CHAPTER LV. 



THE MUSCULAR SYSTEM. 



The general Muscular system has been discussed in considerable 

 detail in our study of the frog while the development of the muscles was 

 taken up in the study of embryology. 



It will be remembered that histologically there are voluntary and in- 

 voluntary muscles, the former being striated, the latter smooth, while 

 the heart muscles are a sort of combination of these two. 



The smooth muscles have their beginnings in mesenchyme, and, 

 being involuntary, are innervated by the sympathetic nervous system. 

 Their action is also much slower than that of the striated muscle. They 

 are found in the skin, in the walls of blood vessels, in the walls of the 

 digestive canal and in the urogenital system. 



The striated muscles have their origin in the walls of the coelom 

 and are of mesothelial origin. They are supplied by the motor nerves of 

 the central nervous system. They are all voluntary except those at the 

 more cephalic end of the digestive tract. Striated fibres may be found 

 in the body walls, in all organs of locomotion, in the head, in the 

 diaphragm, and in the cephalic end of the alimentary canal. 



The voluntary muscles arise from the somites (which divide into 

 myotomes and lateral plates, after the epimeres have given rise to the 

 sclerotomes and dermatomes). 



The myotome grows downward between the hypomere and the skin 

 to meet its fellow on the opposite side in the median ventral line. This 

 produces a completed coat of voluntary muscles which lies beneath the 

 skin. The muscle coat is divided into a dorsal and ventral part by the 

 horizontal skeletogenous partition (Fig. 423) which intersects the skin 

 at the lateral line. The dorsal muscles are called epaxial, and those ven- 

 tral to the septum, hypaxial. 



The muscles originating from the lateral plates, in the gill-arch 

 region, which move the gill arches are called visceral muscles. 



The muscles originating from the myotomes are called parietal or 

 somatic muscles. 



All muscles except the diaphragm and heart (the heart is always 

 included under the circulatory system) are divided into three groups 

 known as parietal, visceral, and dermal muscles. 



From our embryological study it will be remembered that the myo- 

 tomes were cut off from the walls of the coelom, each one forming a 

 closed sac, the inner wall being called the splanchnic layer and the outer 

 the somatic layer. The more dorsal cells of the splanchnic layer develop 

 many nuclei which can be seen in the interior of the cell in the lower 

 vertebrates. They are quite close to the surface in the muscle fibres of 

 mammals. Each myotome has its splanchnic wall converted into a 



