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HEAD MESODERM

AROUND THE OPTIC CUP

The individual extraocular muscles become evident and receive an innervation from cranial nerves III, IV or VI.

BRANCHIAL ARCHES

Arch 1—The muscles of mastication are a large premuscle mass that receives fibers from the mandibular nerve. The premuscle mass is located lateral to the first arch cartilage between the optic cup and the first branchial groove.

Arch 2—The muscles of facial expression appear as sheets of premuscle cells (laminae) that extend into the occipital, cervical and mandibular regions. Deep to each lamina is one or more branches of the facial nerve.

Caudal arches—It is difficult to separate the mesenchyme in the caudal arches. This mesenchyme collectively gives rise to the muscles of the pharynx (stylopharyngeus and pharyngeal constrictors) and larynx, which are innervated by branches of cranial nerves IX and X.

Sternocleidomastoid-trapezius muscles—These muscles become separate masses in the lateral cervical region. Each receives a separate branch of the spinal accessory nerve.

FLOOR OF THE PHARYNX

The intrinsic and extrinsic muscles of the tongue can be identified along with their innervation by the hypoglossal nerve.

MYOTOMIC DERIVATIONS

The myotomic derivations are indicated in Table 7–1.

LATERAL (PLATE) MESODERM

Somatic layer—Experimental studies on lower forms suggest that these mesenchymal cells give rise to the muscles of the limbs and also contribute to the formation of the muscles in the ventral and lateral body wall.

Splanchnic layer—The muscularis layer of the gut differentiates from the condensation around the endodermal tube of the alimentary tract. Cartilages form in the condensation around the endodermal tracheobronchial tree.

LIMB MESODERM

Muscles differentiate in situ from the mesenchymal cells within the limb buds.

Individual muscles first appear proximally in each limb bud.

Extensor muscles develop in the dorsal part of the limb, flexor muscles develop in the ventral part.

In the upper limb the radial nerve supplies the extensors; the musculocutaneous, median and ulnar nerves innervate the flexors.

In the lower limb the pattern is more complex with some nerves innervating both extensors and flexors. The major nerves (femoral, obturator, sciatic and its common peroneal and tibial branches) can be identified.

Source: Atlas of Human Embryos.