160 CONTRACTION OF PLAIN MUSCLES. [BOOK i. 



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 longitudi- 

 nally 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 contrac- 

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

 the contraction-wave is a simple wave propagated along the in- 

 dividual 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 neighbouring fibres or indeed in 

 any way influence neighbouring 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 fibre to fibre, both in the direction of the fibres, as when the 

 whole circumference of the intestine is engaged in the contraction, 

 or when the wave travels longitudinally along the longitudinal coat, 

 and also in a direction at right angles to the axes of the fibres, as 

 when the contraction-wave travels lengthways along the circular 

 coat of the intestine, or when it passes across a breadth of the 

 longitudinal coat; that is to say, the changes leading to contraction 

 are communicated not only in a direct manner across the cement 

 substance uniting the fibres of a bundle but also in an indirect 

 manner, probably by means of nerve fibres, from bundle to bundle 

 across the connective tissue between them. Moreover, it is obvious 

 that even the contraction-wave which passes along a single un- 

 striated fibre differs from that passing along a striated fibre, in 

 the very great length both of its latent period and of the duration 

 of its contraction. Hence, much more even than in the case of a 

 striated muscle, the whole of each fibre must be occupied by the 

 contraction-wave, and indeed be in nearly the same phase of the 

 contraction at the same time. 



Waves of contraction thus passing along the circular and longi- 

 tudinal coats of the intestine constitute what is called peristaltic 

 action. 



Like the contractions of striated muscle the contractions of 

 plain muscles may be started by stimulation of nerves going 

 to the part, the nerves supplying plain muscular tissue, running 

 for the most part as we have said in the so-called sympathetic 

 system, but being as we shall see ultimately connected with 

 the spinal cord or brain. Here however we come upon an im- 

 portant distinction between the striated skeletal muscles, and 

 the plain muscles of the viscera. As a general rule the skeletal 



