THE MUSCLES. 



345 



action. A striated muscle fiber may be I J inches long and 

 2^0- of an inch wide, though usually less. The heart muscle 

 fiber is narrower than the skeletal fiber, and the plain fiber 

 very much smaller than either. (But the figures do npt 

 attempt to give relative proportions with any exactness.) 



Each Muscle Fiber is a Muscle Cell. It is easily seen 

 that each plain muscle fiber is a single cell, having its dis- 

 tinct nucleus. The same is true of the heart muscle fibers, 



-- Nucleus 



."-- Isolated Fibers 



Fibers Joined 



Fig. 110. Plain (unstriated) mus- 

 cular fibers from the bladder. 



Fig. 111. Two striated mus- 

 cular fibers showing thfe ter- 

 minations of the nerves. 



though they are not so simple, being more or less branched. 

 In the development of striated muscle, when the muscular 

 fibers are about to be formed, the cells from which they 

 develop (called muscle plates) become elongated so that 

 each cell is converted into a long protoplasmic fiber, with 

 many nuclei. Most investigators agree that the striated 

 fibers are produced by the elongation of single cells with 

 multiplication of their nuclei, though some have thought 

 that the fiber is formed by the coalescence of several cells 

 end to end. 



Muscles of Expression. The facial expression is due 

 to the action of the muscles of the face, which in turn are 



