THE MUSCULAR SYSTEM 



317 



The development of the human musculature is fully described by 

 W. H. Lewis in Keibel and Mall, vol. i. 



Fundamental Processes. The changes occurring in the myotomes 

 during the formation of adult muscles are referable the operation of the 

 following fundamental processes : 



(1) A change in direction of muscle fibers from the original cranio- 

 caudal orientation in the myotome. The fibers of but few muscles retain 

 their initial orientation. 



(2) A migration of myotomes, wholly or in part, to more or less remote 

 regions. Thus the latissimus dorsi originates from cervical myotomes, but 

 finally attaches to the lower thoracic and lumbar vertebrae and to the crest 

 of the ilium. Other examples are the serratus anterior and the trapezius. 



(3) A fusion of portions of successive myotomes. The rectus abdomi- 

 nis illustrates this process. . 



(4) A longitudinal splitting of myotomes into several portions. Ex- 

 amples are found in the sterno- and omo-hyoid and in the trapezius and 

 sterno-mastoid. 



(5) A tangential splitting into two or more layers. The oblique and 

 the transverse muscles of the abdomen are formed by the common process. 



(6) A degeneration of myotomes, wholly or in part. In this way 

 fascias, ligaments, and aponeuroses may be produced. 



Muscles of the Trunk. Ventral extensions grow out from the cervical 

 and thoracic myotomes, and a fusion that is well advanced superficially 

 occurs between all the myotomes in embryos of 10 mm. A dorsal, longitu- 

 dinal column of fused myotomes, however, can still be distinguished from 

 the sheet formed from the combined ventral prolongations (Fig. 322). 



From the superficial portions of the dorsal column there arise by 

 longitudinal and tangenital splitting the various long muscles of the back and 

 neck, which are innervated by the dorsal rami of the spinal nerves. The 

 deep portions of the myotomes to not fuse, but give rise to the several 

 intervertebral muscles, which thus retain their primitive segmental arrange- 

 ment. 



The muscles of the neck, other than those innervated by the dorsal 

 rami and those arising from the branchial arches (p. 319) differentiate 

 from ventral extensions of the cervical myotomes. In the same way the 

 thoraco-abdominal muscles arise from the more pronounced ventral pro- 

 longations of the thoracic myotomes that grow into the body wall along 

 with the ribs (Fig. 322). Reference has already been made to the prob- 

 able contribution from cervical myotomes to the formation of the dia- 

 phragm (p. 189). 



The ventral extensions of the lumbar myotomes (except the first) 

 and of the first two sacral myotomes do not participate in the formation 



