200 



ANIMAL MECHANICS. 



The first of these cases is one of common occurrence, as the 

 sphincter is often called upon to contract upon a resisting 

 body ; in this case, the cross section of the sphincter ring 

 measures the total amount of the tangential contraction possible, 

 and the body filling the orifice will be compressed by a force 

 which increases as the aperture of the orifice diminishes. 



The best example of sphincter 

 and radiating fibres to be found in 

 nature occurs in the muscles of 

 the iris, which close and open the 

 aperture of the pupil of the eye. 

 These are shown in Fig. 43. 



The sphincter muscle forms a 

 flat, narrow band on the posterior 

 surface of the inner rim of the iris 

 Fig. 43. close to the pupil, and is about 



^th of an inch wide. 



The radiating fibres, which are antagonistic to the sphincter 

 fibres, commence at the ciliary, or outer margin of the iris, 

 and are directed inwards, in bundles, towards the pupil ; at 

 the pupil itself, they blend with the sphincter, some of the 

 radiating fibres reaching as far as its inner margin. 



Under the influence of these opposing muscles, the dia- 

 meter of the pupil ranges from |rd to ^th of an inch. At any 

 given diameter of the pupil, equilibrium exists between the 

 forces T and P, developed by the sphincter and radiating 

 fibres ; the ratio between these forces, however, does not re- 

 main constant, but ranges from 3 to 20, according to the dia- 

 meter of the pupil. 



The sphincter muscle of the iris derives its motor supply 

 from the third pair of nerves, while the dilator muscle is sup- 

 plied from the sympathetic; thus, the opposing forces of these 

 antagonist muscles are instructed to act from distinct nerve 

 centres. 



