. Comparative anatomy. Anatomy, Comparative. THE MUSCULAR SYSTEM 291 into a series of cavities each of which lies in a visceral arch. This hypo- meric segmentation (branchiomerism) is independent of the segmentation of the epimere (mesomerism), and should not be confused with this, although it is possible that the two types of segmentation may originally have coincided. From the mesoderm of the visceral arches arise the muscles, connective tissues, and blood vessels of the arches. In the fishes, these muscles are differentiated into levators, depressors, and constrictors of the gills. In the p
. Comparative anatomy. Anatomy, Comparative. THE MUSCULAR SYSTEM 291 into a series of cavities each of which lies in a visceral arch. This hypo- meric segmentation (branchiomerism) is independent of the segmentation of the epimere (mesomerism), and should not be confused with this, although it is possible that the two types of segmentation may originally have coincided. From the mesoderm of the visceral arches arise the muscles, connective tissues, and blood vessels of the arches. In the fishes, these muscles are differentiated into levators, depressors, and constrictors of the gills. In the process of conversion the epithelium of the hypomere breaks up into mesenchyme and the coelomic cavity disappears. .^MYOTOMES 1-4 ANL. M. TRAPEZIUS + "M. STERNOCLEIDO- MASTOID. MANDIBULAR MUSCLES/ â -â¢^--^^-^ ^^C^ ^::^^ ^'ANLACE DIAPHRAGM 1ST THORACIC MYOTOME''' ^^ Fig. 243.âThe anlagen of the cranial muscles with their nerve relations as seen in a 7 mni. human embryo. (Redrawn from Keibel and Mall, after W. H. Lewis.) In mammals and man, the coelom is absent in the visceral arches and the muscles are formed from masses of mesenchymatous cells. From the first visceral arch arise the muscles innervated by the mandibular branch of the fifth nerve, the masseter, temporalis, pterygoid, mylohyoid, and tensor veli palatini. From the same source come the tensor tympani of the ear and the anterior belly of the digastricus. The muscles inner- vated by the facial nerve are derived from the second visceral arch, the hyoid. They include the muscles of expression, the stylo-hyoid, stapedius, and the posterior belly of the digastricus. From the third visceral arch arise the stylopharyngeus muscle innervated by the glossopharyngeus nerve, and the constrictors of the pharynx innervated by the vagus nerve. The laryngeal muscles innervated by the vago-accessory nerve originate from the fourth and fifth visceral arches. As already explained the muscles of the tongue and throat innervated
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