The Muscles of Respiration
Muscles of the Anterior Abdominal Wall
Muscles of the Back
Back muscles that act on the trunk rotate and extend the vertebral column and flex it laterally. They are categorized into deep, intermediate, and superficial groups. Muscles in the deep group connect vertebrae together; the erector spinae muscle is the prime mover in extension of the spine. It enables standing upright. The erector spinae is further divided into three complex vertical muscles: the iliocostalis, the longissimus, and the spinalis. These include cervical, thoracic, and lumbar portions and produce movements ranging from the head to the lower back. Erector spinae muscles attach across a variety of structures associated with ribs and vertebrae. The primary deep back muscle in the thoracic region is the semispinalis, which extends and bends the trunk laterally. It is divided into the semispinalis cervicis and semispinalis thoracis. The primary deep lumbar muscle is the quadratus lumborum. The quadratus lumborum and erector spinae are enclosed together in a fibrous sheath known as the thoracolumbar fascia. The multifidus muscle lies beneath this sheath. Most back muscles are innervated by the dorsal rami of the spinal nerve. The dorsal ramus is the dorsal or posterior division of the spinal nerve and it forms the dorsal nerve root after emerging from the spinal cord.The intermediate group consists of the serratus posterior inferior and the serratus posterior superior. Rib movements during breathing are controlled by these serratus posterior muscles which lie over the erector spinae. Serratus posterior muscles attach from the spines of cervical and thoracic vertebrae to the ribs. The superficial back muscles are the trapezius, which elevates, depresses, and retracts the scapulae; the latissimus dorsi, which extends the arms and rotates them medially; the levator scapulae, which elevate the scapulae; and the rhomboideus major and minor, which retract, elevate, and rotate the scapulae.
Muscles of the Pelvic Floor
The pelvic floor is a wall of muscle spanning the space formed by the pelvic bone. It consists of three layers of muscles combined with a sheet-like connective tissue. The pelvic floor layer just under the skin includes the superficial muscles of the perineum, which is the area between the anus and the scrotum in men, or the vulva in women. Some of these include the gluteus maximus and the superficial transverse perineal. The middle layer comprises muscles forming the urogenital triangle (for example, the ischiocavernosus and bulbospongiosus) and the anal triangle (consisting of the external anal sphincter and anal canal). Muscles making up the pelvic diaphragm form the deepest layer of the pelvic floor. This includes the levator ani, consisting of the pubococcygeus and the iliococcygeus. Most pelvic floor muscles are innervated by the pudendal nerve, which carries both sensation from the external genitalia as well as motor supply to the pelvic muscles and external urethral sphincter in both men and women.The urethra, anal canal, and vagina pass through the pelvic floor. They open into the diamond-shaped perineum, which consists of the urogenital triangle anteriorly and anal triangle posteriorly. The perineum lies between bony structures on all sides: the pubic symphysis in front; coccyx, or tailbone, behind; and an ischial tuberosity on each side. Three muscles make up the superficial perineal space. The superficial transverse perineus attaches from the ischial tuberosities to the perineum's central tendon, supporting the pelvic floor. Paired ischiocavernosus muscles originate at the ischial tuberosities and angle to meet at the penis or clitoris, where they compress to initiate erection. The bulbospongiosus muscle originates at the perineal central tendon and plays different roles in males and females. In males it compresses the urethra, which expels semen during ejaculation. It females it contracts the vagina during intercourse. In both sexes the bulbospongiosus can be contracted to void, or expel, any remaining urine from the canal of the urethra. It exerts its action at the end of the urination process. In the middle layer or compartment of the pelvic floor, two muscles and a fibrous membrane make up the thin urogenital diaphragm that lies across the urogenital triangle. The deep transverse perineal muscle anchors at the central tendon. It supports the pelvic floor and is responsible for the expulsion of the last drops of urine from the urethra in both men and women as well as the expulsion of semen in men. The external urethral sphincter can be voluntarily constricted to prevent urination. Also in the middle pelvic compartment, the external anal sphincter muscle is in the anal triangle. Originating at the coccyx, it voluntarily constricts the anal opening to prevent defecation. The deepest pelvic layer, the pelvic diaphragm, contains two pairs of muscles. The levator ani supports pelvic organs and elevates the anus during defecation. It attaches from the pubic bone in the front of the pelvis and extends across to the sacrum and coccyx, or tailbone. The coccygeal muscle supports and lifts the pelvic floor. It originates at the ischial spine (a pointed projection of bone just inferior to the acetabulum, or hip bone) and inserts at the coccyx.