250 



TWENTY-FOURTH LECTURE. 



boundary region of the cornea and sclerotica. Meridional 

 bundles of the former radiate in a posterior direction into the 

 ciliary body. Below and inwards occur interwoven filaments, 

 and still further inwards, circular bundles (Mueller's ring 

 muscle). 



We meet with colorless connective-tissue cells in the con- 

 nective-tissue substratum of the iris of light eyes, and pig- 

 mented cells in that of dark ones. Besides these, smooth 

 muscular elements occur. Annular bundles (Fig. 201, a) 



Fig. 201. — Surface of the human iris ; a, the sphincter ; b, the dilator of the pupil. 



form the constrictor or sphincter of the pupil. From it pro- 

 ceeds the dilator pupillse, an object of controversy of later 

 years. 



Muscular bundles, which are at first separated, form more 

 peripherically a connected radial layer of fibres (b). At the 

 ciliary, that is the outer border, we finally meet with an an- 

 nular muscular layer. 



This external or ciliary border of the iris gives rise at its 

 anterior surface to another peculiar tissue, the ligamentum 

 pectinatum iridis (Huek). 



We have already learned (p. 56) that the posterior surface 

 of the cornea is covered by a hyaline membrane, the mem- 

 brana Descemetica or Demoursii. At its periphery, this 

 posterior covering layer passes over into a peculiar reticular 

 tissue (probably, in man, most intimately connected with the 

 elastic tissue), which passes through the outer margin of the 



