THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 295 



of the muscles and concerning the innervation of the muscles, certain factors 

 can be recognized, to one or more of which the changes in the myotomes may 

 be referred. These factors are as follows: 



1. Migration. The myotomes may migrate in whole or in part, and the 

 muscles derived from them may be situated far beyond their limits. For 

 example, the latissimus dorsi is derived from cervical myotomes but ultimately 

 becomes attached to the lumbar vertebrae and to the crest of the ilium. To this 

 factor, possibly more than to any other, is due the loss of the segmental character 

 in the musculature. 



2. Fusion. Portions of two or more myotomes may fuse to form one muscle. 

 For example, each oblique abdominal muscle is derived from several thoracic 

 myotomes. 



3. Longitudinal Splitting. Very frequently a myotome or a developing 

 muscle splits longitudinally into two or more portions. The sternohyoid and 

 the omohyoid, for example, are formed in this manner. 



4. Tangential Splitting. A developing muscle may split tangentially into 

 two or more plates or layers. The two oblique and the transverse abdominal 

 muscles, for example, are formed in this way. 



5. Degeneration. Myotomes may degenerate as a whole or in part and be 

 converted into some form of connective tissue, such as fascia, ligament or 

 aponeurosis. The aponeuroses of the transverse and oblique abdominal 

 muscles are probably due to a degeneration of portions of the myotomes from 

 which the muscles are derived. 



6. Change of Direction. The muscle fibers may change their direction. 

 As a matter of fact, the fibers of very few muscles retain their original direction. 



Muscles of the Trunk. 



The myotomes are at first arranged serially along each side of the notochord and 

 spinal cord (compare Fig. 262 with Figs. 143 and 261). By the end of the second 

 week fourteen myotomes are differentiated in the human embryo. Differen- 

 tiation continues until, by the end of the fourth week, the total number thirty- 

 eight is present. Of the thirty-eight, three are occipital, eight cervical, twelve 

 thoracic, five lumbar, five sacral, and five (or six) coccygeal. The occipital 

 myotomes are transient structures that appear in relation with the hypoglossal 

 (XII) nerve. The cervical, thoracic, lumbar, sacral and coccygeal myotomes 

 correspond individually to the spinal nerves (Fig. 262). As stated on page 184, 

 the myotomes alternate with the anlagen of the vertebrae. Consequently in the 

 cervical region there are eight myotomes, corresponding to the eight cervical 

 spinal nerves, and only seven vertebrae. The myotomes in the neck and body 

 regions are destined to give rise to the dorsal musculature, to the thoraco- 



