ON SOME OTHER FORMS OF CONTRACTILE TISSUE. 117 



glycogen, and, indeed, it seems probable that the whole metabolism of 

 plain muscular tissue is fundamentally the same as that of the striated 

 muscles. 



88. In their general physical features plain muscular fibres also resem- 

 ble 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 bloodvessels, 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 intestine. The longitudi- 

 nally 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 

 contraction 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 relaxa- 

 tion slowly takes place. We may say then that over the circularly disposed 

 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, the wave being propagated from fibre to fibre. From 

 the spot so directly stimulated the contraction may pass also as a wave (with 

 a length of 1 cm. and a velocity of from 20 to 30 millimetres a second in 

 the ureter), along the circular coat both upwards and downwards. The 

 longitudinal fibres at the spot stimulated are, as we have said, also thrown 

 into contractions of altogether similar character, and a wave of contraction 

 may thus also travel longitudinally along the longitudinal coat both upwards 

 and downwards. It is evident, however, that the wave of Contraction of 

 which we are now speaking is, in one respect, different from the wave of 

 contraction treated of in dealing with striated muscle. In the latter case 

 the contraction-wave is a simple wave propagated along the individual fibre, 

 and starting from the end-plate or, in the case of direct stimulation, from 

 the part of the fibre first affected by the stimulus ; we have no evidence that 

 the contraction of one fibre can communicate contraction to neighboring 

 fibres, or, indeed, in any way influence neighboring fibres. In the case of 

 the intestine or ureter the wave is complex, being the sum of the contraction- 

 waves of several fibres engaged in different phases, and is propagated from 



