Trunk and Perineum

Muscles of the Trunk: (Name, Origin, Insertion, Action)

Diaphragm: xyphoid process and last six costal cartilages and anterior surfaces of lumbar vertebrae – central tendon – flattens to enlarge chest cavity for inhalation
External Intercostals: superior rib – inferior rib – pulls ribs up and out (inhalation)
External oblique: lower eight ribs – iliac crest and linea alba – rotates and flexes vertebral column and compresses abdomen
Internal intercostals: inferior rib – superior rib – pulls ribs down and in (forced exhalation)
Internal oblique: iliac crest and adjacent connective tissues – lower ribs, xiphoid of sternum, and linea alba – compresses abdoment, depresses ribs, flexes or laterally flexes vertebral column
Rectus abdominus: superior surface of pubis around symphysis – inferior surfaces of 5th - 7th costal cartilages and xiphoid process – depresses ribs and flexes vertebral column
Sacrospinalis group (deep): ilium, lumbar, and some thoracic vertebrae – ribs, cervical, and thoracic vertebrae – extends vertebral column
Transverse abdominis: cartilages of lower ribs, iliac crest, and adjacent connective tissues – linea alba and pubis – compresses abdomen

Muscles of the Perineum: (Name, Origin, Insertion, Action)

Bulbospongiosus:
- Male – base of penis and fibers that cross over the urethra – midline and central tendon of perineum – compresses base, stiffens penis, and ejects urine or semen
- Female – base of clitoris and fibers that run on either side of urethral and vaginal openings – central tendon of the perineum – compresses and stiffens clitoris and narrows the vaginal opening
Ischiocavernosus: inferior medial surface of ischium – symphysis pubis anterior to base of penis or clitoris – compresses and stiffens penis or clitoris
Transverse perineus: inferior, medial surface of ischium – central tendon of perineum – stabilizes central tendon of perineum
External urethral sphincter:
- Male - inferior, medial surfaces of ischium and pubis – midline at base of penis and inner fibers that encircle urethra – closes urethra and compresses prostate and bulbourethral glands
- Female – base of clitoris and fibers that run on either side of urethral and vaginal openings – central tendon of perineum – compresses and stiffens clitoris and narrows vaginal opening
External anal sphincter: via tendon from coccyx – encircles anal opening – closes anal opening
Levator ani: ischial spine and pubis – coccyx – tenses floor of pelvis, supports pelvic organs, flexes coccyx, and elevates and retracts anus.

This page was updated in December 2005.