Chap, xxxvii.] THE IRIS. 301 



(3) The substantia propria : this is the ground- 

 substance ; it consists of fibrous connective tissue in 

 bundles, accompanying the blood-vessels, which are 

 very numerous in the tissue of the iris. Many 

 connective tissue corpuscles are found in the sub- 

 stantia propria ; they are more or less branched, and 

 many of them contain, in all but albino and blue 

 eyes, yellowish-brown pigment granules. The colour 

 of the iris varies according to the number of these 

 pigmented connective tissue cells, and to the amount 

 of the pigment granules present in them. 



(4) A hyaline delicate basement membrane limits 

 the substantia propria at the posterior surface ; this 

 is an elastic membrane, and is continued over the 

 ciliary processes and choroid as the lamina vitrea. 



420. (5) The last layer is the epithelium of the 

 posterior surface : this is a layer of polyhedral cells, 

 filled with dark pigment granules, except in albinos, 

 where there are no pigment granules. This endo- 

 thelium is called the uvea, or tapetum nigrum. The 

 interstitial cement substance between the cells is not 

 pigmented, but transparent. 



The name " uvea " is sometimes applied to the 

 whole of the iris, ciliary processes, and choroid mem- 

 brane. 



In blue eyes the posterior epithelium is the only 

 pigmented part of the iris, and so it is also in the 

 iris of new-born children ; hence, their eyes are blue. 

 Such iris appears blue because its dull tissue is viewed 

 on dark ground i.e., on the pigmented epithelium of 

 the posterior surface. 



421. Near the pupillary border the posterior sec- 

 tion of the substantia propria contains a broad layer 

 of circular bundles of non-striped muscular tissue : 

 this is the sphincter pupillce. In connection with this 

 are bundles of non-striped muscular fibres, passing in 

 a radiating direction towards the ciliary margin of the 



