]'26 THE MECHANICAL FUNCTIONS. 



the central aperture, or pupil, the size of which 

 they dimiuish when they contract; while on the 

 contrary, the radiating fibres, acting on the inner 

 circle, and drawing it nearer to the outer circum- 

 ference, which is fixed, lessen the breadth of the 

 ring, and consequently enlarge the circular aperture. 



A similar combination of radiating and circular 

 fibres is employed in the construction of flat, or 

 slightly concave muscular disks, which are thus 

 rendered capable of exerting a strong force of ad- 

 hesion to the surfaces to which they are applied. 

 In these organs the circular fibres are placed at the 

 circumference, and the radiating fibres in the inte- 

 rior 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 cir- 

 cular fibres is exerted to remove the central por- 

 tions 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. 



Another kind of circular disposition of fibres is 

 that which occurs in the muscular coats surround- 

 ing canals of various kinds, such as the blood ves- 

 sels, 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 longitudinally, as shown in Fig. 

 49 ; the circular fibres being seen in Fig. 50. The 

 action of the longitudinal fibres is evidently to 



