106 



THE MECHANICAL FUNCTIONS. 



the circumference, and the radiating fibres in the interior of 

 the sucker, (see Fig. 48;) so that, while the margin of the 

 disk is closely applied to the object, the force resulting 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 at- 

 mospheric pressure, which acts on their outer sides. An ap- 

 paratus of this kind, as we shall afterwards find, is met with 

 very frequently among the lower orders of the animal king- 

 dom. 



Another kind of circular disposition of fibres is that which 

 occurs in the muscular coats surrounding 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 

 2;enerally at the same time provided another layer of fibres, 

 disposed longitudinally, as shown in Fig. 49; the circu- 

 lar 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 re- 

 action of the contents of the vessel, has a tendency to 

 extend it. The Jiscidia, which is a species of marine worm, 

 is an example of an animal whose skin contains a union of 

 straight and circular fibres, by which all its movements are 

 readily performed. Many instances occur in the cylindrical 

 envelopes of animals, of the combination of a third series of 

 fibres, passing obliquely, with those which have transverse 

 and longitudinal directions. In the muscular skin of the 

 Leechy for example, besides two internal layers of longitudi- 

 nal fibres, an external one has lately been discovered, which 

 is composed of oblique or spiral fibres, crossing one another 



