the sheath is lost, as the transversus ab-dominis passes internal to the costal cartilages. (The internal oblique is attached to the costal margin and does not extend posterior to the rectus abdominis.) At this level, the aponeurosis of the external oblique continues anterior to the rectus and gives attachment to the lowest fibres of the pectoralis major [Fig. 8.9].
12. Inferior to a horizontal line midway between the umbilicus and the symphysis pubis, the aponeuro-ses of all three muscles pass anterior to the rectus abdominis. The rectus then lies on the transversalis fascia posteriorly. The inferior edge of the posterior layer of the rectus sheath often forms a sharp mar-gin—the arcuate line. This will be seen when the rectus sheath is opened.
13. Remove the fascia from the surface of the trans-versus abdominis and from the nerves and vessels which lie on it. Confirm the continuity of the lateral cutaneous branches of these nerves.
14. Define the origins of the transversus. Follow its aponeurosis medially. Above the arcuate line, it fuses with the aponeurosis of the internal oblique, posterior to the rectus abdominis. Below the arcuate line also, it fuses with the aponeurosis of the internal oblique, but anterior to the rectus [Fig. 8.9].
15. Note that, below the arcuate line, the aponeurosis of the external oblique is less firmly fused with that of the internal oblique.
16. Open the rectus sheath by a vertical incision along the middle of the muscle. Reflect the anterior layer of the sheath medially and laterally, cutting its at-tachments to the tendinous intersections in the
anterior part of the rectus muscle. Lift the rectus muscle and identify the intercostal and subcostal nerves entering the sheath and piercing the mus-cle. Confirm the mode of formation of the rectus sheath.
17. On the lower part of the rectus, identify the pyrami-dalis muscle, if present. This small triangular mus-cle arises from the upper surface of the pubic crest and symphysis. It lies anterior to the rectus and is inserted into the lowest part of the linea alba. It is supplied by a small branch of the subcostal nerve.
18. Divide the rectus abdominis transversely at its middle. Identify its attachments. Expose the pos-terior wall of the rectus sheath by turning its parts superiorly and inferiorly, cutting the nerves as they enter it. Identify and follow the superior and inferior epigastric arteries running longitudinally deep to the muscle within the rectus sheath [Fig. 8.6]. The tendinous intersections are only in the anterior part of the rectus abdominis, so they do not interfere with this longitudinal anastomosis between the su-perior and inferior epigastric arteries. Try to define the arcuate line on the posterior wall of the rec-tus sheath. The inferior epigastric artery enters the sheath by passing anterior to this line.
Fig. 8.9 Transverse sections of the anterior abdominal wall to show the formation of the rectus sheath at different levels.
(A) Above the costal margin. (B) Upper two-thirds of the abdominal wall. (C) The lower one-third of the wall.
Anterior wall of sheath (A)
(B)
(C)
Anterior wall of sheath Rectus abdominis External oblique
External oblique Internal oblique Transversus abdominis
External oblique Internal oblique Transversus abdominis
Inferior epigastric artery Fifth costal
cartilage
Xiphoid process
Diaphragm Diaphragmatic fascia Linea alba Fascia transversalis Posterior wall of sheath Rectus abdominis
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Three horizontal tendinous intersections present in the anterior part of the rectus abdominis attach the muscle to the anterior layer of the rectus sheath. They are at the level of the umbilicus, at the tip of the xiphoid process, and midway between these two. Occasionally, a fourth intersection is present between the umbilicus and the pubis.
Nerve supply: the lower five or six intercostal nerves and the subcostal nerve. Action: the rectus abdominis is a powerful flexor of the vertebral col-umn. It may be made to stand out when attempt-ing to raise the head and shoulders (or lower limbs and pelvis) from the floor when lying on the back (supine position). When the extensors of the verte-bral column (erector spinae) contract at the same time, the rectus muscles tighten the anterior ab-dominal wall against blows, provided the ribs are
fixed by inspiratory muscles. When the ribs are not fixed, the rectus acts as an expiratory muscle.