In humans, a striated cloacal sphincter muscle and levator ani muscle (pelvic diaphragm) arise from the third sacral to the first coccygeal myotomic hypomeres and are well developed by the eighth week. The striated external anal sphincter, perineal, and external genital muscles arise from the cloacal sphincter muscle by its rearrangements and additions during the establishment of the urogenital and anal openings. The deep, or inner, fibers of the cloacal sphincter muscle give rise to the urethral sphincter muscle. Although the muscles of the external genitalia are the same in both sexes, they, of necessity, must undergo a different arrangement in each sex. The mature pelvic muscle arrangement in the two sexes is present by the 16th week of development. However, not until sometime during the second year after birth do the urethral and external anal sphincter muscles come under voluntary control.
During their early development, the limbs are literally ectodermal sacs that become stuffed with mesenchyme. As the limb buds grow, the proliferating local somatic mesenchyme eventually gives rise to all skeletal rudiments. Myotome cells from the adjacent somites invade the limb buds to give rise to all the skeletal muscles. When the ingrowth of myotome cells, nerve fibers, neurilemmal cells, pigment cells, and, possibly, the endothelium of the blood and lymphatic systems are excluded, the limb buds would still have the capacity for self-differentiation to become limbs containing all the normal skeletal rudiments. The muscles of the pectoral and pelvic girdles are also of myotomic origin.
Early in the seventh week, the mesenchymal premuscle masses of the girdle musculature are formed in the human embryo. As the rudiments of the appendicular skeleton become differentiated within the developing limb, the mesenchyme from which the limb muscles arise is aggregated into masses grouped dorsal and ventral to the developing skeletal parts. The progressive formation of distinct muscles reaches the level of the hand and foot during the seventh week. The muscles of the upper limb develop slightly ahead of those of the lower limb.
The early limbs are flattened dorsoventrally and look like paddles projecting straight out from the body. They each have a cephalic (preaxial) border and a caudal (postaxial) border, as well as a craniocaudal attachment to the body opposite a number of myotomes (see Plate 1-9). Each upper limb bud lies opposite the lower five cervical and the first thoracic myotomes. Each lower limb bud is opposite the second and fifth lumbar and the upper three sacral myotomes. The branches of the spinal nerves supplying these myotomes reach the base of their respective limb bud. As the bud elongates to form a limb, the nerves grow into it in such a manner that the group of limb muscles along the preaxial border of the upper limb becomes innervated by the fourth to the seventh cervical nerves, and those of the postaxial border, by the eighth cervical and the first thoracic nerves. In the lower limb, the group of muscles along the preaxial border receives innervation from the second to the fifth lumbar nerves and the group of muscles along the postaxial border receives innervation from the first to the third sacral nerves.
The preaxial and postaxial groups of developing muscles become split and rearranged. In so doing, they both contribute to the formation of the ventral, or anterior, limb-flexor group of muscles and a dorsal, or posterior, limb-extensor group (see Plate 1-18). The original preaxial and postaxial nerves of the limbs are similarly divided into anterior and posterior divisions, supplying the flexors and extensors, respectively. Thus, the ulnar and median nerves in the upper limb, which contain both preaxial and postaxial nerve fibers, are branches of the anterior divisions of the trunks of the brachial plexus and innervate flexor muscles. Likewise, the radial nerve, containing both preaxial and postaxial nerve fibers, is derived from the posterior divisions of the trunks of the brachial plexus and innervates ex-tensor muscles.
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