CHAP. IL] THE CONTRACTILE TISSUES. 159 



91. As 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. 



92. 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 

 brought to bear on a part of a fresh living still warm intestine (the 

 small intestine is the best to work with) a circular contraction is 

 seen to take place at the spot stimulated ; the intestine seems 

 nipped in ringwise, as if tied round with an invisible cord ; and the 

 part so constricted, previously vascular and red, becomes pale and 

 bloodless. The individual fibres of the circular coat in the region 

 stimulated have each become shorter, and the total effect of the 

 shortening of the multitude of fibres all having the same circular 

 disposition is to constrict or narrow the lumen or tube of the in- 

 testine. The longitudinally disposed fibres of the outer longitudinal 

 coat will at the same time similarly contract or shorten in a 

 longitudinal direction, but this coat being relatively much thinner 

 than the circular coat, the longitudinal contraction is altogether 

 overshadowed by the circular contraction. A similar mode of con- 

 traction is also seen when the ureter is similarly stimulated. 



The contraction thus induced is preceded by a very long latent 

 period and lasts a very considerable time, in fact several seconds, 

 after which relaxation slowly takes place. We may say then that 

 over the circularly dispersed fibres of the intestine (or ureter) at 

 the spot in question there has passed a contraction- wave remarkable 

 for its long latent period and for the slowness of its development, 



