CHAPTER XIII 



THE MUSCULAR SYSTEM 



The muscular system is the largest in the body, the skeletal 

 muscles alone representing 45 per cent, of the body weight. 



The regulation of the blood supply, the movements of the 

 skeleton, the contraction of the heart, and the transport of the 

 ingesta along the intestinal canal, are all examples of muscular 

 movement, and, further, they are examples of different kinds of 

 movement ; the slowly moving intestinal canal is very different 

 from the active skeletal muscles, and these, with their long 

 periods of activity and rest, are greatly in contrast with the 

 rhythmical movements of the heart. 



Structure of Muscle. — There are three varieties of muscle in the 

 body : 



1. Voluntary, skeletal, striped, or red muscle. 



2. Involuntary, pale, smooth, or unstriped muscle. 



3. Heart muscle. 



The voluntary muscles are generally in large masses known as 

 ' flesh,' and their function is to move the skeleton. The muscle 

 mass consists of bundles, the bundles are composed of smaller 

 bundles, the smaller bundles are made up of fibres. The fibre of 

 a muscle does not run the length of the bundle ; on the other hand, 

 a primitive fibre is only about 4 or 5 centimetres (ij to 2 inches) 

 in length, and of microscopic thickness — viz., ^-J^ inch on an 

 average. The fibre is as a rule unbranched, its ends are rounded, 

 and it is enclosed within a sheath, or sarcolemma. The contents 

 of the fibre consist of a semifluid, contractile substance, marked 

 with alternate dark and light bands (Fig. no). Each fibre is 

 made up of bundles of fibrils or sarcostyles, between which is a 

 coarse network or reticulum, known as sar ooplasm. The sarco- 

 styles show the light and dim bands seen in the fibre. Through 

 the middle of each light stripe is a transverse line known as 

 Dobie's or Krause's membrane (Fig. in). The dim band is 

 the sarcous element ; each is separated by a line known as 

 Hensen's, and one portion of the dim band, together with a 



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