THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 303 



platysma and epicranius, the muscles of expression — quadratus labii superioris, 

 risOrius, triangularis, mentalis, etc.; also two muscles connected with the hyoid 

 bone — digastricus (venter posterior) and stylohyoideus — and the stapedius of the 

 middle ear. The facial (VII) nerve corresponds to the second arch and sup- 

 plies all these muscles. 



The glossopharyngeal (IX) nerve corresponds to the third branchial arch, 

 and this fact indicates the muscles derived from that arch. Some, at least, of 

 the constrictor muscles of the pharynx are derived from the third arch. The 

 stylo-pharyngeus is also a derivative of the same arch. 



The vagus (X) nerve is associated with the fourth and fifth arches and con- 

 sequently innervates the muscles derived from these arches, viz., the rest of the 

 constrictors of the pharynx (see above), the laryngeal muscles and the muscles 

 of the soft palate (except the tensor veli palatini which is derived from the first 

 arch (p. 302) . The glossopalatinus and chondroglossus are also derived from 

 the fourth and fifth arches, while the rest of the extrinsic muscles of the tongue 

 are of myotomic origin. 



Two other muscles are probably derived in part from the branchial arches, 

 for fibers of the spinal accessory (XI) nerve afford a part of their innervation. 

 These are the trapezius and the sternomastoideus, the remaining parts of which 

 are of myotomic origin (p. 298). 



Muscles of the Extremities. 



The question as to whether the muscles of the extremities are derivatives of 

 the myotomes or of the mesenchymal tissue in the limb buds has not been 

 settled. In some of the lower Vertebrates, especially in some of the Fishes, it 

 seems to have been pretty clearly demonstrated that bud-like processes from 

 the myotomes grow into the anlagen of the extremities (fins), and there give 

 rise to muscles. In other lower forms no such buds from the myotomes have 

 been demonstrated, but the muscles are apparently derived directly from 

 the mesenchymal tissue in the anlagen of the extremities. In the higher verte- 

 brates, especially in Mammals, no distinct myotome buds have been traced into 

 the extremities. Some investigators hold, however, that instead of myotome 

 buds some cells from the myotomes — myoblasts — wander into the limb buds 

 and give rise to muscles. Other investigators are inclined to the view that the 

 musculature of the extremities is not of myotomic origin, but that it is derived 

 from the mesenchymal tissue of the limb buds. 



A most striking feature of the musculature of the extremities is its distinctly 

 segmental nerve supply. This, of course, is in favor of, although it does not 

 prove, its myotomic origin. If the muscles of the extremities are of myotomic 

 origin, it is very probable that several myotomes take part in their formation. 



