THE MUSCULAR SYSTEM 317 



The development of the human musculature is fully described by W. H. Lewis 

 in Keibel and Mall, vol. 1. 



Fundamental Processes. — The changes occurring in the myotomes during 

 the formation of adult muscles are referable to the operation of the following fun- 

 damental processes : 



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

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

 orientation. 



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

 Thus the latissimus dor si originates from cervical myotomes, but finally attaches 

 to the lower thoracic and lumbar vertebrae and to the crest of the ihum. Other 

 examples are the serratus anterior and the trapezius. 



(3) K fusion of portions of successive myotomes. The rectus abdominis illus- 

 trates this process. 



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

 verse muscles of the abdomen are formed by this common process. 



(6) A degeneration of myotomes, wholly or in part. In this wa-yfascias, liga- 

 ments, and aponeuroses may be produced. 



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

 thoracic myotom.es and a fusion that is well advanced superficially occurs between 

 all the myotomes in embryos of 10 mm. A dorsal, longitudinal 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 tangential sphtting the various long muscles of the back which are innervated 

 by the dorsal rami of the spinal nerves. The deep portions of the myotomes do 

 not fuse, but give rise to the several intervertebral muscles, which thus retain their 

 primitive segmental arrangement. 



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

 those arising from the brancliial arches, differentiate from ventral extensions of 

 the cervical myotomes. Reference has already been made to the probable con- 

 tribution from cervical myotomes to the formation of the diaphragm (p. 188). 

 In the same way the fhoraco-abdominal muscles arise from the more pronounced 

 ventral prolongations of the thoracic myotomes which grow into the body wall 

 along with the ribs (Fig. 322). 



