SMOOTH MUSCLE-FIBRES. 



97 



<?., the walls of which are composed of these smooth 

 muscle-fibres. In such cases they are usually arranged 

 in two payers, one of which consists of ring-shaped fibres 

 surrounding the tube, while the other consists of fibres 

 arranged parallel to the tube. When, therefore, these 

 muscle-fibres contract, they are able both to reduce 



Fro. 25. SMOOTH MUSCL.E-FIBIJKS (300 TIMES ENLARGED). 



the circumference, and to shorten the length of the 

 walls of the tube in which they occur. This is of great 

 importance in the case of the smaller arteries, in which 

 the smooth muscle-fibres, arranged in the form of a 

 ring, are able greatly to contract, or even entirely to 

 close the vessels, thus regulating the current of blood 

 through the capillaries. In other cases, as in the in- 

 testine, they serve to set the contents of the tubes in 

 motion. In the latter cases the contraction does net 



