THE MUSCULAR SYSTEM 205 



lower animals. The pyramidalis abdominis muscle is an example. When 

 present in man the pyramidalis arises from the pubic bone anterior (ven- 

 trad) to the rectus abdominis muscle. Its length varies greatly in individu- 

 als. It may occur on one or both sides or may be wanting. In 

 non-placental mammals the pyramidalis is powerfully developed in con- 

 nexion with the marsupial bones which it serves to support. Even in 

 insectivores in which the marsupium has disappeared the pyramidalis 

 muscle is well developed. The presence in man of such a useless rudiment 

 suggests the animal origin of the human body. 



Rudimentary integumentary muscles occasionally appear in individu- 

 als. Among these are the stemalis muscle of the chest and the axillary 

 muscle connected with the pectoralis in the axillary region. They are 

 normally present in apes, but occur in human individuals only excep- 

 tionally. They are interpreted as remnants of the panniculus camosus 

 of lower mammals. 



Of similar significance is the fact that, although metamerism is evident 

 in few adult mammalian muscles (intercostals, intervertebrals and the 

 rectus abdominis), nevertheless all the skeletal muscles arise from the 

 metameric somites of the embryo. Why are embryonic myotomes meta- 

 meric when the muscles which develop from them are not metameric? 

 The primary metamerism corresponds with the muscular metamerism of 

 the skeletal muscles of the lower vertebrates. Does this fact not give the 

 clue to the muscular metamerism of the human embryo? 



Developm.ent of the Muscles. Classified on the basis of their onto- 

 genetic development, muscles are of two kinds: (i) Somatic Muscles, 

 derived from the mesodermal epimere, and (2) Visceral Muscles, which 

 develop from the hypomere. With the exception of the smooth muscles 

 of the eyeball, which are of ectodermal origin, all muscles are mesodermal. 



We may then describe first the development of the Somatic Muscles, 

 derived from the Epimere or Somite. 



Very early in ontogenesis the mesoderm becomes divided into a 

 metameric series of "somites." In all chordates above the cephalo- 

 chordates (Amphioxus) the metamerism affects only the dorsal portion of 

 the mesoderm, that is, the portion known as the epimere. In the trunk 

 region — but not in the head — of vertebrate embryos the adjacent mesomere 

 becomes segmented as the nephrotome. The epimere becomes later 

 differentiated into (i) myotome which forms muscle, (2) sclerotome 

 which forms skeletal material, and (3) dermatome which gives rise to loose 

 connective tissue. 



In the embryos of the lower vertebrates, as shown in Fig. 197, the somites 

 extend in an unbroken series throughout head and trunk. In embryos of 

 the higher vertebrates, however, the metamerism of the mesoderm in the 

 head region disappears. Only in the embryos of Amphioxus and cyclo- 



