SEC. 6. ON SOME OTHER FORMS OF CONTRACTILE 



TISSUE. 



Plain, Smooth or Uhstriated Muscular Tissue. 



§ 82. This, in vertebrates at all events, rarely occurs in isolated 

 masses or muscles, as does striated muscular tissue, but is usually 

 found taking part in the structure of complex organs, such for 

 instance as the intestines ; hence the investigation of its properties 

 is beset with many difficulties. 



§ 83. So far as we know plain muscular tissue in its chemical 

 features resembles striated muscular tissue. It contains albumin, 

 some forms of globulin, and antecedents of myosin which upon the 

 death of the fibres become myosin ; for plain muscular tissue after 

 death becomes rigid, losing its extensibility and probably becoming 

 acid, though the acidity is not so marked as in striated muscle. 

 Kreatin has also been found, as well as glycogen, and indeed it 

 seems probable that the whole metabolism of plain muscular 

 tissue is fundamentally the same as that of the striated muscles. 



§ 84. In their general physical features plain muscular fibres 

 also resemble striated fibres, and like them they are irritable and 

 contractile ; when stimulated they contract. The fibres vary in 

 natural length in different situations, those of the blood vessels for 

 instance being shorter and stouter than those of the intestine ; but 

 in the same situation the fibres may also be found in one of two 

 different conditions. In the one case the fibres are long and thin, 

 in the other case they are reduced in length, it may be to one half 

 or even to one third, and are correspondingly thicker, broader 

 and less pointed at the ends, their total bulk remaining unaltered. 

 In the former case they are relaxed or elongated, in the latter case 

 they are contracted. 



The facts of the contraction of plain muscular tissue may be 

 studied in the intestine, the muscular coat of which consists of an 

 outer thin sheet composed of fibres and bundles of fibres disposed 

 longitudinally and of an inner much thicker sheet of fibres disposed 

 circularly ; in the ureter a similar arrangement of two coats obtains. 



If a mechanical or electrical (or indeed any other) stimulus be 



