40 NUTRITIONAL PHYSIOLOGY 



of nerve-fibers placing it in connection with the brain or 

 the spinal cord and under their control. If its connection 

 is severed it becomes paralyzed and remains inactive, 

 unless special local means, like electricity, are employed 

 to excite it. Ordinarily we are justified in saying that 

 skeletal muscle is not automatic, meaning that every move- 

 ment which it makes is an indication of a previous act, or, 

 as we say, a discharge on the part of the nervous system. 



Fig. 4. The above represents somewhat diagrammatically a 

 very small fraction of the length of a fiber of skeletal muscle. To 

 include the entire element with the length proportional to the width 

 we should have to extend this drawing to a length of several yards. 

 The fiber is cylindric and enclosed by a more definite membrane than 

 is usual with animal cells. The cross-marking is not a feature of this 

 membrane, but stands for a peculiar organization of the protoplasm 

 inside. Nuclei are seen here and there near the surface. The seg- 

 ment shown is supposed to be the particular one about in the middle 

 of the fiber within which falls the connection with the nervous sys- 

 tem. A nerve-fiber (n.f.) is seen making a junction with the muscle- 

 fiber (M) through the so-called end-plate (e.p.). 



In somewhat sharp contrast is the behavior of the 

 muscle composing the heart and of the form which is 

 found in the viscera. These two kinds of contractile 

 tissue are described as automatic, in the sense that they 

 show a tendency to rhythmic contraction and relaxation 

 even when deprived of their nervous connections. The 

 automatic property of the heart is the cause of its beating. 



