Abdomen

Muscles of abdominal wall Rectus abdominus--vertical midline Three layers form tube External oblique (“hands-in-pocket”) Internal oblique (fibers perpendicular to external) Transverse abdominus (wraps around)

Muscles of abdominal wall--details

External Oblique It arises from eight fleshy digitations, each from the external surfaces and inferior borders of the fifth to twelfth ribs Those from the lowest ribs pass nearly vertically downward, and are inserted into the anterior half of the outer lip of the iliac crest; the middle and upper fibers, directed downward (inferiorly) and forward (anteriorly), become aponeurotic at approximately themidclavicular line. This aponeurosis formed from fibres from either side of the external oblique decussates at the linea alba.

External oblique The external oblique muscle is innervated by ventral branches of the lower 6 intercostal (thoracoabdominal) nerves and the subcostal nerve on each side

Ext oblique - Action The external oblique functions to pull the chest downwards and compress the abdominal cavity, which increases the intra-abdominal pressure as in a valsalva maneuver. It also has limited actions in both flexion and rotation of the vertebral column. One side of the obliques contracting can create lateral flexion. It also contributes in compression of abdomen.

Internal oblique The internal oblique muscle (of the abdomen) is the intermediate muscle of the abdomen, lying just underneath the external oblique and just above (superficial to) the transverse abdominal muscle.

Internal oblique Its fibers run perpendicular to the external oblique muscle, beginning in the thoracolumbar fascia of the lower back, the anterior 2/3 of the iliac crest (upper part of hip bone) and the lateral half of the inguinal ligament. The muscle fibers run from these point superiomedially (up and towards midline) to the muscle's insertions on the inferior borders of the 10th through 12th ribs and the linea alba (abdominal midline seam).

Internal oblique The internal oblique is innervated by the lower intercostal nerves, as well as the iliohypogastric nerve and the ilioinguinal nerve. The internal oblique performs two major functions. First, it acts as an antagonist (opponent) to the diaphragm, helping to reduce the volume of the thoracic (chest) cavity during exhalation.  Secondly, its contraction rotates and side-bends the trunk by pulling the rib cage and midline towards the hip and lower back, of the same side.

Transverse abdominis Deep to (layered below) the internal oblique muscle.

Transverse abdominis It arises, as fleshy fibers, from the lateral third of the inguinal ligament, from the anterior three-fourths of the inner lip of the iliac crest, from the inner surfaces of the cartilages of the lower six ribs, interdigitating with the diaphragm, and from the lumbodorsal fascia. The muscle ends anteriorly in a broad aponeurosis, the lower fibers of which curve inferomedially (medially and downward), and are inserted, together with those of the internal oblique muscle, into the crest of the pubis and pectineal line, forming the inguinal aponeurotic falx, also called the conjoint tendon. 

Transverse abdominis The transversus abdominis is innervated by the lower intercostal nerves (thoracoabdominal, nerve root T7-T11), as well as the iliohypogastric nerve and the ilioinguinal nerve. The transversus abdominis (TVA) helps to compress the ribs and viscera, providing thoracic and pelvic stability.

Rectus abdominis The rectus abdominis muscle, also known as the "6-pack," is a paired muscle running vertically on each side of the anterior wall of the human abdomen. There are two parallel muscles, separated by a midline band of connective tissue called thelinea alba (white line). It extends from the pubic symphysis/pubic crest inferiorly to the xiphisternum/xiphoid process and lower costal cartilages (5–7) superiorly. It is contained in the rectus sheath.

Rectus abdominis The rectus is usually crossed by three fibrous bands linked by the tendinous intersections. While the "sixpack" is by far the most common configuration of the muscle bellies of the rectus, there exist rare anatomic variations which result in the appearance of eight ("eightpack"), ten, or asymmetrically arranged segments. All these variations are functionally equivalent.

Rectus abdominis The rectus abdominis is an important postural muscle. It is responsible for flexing the lumbar spine, as when doing a "crunch". The rib cage is brought up to where the pelvis is when the pelvis is fixed, or the pelvis can be brought towards the rib cage (posterior pelvic tilt) when the rib cage is fixed, such as in a leg-hip raise.  The muscles are innervated by thoracoabdominal nerves, which pierce the anterior layer of the rectus sheath.

Rectus sheath The Rectus sheath is formed by the aponeuroses of the Obliqui and Transversus. It contains the Rectus abdominis and Pyramidalis muscles. It can be divided into anterior and posterior laminae.

Region Description Above the arcuate line At the lateral margin of the Rectus, the aponeurosis of the Obliquus internus divides into two lamellae:one of which passes in front of the Rectus, blending with the aponeurosis of the Obliquus externus. the other, behind it, blending with the aponeurosis of the Transversus, and these, joining again at the medial border of the Rectus, are inserted into the linea alba. Below the arcuate line Below this level, the aponeuroses of all three muscles (including the internus) pass in front of the Rectus.

Muscles of Posterior Abdominal Wall

Quadratus Lumborum It arises by aponeurotic fibers from the iliolumbar ligament and the adjacent portion of the iliac crest for about 5 cm., and is inserted into the lower border of the last rib for about half its length, and by four small tendons into the apices of the transverse processes of the upper four lumbar vertebrae.

Quadratus Lumborum The quadratus lumborum can perform four actions: Lateral flexion of vertebral column, with ipsilateral contraction Extension of lumbar vertebral column, with bilateral contraction Fixes the 12th rib during forced expiration Levates ilium, with ipsilateral contraction

Psoas major The psoas major is a long fusiform muscle located on the side of the lumbar region of the vertebral column and brim of the lesser pelvis. It joins the iliacus muscle to form the iliopsoas. In less than 50 percent of human subjects the psoas major is accompanied by the psoas minor.

Psoas major The psoas major originates from the transverse processes of lumbar vertebrae I-V, lateral surfaces of the last thoracic vertebra, lumbar vertebrae I-IV, and from neighboring invertebral discs. Joined by the iliacus, psoas major forms the iliopsoas which is surrounded by the iliac fascia. The iliopsoas runs across the iliopubic eminence through the muscular lacuna to its insertion on the lesser trochanter of the femur. 

Psoas major Innervation of the psoas major is through the anterior rami of L1 to L3 As part of the iliopsoas, psoas major contributes to flexion and external rotation in the hip joint. On the lumbar spine, unilateral contraction bends the trunk laterally, while bilateral contraction raises the trunk from its supine position.

Inguinal Canal The inguinal canal is a passage in the anterior (toward the front of the body) abdominal wall which in men conveys the spermatic cord and in women the round ligament. The inguinal canal is situated just above the medial half of the inguinal ligament. Length is approximately 3.75cm It is oblique directed inferiorly, anteriorly and medially. Extends between superficial and deep inguinal rings.

Inguinal Region Inguinal Canal: Superficial inguinal ring: Triangular defect in the aponeurosis of the external oblique muscle layer. Superficial opening of the inguinal canal. Lies above and lateral to pubic tubercle. Larger in males: Transmits spermatic cord in males. Transmits round ligament of uterus in females.

Inguinal Region Inguinal Canal: Deep inguinal ring: Opening of the evagination of the transversalis fascia. Lies above inguinal ligament midway between anterior iliac spine and pubic tubercle.

Margins of the inguinal canal superior wall (roof): internal oblique transversus abdominis anterior wall: aponeurosis of external oblique aponeurosis of internal oblique (lateral third of canal only) superficial inguinal ring (medial third of canal only) (inguinal canal) posterior wall: transversalis fascia conjoint tendon (medial third of canal only) deep inguinal ring (lateral third of canal only) inferior wall (floor): inguinal ligament lacunar ligament (medial third of canal only) iliopubic tract (lateral third of canal only)

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