After the transverse processes appear, the ribs form in the sclerotomal tissue that extends by differentiation into the ventral portions of the original clefts between the somites. The maximum development of the ribs is in the thoracic region; consequently, of all the muscles in the adult, the intercostal muscles retain to the greatest degree the original segmental pattern of the hypaxial musculature.
The epaxial column of epimeres divides further into a medial, or deep, and a lateral, or superficial, group of muscles that eventually give rise to the extensors of the vertebral column. The medial group of muscles, supplied by the medial branches of the posterior primary rami of the spinal nerves, retains a resemblance to the primitive segmental arrangement by arising from the fusion of only a few consecutive segments. By subsequent longitudinal and tangential splitting, they become the short oblique muscles of the vertebral column (the semispinalis, multifidus, and rotatores muscles and a longer muscle, the spinalis division of the erector spinae muscle). The lateral, more superficial group of muscles, which is supplied by the lateral branches of the posterior primary rami of the spinal nerves, arises by the fusion of a larger number of consecutive segments and subsequent splitting to become the long extensor muscles of the back (the iliocostalis and longissimus divisions of the erector spinae muscle).
The hypaxial column of hypomeres invades the region ventral to the vertebrae to give rise to the psoas and quadratus lumborum muscles (see Plate 1-18). The hypomeres also extend into the lateral and ventral body wall to form the layered muscles of the thorax and abdomen (see Plate 1-16). In the thorax, they are the intercostals; in the abdomen, they are the external and internal oblique, transverse abdominal, and rectus abdominis muscles (see Plate 1-18). The rectus abdominis muscle develops from the most ventral extension of the lower thoracic and first abdominal hypomeres that fuse in a cephalocaudal direction to become a single longitudinal muscle on either side of the midline of the body, which is separated in the abdomen by the linea alba of dense connective tissue. The tendinous intersections (inscriptions) are indicative of the original segmental character of the rectus abdominis muscle (see Plate 1-16). Also, the fibers of this muscle retain the cephalocaudal orientation of the original myotomic fibers. In the upper thoracic region, there is also a longitudinal muscle sheet that is continuous with the sheet that gives rise to the rectus abdominis muscle. It normally disappears but is occasionally retained as the sternalis muscle. All muscles derived from the hypomeres are primarily flexors of the vertebral column.
The formation of the muscles derived from both the epimeres and the hypomeres is well advanced by the seventh week, except for the muscles of the pelvic diaphragm, perineum, and external genitalia (see Plate 1-19). These muscles develop later because of the late division of the single cloacal opening into a urethral and anal opening in the male and female and the acquisition of an additional opening in the female—the vagina.
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