704 



THE MUSCULAR SYSTEM 



MESENCHYME GIVES 

 ORIGIN TO SMOOT 

 MUSCLE TISSUE 



ENTODERM 



VENTRAL SEPTUM 



PLEURAL R 



MYOSEPTjM 



Fig. 326. Arrangement of muscle tissues. (A) Ventricles of alligator heart, ventral 

 aspect, showing spiral arrangement of superficial muscle layers. (Redrawn from Shaver, 

 Anat. Rec, 29.) (B) Arrangement of smooth muscle layers of the stomach. (Redrawn 

 from Bremer, 1936, Textbook of Histology, Philadelphia, Blakiston. after Spalteholz. ) 

 (C) Transverse section of tail of Squalus acanthias showing arrangement of epaxial and 

 hypaxial muscle groups. (D) Primitive arrangement of myotomes into epaxial and 

 hypaxial groups in relation to the myocommata or myosepta. Observe that the myoseptum 

 attaches to the middle of the vertebra. (Redrawn and modified from Goodrich, Vertebrate 

 Craniata, 1909, New York, Macmillan Co., and Kingsley, Comparative Anatomy of 

 Vertebrates, 1912, Philadelphia, Blakiston. 



continue to increase, they become aggregated into groups and are arranged 

 in such a manner that the dark and light bands of adjacent fibrils form regular 

 dark and light bands across the muscular strands. The intercalated discs 

 finally make their appearance here and there across the muscle strands (fig. 

 325C). In some areas, there are no nuclei within the muscle strand between 

 the intercalated discs. 



3. Smooth Muscle 



Smooth muscle cells arise from mesenchyme. In doing so, the mesenchymal 

 cells lose their stellate shapes, elongate, and eventually become spindle shaped. 

 Accompanying these changes, the nuclei experience some extension in the 

 direction of the elongating cells (fig. 325B). Fibrils appear in the cytoplasm, 

 first at the periphery in the form of coarse fibers, to be followed somewhat 

 later by the true myofibrils of finer texture. It is possible that the coarser 

 fibrils, the so-called myoglial fibers, represent bundles of myofibrils. The 



