702 THE MUSCULAR SYSTEM 



tapers gradually from the middle area and may terminate in a pointed or 

 slightly truncate tip (fig. 325B). 



Smooth muscle cells may contain two kinds of fibrils: 



( 1 ) fine myofibrils, presumably concerned with contraction phenomena, 

 within the cytoplasm and 



(2) myoglial or border fibrils, coarser than the myofibrils, in the peripheral 

 areas of the cell. 



The myoglial fibrils are not usually demonstrable in adult tissues. 



A connective-tissue mass of fibers between the smooth muscle fibers which 

 binds the fibers into bundles as in skeletal muscle is not readily demonstrated. 

 It may be that a kind of adhesiveness or stickiness (Lewis, W. H., '22) asso- 

 ciates these muscle fibers into a mass, within which each muscle cell is a 

 distinct entity and not part of a syncytium. However, around the muscle 

 bundles, elastic and white fibers (Chap. 15) seem to hold the muscle tissue 

 in place and some elastic fibers may be present between the cells, especially 

 in blood vessels. 



B. Histogenesis of Muscle Tissues 



1. Skeletal Muscle 



The primitive embryonic cell which gives origin to the later muscle cells is 

 called a myoblast. The myoblasts which give origin to skeletal muscle fibers 

 are derived from two sources: 



( 1 ) mesenchyme and 



(2) myotomes. 



(See Chap. 11 for origin of m.esenchyme and myotomes; also consult 

 fig. 252.) 



In striated-muscle-fiber formation, the myoblasts begin to elongate and 

 eventually produce cylinder-like structures. As the cell continues to elongate, 

 the nuclei increase in number, and, hence, the myoblast becomes converted 

 into a multinuclear affair in which the nuclei at first lie centrally along the 

 axis of the cell. Later, the myofibrils increase, and the nuclei move peripherally. 



As the myofibrils grow older, dark and light areas appear along the fibrils. 

 These dark and light bands are shown in figure 325E. Observe that the light 

 band is bisected by the slender membrane, known as Krause's membrane, 

 shown in the figure as the dark line, Z., and the dark band is bisected by 

 Hensen's membrane. 



2. Cardiac Muscle 



The musculature of the vertebrate heart takes its origin from the two mesial 

 walls of hypomeric fnesoderm (i.e., the splanchnic layers of mesoderm) which 

 come to surround the endocardial primordia or primitive blood capillaries 



