At 6 weeks, the flexed limbs have not yet rotated out of their primary position (see Plate 1-8). Because the upper and lower limbs later undergo opposite rotations to reach their definitive positions, the eventual anterior, or ventral, flexor muscle compartment of the mature arm corresponds to the posterior, or dorsal, flexor muscle compartment of the mature thigh. Also, the eventual anterior, or ventral, flexor muscle compartment of the mature forearm corresponds to the posterior, or dorsal, flexor muscle compartment (calf) of the mature leg. Because of the twist of the lower limb during development that results in permanent pronation of the foot, extension of the mature wrist corresponds to the so-called dorsiflexion of the ankle that is actually its extension.

HEAD AND NECK MYOTOMIC MUSCLES

The formation of the three preotic somites is slurred over in the human embryo. What would have been their myotomes appear as three closely apposed aggregations of mesenchyme in the region of the developing eye that give rise to the extrinsic ocular muscles (see Plate 1-20). The three surviving postotic occipital somites of the original four give rise to typical myotomes.

Comparative anatomy indicates that during evolution, the tongue muscles first appeared in amphibian forms because, in fish, the tongue is a membranous sac lacking muscle. In ancestral forms, the tongue muscles are derived from the occipital myotomes that are innervated exclusively by the hypoglossal (XII) nerves.

In the human embryo, the origin of the tongue muscles is abbreviated and slurred over. The muscles arise directly from an ill-defined mass of mesenchyme located adjacent to the pharynx in the region of the branchial arch mesenchyme from which the branchiomeric skeletal muscles arise (see Plate 1-20). However, because of the close relationship of the hypoglossal nerves to the occipital somites when they first form in the human embryo, the tongue muscles are regarded as being derived from occipital myotomes even though they appear to arise directly from mesenchyme in the region of the tongue rudiment.

Another muscle mass that has slurred-over development gives rise to the trapezius and sternocleidomastoid muscles. It forms in mesenchyme situated be-tween the occipital myotomes and the branchiomeric mesenchyme of the most caudal branchial arch. The innervation of the muscle mass is unique because it arises as a number of motor roots from the side of the upper five segments of the cervical spinal medulla (cord) between the dorsal and ventral roots of the cervical spinal nerves, which eventually become the spinal part of the accessory (XI) nerve.