CHAPTER XII 



MUSCULAR SYSTEM 



Muscle tissues are of mesodermal origin and are specialized for 

 contractility. In all groups of animals above the coelenterates 

 it is these tissues which are concerned with locomotion. In the 

 vertebrates a study of the muscular system is usually limited to 

 the striated skeletal muscles, for by common consent the smooth 

 muscles are considered with the digestive tract, and the cardiac 

 muscles with the vascular system. 



A. Development and Classification of Muscles 



The mesoderm arises as metameric blocks on either side of 

 the notochord. Each of these segments soon becomes differenti- 

 ated into epimere, an intermediate cell mass or nephrotome, and 

 the hypomere (illustrations on page 68). From the hypomere 

 arise the cardiac and smooth muscles, and the striated muscles 

 of the gills; and also the mesenteries and peritoneum. The cav- 

 ity which forms within the hypomere becomes the coelomic cav- 

 ity of the body. 



The epimere gives rise to the dermis of the skin, the con- 

 nective tissues and bone, and the skeletal muscles. The last de- 

 velop from the central portion of the epimere (the myotome) 

 which grows ventrally between the peritoneum and the ectoderm 

 of the embryo. When each myotome meets its mate from the 

 opposite side the metameric structure of the body is completed. 

 The myotomes are heavier in the dorsal region than in the 

 ventral. Slightly later in development the myotomes are divided 

 into dorsal and ventral regions by an axial line from anterior 

 to posterior which parallels the ventral border of the notochord. 

 The muscles which develop from the myotomes above this line 

 are the epaxial group, the ventral muscles being the hypaxial 



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