MUSCULAR POWER. 137 



ing from the contraction of the circular fibres 

 is exerted to remove the central portions from 

 the surface of attachment, and thereby tends 

 to create a vacuum underneath the disk ; the 

 two surfaces remain, therefore, strongly attached 

 by the atmospheric pressure, which acts on their 

 outer sides. An apparatus of this kind, as we 

 shall afterwards find, is met with very frequently 

 among the lower orders of the animal kingdom. 



49 



Another kind of circular disposition of fibres 

 is that which occurs in the muscular coats sur- 

 rounding canals of various kinds, such as the 

 blood vessels and the alimentary tube. Their 

 action tends to contract the diameter of the 

 canal, and to exert pressure on its contents. In 

 these cases, there is generally at the same time 

 provided another layer of fibres, disposed longi- 

 tudinally, as shown in Fig. 49 ; the circular fibres 

 being seen in Fig. 50. The action of the longi- 

 tudinal fibres is evidently to shorten the canal ; 

 while that of the circular fibres, by the yielding 

 and the partial reaction of the contents of the 

 vessel, has a tendency to extend it. The Ascidia, 



