The formation of muscles from the cervical hypaxial column of hypomeres, however, is quite different from what happens in the thorax; this is due to the development of the adjacent upper limbs, to the caudal recession of the coelomic, or body, cavity that originally extended into the head region, and to the presence of the branchial arches. It is interesting that the muscle mass giving rise to the infrahyoid muscles is continuous with the mass giving rise to the tongue muscles and that the infrahyoid muscle mass is also continuous caudally with the muscle mass that becomes the diaphragmatic striated muscle.
The diaphragm is originally located in the neck region. Because of its caudal migration, mainly due to differential growth, its cervical spinal innervation via the phrenic nerves has to elongate markedly.
HEAD AND NECK BRANCHIOMERIC MUSCLES
During evolution, the switch from water breathing to air breathing resulted in the loss of the branchial arch, gill slit, and aqueous respiratory apparatus and the acquisition of a definitive face and neck. Many of the branchial arch structures, especially the skeleton, underwent modification and were retained in the resulting air-breathing upper respiratory system and acoustic system. A résumé of these modifications is recapitulated in the human embryo. Of the six branchial arches of primitive vertebrates, the fifth and sixth arches are completely rudimentary in humans. Even so, the deep tissue in the territories of the fifth and sixth arches gives rise to certain primitive structures that undergo modifications and are retained in the adult. The four definitive arches are present by the fifth week.
During the fifth week, condensations of mesoderm appear in the dorsal end of each of the four branchial arches, including the territories of the fifth and sixth arches. In the development of primitive vertebrates, there is continuity between the mesodermal condensations of each arch and one of the head somites, indicating that the condensations represent the hypaxial portion of the head somites. However, in the human embryo, this phase of development is slurred over because no such continuity occurs between the condensations and the somites. Therefore, the myoblasts that differentiate directly from the mesodermal condensations of the arches give rise to skeletal striated muscles that are regarded as branchiomeric in origin. The voluntary motor part of a special visceral cranial nerve grows into each of the muscle rudiments of the arches, including those of the territories of the fifth and sixth arches.
The muscles of branchiomeric origin retain their original cranial nerve innervation as they migrate to their final destinations (see Plates 1-20 and 1-21). The muscles that arise from the primordial mesenchymal mass of the first or mandibular branchial arch become innervated by the motor neurons of the trigeminal (V) nerve. These muscles become the masticatory muscles (the temporal, masseter, and pterygoid muscles) as well as the mylohyoid, anterior belly of the digastric, tensor veli palatini, and tensor tympani muscles. The muscles arising in the region of the second, or hyoid, branchial arch become the muscles of facial expression and receive their motor innervation from the facial (VII) nerve. Other muscles arising from the second arch mesenchyme and innervated by the facial nerve are the posterior belly of the digastric, stylohyoid, and stapedius muscles. The glossopharyngeal (IX) nerve supplies motor innervation to the muscle mass of the third branchial arch, which becomes the stylopharyngeus muscle.
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