136 THE MECHANICAL FUNCTIONS, 



circular direction, forming what is called an 

 orbicular \ or sphincter muscle, of which an example 

 occurs in that which surrounds and closes the 

 eye. (Fig. 46.) Very frequently these two last 

 modes of arrangement are united in some part, 

 as appears to be the case in the membrane of 

 the eye, called the Iris. (Fig. 47.) The circular 

 fibres of the iris surround the central aperture, 

 or pupil, the size of which they diminish when 

 they contract ; while on the contrary, the radi- 

 ating fibres, acting on the inner circle, and 

 drawing it nearer to the outer circumference, 

 which is fixed, lessen the breadth of the ring, 

 and consequently enlarge the circular aperture. 



47 48 



A similar combination of radiating and cir- 

 cular fibres is employed in the construction of 

 flat, or slightly concave muscular disks, which 

 are thus rendered capable of exerting a strong 

 force of adhesion to the surfaces on which they 

 are applied. In these organs the circular fibres 

 are placed at the circumference, and the radi- 

 ating 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 result- 



