CHAPTER VI 

 THE MUSCULAR SYSTEM 



" When we no longer look at an organic being as a 

 savage looks at a ship, as something wholly beyond 

 his comprehension ; when we regard every produc- 

 tion of nature as one which has had a long his- 

 tory; when we contemplate every complex structure 

 and instinct as the summing up of many contrivances, 

 each useful to the possessor, in the same way as any 

 great mechanical invention is the summing up of the 

 labor, the experience, the reason, and even the 

 blunders of numerous workmen ; when we thus view 

 each organic being, how far more interesting . . . 

 does the study of natural history become ! " 



CH. DARWIN, Origin of Species. Chapter XV. 



VERTEBRATES possess two sorts of muscular tissue, un- 

 striated, or involuntary, in the form of cells, and striated or 

 voluntary, in the form of long fibers usually formed from 

 cell-complexes. The cells of the involuntary type are mesen- 

 chymatous in origin and are usually associated together to 

 form layers that supply the walls of certain internal organs and 

 the larger blood vessels with the power of expansion and con- 

 traction. All voluntary muscles, on the other hand, are de- 

 rived directly from the mesoderm, and the ultimate contractile 

 organs are here not the cells themselves, but long fibrils of 

 contractile substance built up by the cells, or often by long 

 rows of cells, and organically connected with them. The mus- 

 culature of the heart, in some characteristics seemingly inter- 

 mediate between the two classes of muscular tissue, is a modi- 

 fication of the first or involuntary type, the cells of which 

 possess a peculiar shape and become marked with striae. This 

 derivation is rendered clear through the embryological develop- 

 ment of the heart, which is seen to be originally an expanded 

 blood vessel with an hypertrophied muscular coat. 



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