MORPHOGENESIS OF MUSCULAR SYSTEM 717 



the pattern of the eye-muscle development from three pre -otic pairs of somites 

 as manifested in the shark embryo. 



2) Muscles of the Visceral Skeleton and Post-branchial area: a) Tongue 

 AND Other Hypobranchial Musculature. As indicated in figures 253 

 and 327A, a variable number of post-otic or met-otic somites are concerned 

 with the composition of the head of the gnathostomous vertebrate. In the 

 dogfish, Squalus acanthias, about six pairs of post-otic somites contribute to 

 the structure of the head (De Beer, '22). For most vertebrates, about three 

 pairs of post-otic somites, a conservative estimate, appear to enter into the 

 head's composition. The hypobranchial musculature in the elasmobranch 

 embryo arises as myotomal buds from the myotomes of posterior head area. 

 These muscle buds migrate ventrad from these myotomes to the hypobranchial 

 region as indicated in figure 253. Associated with this migration of myotomal 

 material is the migration and distribution of the hypoglossal nerve, com- 

 pounded from the ventral roots of post-otic spinal nerves to this area (fig. 

 253). In the human, W. H. Lewis ('10) favors the view that the tongue 

 musculature arises in situ from the hypobranchial mesenchyme, but Kingsbury 

 ('15) suggests the post-otic origin of the tongue musculature for all verte- 

 brates. Regardless of its origin, the tongue musculature is innervated by 

 ventral nerve roots of post-otic segments in higher vertebrates, i.e., the 

 hypoglossal or twelfth cranial nerve. The tongue musculature becomes asso- 

 ciated with the basihyal portion of the hyoid arch, which acts as its support. 

 In mammals, the sternohyoid, sternothyroid, and omohyoid muscles are in- 

 nervated also by the hypoglossal or twelfth cranial nerve. These muscles 

 probably arise from the post-otic myotomes in a manner similar to the tongue 

 musculature. 



b) Musculature of the Mandibular Visceral Arch. The mesoderm, 

 associated with this arch, gives origin to the muscles of mastication, and as 

 a result these muscles are innervated by special visceral motor fibers located 

 in the trigeminal or fifth cranial nerve. In the shark, the muscles arising from 

 the mandibular visceral arch tissue are the adductor mandibulae and the 

 first ventral constrictor muscles (fig. 327B); in the frog, the temporal, mas- 

 seter, pterygoid, and mylohyoid muscles; in the chick, the pterygotemporal, 

 temporal, and digastric muscles; and, in mammals, the temporal, masseter, 

 pterygoid, anterior portion of the digastric, mylohoid, tensor tympani, and 

 tensor veli palatini muscles (fig. 327D, E', E", F, F'). 



c) Musculature of the Hyoid Visceral Arch. The musculature, which 

 develops from mesenchyme associated with the embryonic hyoid arch, be- 

 comes distributed as indicated in figures 327 and 330. It is to be observed 

 that, in the adult shark (fig. 327B), this musculature functions in relation 

 to the hyoid arch. In the adult frog (fig. 327D), it is represented by deep 

 facial musculature or the depressor mandibulae and subhyoideus muscles. In 

 the adult goose (fig. 327F), it is present as the M. sphincter colli, which 



