372 MYOLOGY 



^ The Levator scapulae, Serratus anterior and the Rhomboids arise from premuscl 

 tissue in the lower cervical region and undergo extensive migration. 



The Latissimus dorsi and Teres major are associated in their origin from the 

 premuscle sheath of the arm as are also the two Pectoral muscles when the arm 

 bud lies in the lower cervical region. 



The intrinsic muscles of the arm develop in situ from the mesoderm of the arm 

 bud and probably do not receive cells or buds from the myotomes. The nerves 

 enter the arm bud when it still lies in the cervical region and as the arm shifts 

 caudally over the thorax the lower cervical nerves which unite to form the brachial 

 plexus, acquire a caudal direction. 



The Muscles of the Leg. The muscles of the leg like those of the arm develop 

 in situ from the mesoderma of the leg bud, the myotomes apparently taking no 

 part in their formation. 



The Muscles of the Head. The muscles of the orbit arise from the mesoderm over 

 the dorsal and caudal sides of the optic stalk. 



The muscles of mastication arise from the mesoderm of the mandibular arch. 

 The mandibular division of the trigeminal nerve enters this premuscle mass before 

 it splits into the Temporal, Masseter and Pterygoideus. 



The facial muscles (muscles of expression) arise from the mesoderm of the hyoid 

 arch. The facial nerve enters this mass before it begins to split, and as the muscle 

 mass spreads out over the face and head and neck it splits more or less incompletely 

 into the various muscles. 



The early differentiation of the muscular system apparently goes on independ- 

 ently of the nervous system and only later does it appear that muscles are dependent 

 on the functional stimuli of the nerves for their continued existence and growth. 

 Although the nervous system does not influence muscle differentiation, the nerves, 

 owing to their early attachments to the muscle rudiments, are in a general way 

 indicators of the position of origin of many of the muscles and likewise in many 

 instances the nerves indicate the paths along which the developing muscles have 

 migrated during development. The muscle of the diaphragm, for example, has its 

 origin in the region of the fourth and fifth cervical segments. The phrenic nerve enters 

 the muscle mass while the latter is in this region and is drawn out as the diaphragm 

 migrates through the thorax. The Trapezius and Sternocleidomastoideus arise 

 in the lateral occipital region as a common muscle mass, into which at a very early 

 period the nervus accessorius extends and as the muscle mass migrates and extends 

 caudally the nerve is carried with it. The Pectoralis major and minor arise in the 

 cervical region, receive their nerves while in this position and as the muscle mass 

 migrates and extends caudally over the thorax the nerves are carried along. The 

 Latissimus dorsi and Serratus anterior are excellent examples of migrating muscles 

 whose nerve supply indicates their origin in the cervical region. The Rectus 

 abdominis and the other abdominal muscles migrate or shift from a lateral to a 

 ventrolateral or abdominal position, carrying with them the nerves. 



The facial nerve, which early enters the common facial muscle mass of the second 

 branchial or hyoid arch, is dragged about with the muscle as it spreads over the head 

 and face and neck, and as the muscle splits into the various muscles of expression, 

 the nerve.is correspondingly split. The mandibular division of the trigeminal nerve 

 enters at an early time the muscle mass in the mandibular arch and as this mass 

 splits and migrates apart to form the muscles of mastication the nerve splits into 

 its various branches. 



The nerve supply then serves as a key to the common origin of certain groups of 

 muscles. The muscles supplied by the oculomotor nerve arise from a single mass 

 in the eye region; the lingual muscles arise from a common mass supplied by the 

 hypoglossal nerve. 



