CHAPTER XI 

 THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



Anatomy and Histology show that there are, in a sense, two muscular 

 systems in the body, and Embryology teaches that the two systems have dif- 

 ferent origins. 



i . The skeletal musculature. — This, as the name indicates, is closely associated 

 with the skeletal system. It is made up of striated muscle fibers arranged to 

 form definite bundles or muscles. The skeletal musculature is under the 

 voluntary control of the central nervous system. 



2. The visceral musculature. — This is found in connection with and forms 

 integral parts of certain organs. It is made up of two different kinds of fibers — 

 smooth muscle fibers or cells and striated fibers or cells (heart-muscle cells). 

 The latter are found only in the wall of the heart. The visceral musculature is 

 involuntary, being under the control of the sympathetic nervous system. 



Both systems are derived from mesoderm but from distinct parts of the 

 mesoderm. Furthermore, their developmental histories are quite different, as 

 will be seen in the following paragraphs. 



THE SKELETAL MUSCULATURE. 



In the chapter on the development of the germ layers it was said (p. 72) 

 that throughout the length of the body region of the embryo the mesoderm on 

 each side of the neural tube and notochord becomes divided into a definite 

 number of segments — the primitive segments or mesodermic somites (Figs. 57, 

 72, 74). These indicate the segmentation of the body, and the history of the 

 greater part of the skeletal musculature dates from their differentiation from 

 the axial mesoderm. Thus the skeletal musculature is, for the most part, 

 primarily segmental in character. 



At first the primitive segments are composed of closely packed, epithelial- 

 like cells, and each segment contains a small cavity which represents a portion 

 of the ccelom (Fig. 141). The ventro-medial parts of the segments become 

 differentiated to form the sclerotomes which are composed of more loosely ar- 

 ranged cells (Fig. 261), and which are destined to give rise to the vertebras and 

 to the various kinds of connective tissue in their neighborhood. The lateral 

 parts of the segments become differentiated to form the cutis plates which are 

 destined to give rise to a part of the corium of the skin. The remaining portions 



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