THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



269 



and fourth sacral myotomes give rise to the levator ani, the coccygeus, the 

 sphincter ani externus and the perineal muscles. The dorsal parts of the myo- 

 tomes as far as the fifth sacral probably give rise to the sacrospinalis (Fig. 228). 

 THE DIAPHRAGM. In addition to certain structures which are considered 

 in connection with the pericardium (parietal mesoderm, mesocardium and 

 common mesentery Chapter XIV), two myotomes on each side enter into 



FIG. 229. Drawing from a reconstruction of a human embryo of 20 mm. (about 7 weeks). 



Bardeen and Lewis. 



The superficial tissues have been removed from the extremities, the body wall, and the back. 



the formation of the diaphragm. These are the third and fourth cervical myo- 

 tomes, parts of which grow into the developing diaphragm in the earlier stages 

 when it is situated far forward in the cervical region (p. 346 and Fig. 298), and 

 give rise to its muscular elements. 



Muscles of the Head. 



Primitive segments (mesodermic somites) are not clearly demonstrable in 

 the heads of human embryos, nor, in fact, in the heads of any of the higher 

 Vertebrates. In some of the lower forms, however, they are very distinct. It 

 seems possible, even probable, that their indistinctness in the higher animals 



