MUSCLES AND PHENOMENA OF CONTRACTION. 121 



When a muscle contracts it may be seen that the white 

 band becomes narrower, but that the teeth-like projections 

 of the sarcosome become pushed further apart and dis- 



S.E. 



Fig. 63. DIAGRAM OF A SINGLK SARCOMERK. 



A, in relaxed condition; B, in contraction; K, K, membrane of Krause; H, membrane 

 of Hensen ; S, E, the darker sarcous element. 



tended. The phenomena of contraction according to this 

 view are therefore explained in this manner. When a nerv- 

 ous impulse reaches the muscle directing it to contract, in 

 some way yet entirely unknown, the hyaloplasm by its 

 activity flows in between the prongs of the sarcosome and 

 distends these. In this way each sarcomere is slightly 

 shortened. The multiplication of this shortening through 

 all of the sarcomeres that make up a muscle fibre produces 

 a movement of the entire muscle. Whether this hyalo- 

 plasm forces itself in between these prongs, or whether it 

 distends the sarcosome by flowing into these prongs, which 

 in the latter view would be hollow, is not known. While 

 this serves in a physical way to make clear to us the real 

 manner in which a muscle fibre shortens, it leaves as un- 

 answered as before what causes the hyaloplasm in this 

 active way to flow into the sarcosome and bulge it out 

 laterally, thus shortening the long diameter of the fibre. 



Plain Muscular Tissue. 



Plain muscular tissue also is composed of fibres; but 

 these fibres are very much smaller than those of voluntary 

 muscular tissue, and possess no cross markings whatever. 

 They are so much smaller than the voluntary that on an 

 average the length of an involuntary is less than the width 

 of a voluntary. These plain muscle fibres are about one 



