736 SPECIAL HISTOLOGY. 



(sphincter pupilla*) in the form of a smooth ring, J of a line wide, close 

 to the pupillary margin of the iris, and somewhat nearer to the pos- 

 terior surface, which, in a blue iris, may be readily recognized after the 

 removal of the posterior pigment, with and without the application of 

 acetic acid, and may also be torn up into its elements, 0-02-0-03 of a 

 line long. Besides this larger muscular ring, I find, close to the annu- 

 lus iridis minor, another very narrow ring, nearer the anterior surface 

 of the iris, not more than 1-40 of a line in breadth. Brlicke traces the 

 dilator pupillce as far as the ligamentum pectinatum and the border of 

 the vitreous lamella of the cornea, but I am unable to do so ; and it 

 rather appears to me to commence in the substance of the iris at the 

 ciliary margin. From what the difficulty of the investigation has 

 allowed me to see of this muscle, it consists of numerous slender fasci- 

 culi, which, far from constituting a continuous membrane, run inwards, 

 each separately between the vessels, and are inserted at the border of 

 the sphincter* 



The iris differs from the choroid also, in possessing a cellular layer on 

 the anterior and posterior surfaces. The latter, the so-termed uvea of 

 authors, or t\iQ pigmentum nigrum of the iris (Fig. 295 n) is a stratum, 

 0-0089 of a line thick, of minute, closely filled pigment-cells, like those 

 of the ciliary body, with which they are also uninterruptedly connected, 

 and which lines the whole of the posterior surface of the iris, extending 

 as far as the border of the pupil. When the iris is folded, the pig- 

 mentary stratum, or its free surface, appears to be bounded by a deli- 

 cate, but sharply defined line, which has been described by several 

 authors as a special membrane (membrana pigmenti, Krause, m. 

 limitans, Pacini [?], Briicke, M. Jacobi, Arnold), and, in fact, in eyes 

 that have been kept for some time, and on the addition of alkalies, may 

 be raised in places from the pigment. But since, in such instances, the 

 pigmentary layer is always without any defined outline, and its granules 

 are exposed and dispersed, it appears to me that this membrane is 

 nothing more than the conjoined outer cell-walls of the pigment-cells, 

 which, as is known to be the case elsewhere (intestinal villi for instance) 

 are raised in their totality, and apparently as a special membrane. The 

 cellular layer of the anterior surface of the iris is a simple epithelium of 

 rounded and much flattened cells, which, when viewed in a fold of the 

 iris, are seen to constitute, not a continuous, clear border of uniform 

 breadth throughout, but on the contrary, only distinct, slight elevations. 

 This layer is better seen after the removal of the posterior pigment, in 

 a horizontal view, and also by scraping or shaving off the anterior sur- 

 face of the m's.f The color of the iris, in* blue eyes, depends simply 

 upon the posterior pigment seen through its substance; whilst in 



* [Compare Mr. Lister's " Observations on the contractile tissue of the Iris," " Quarterly 

 Journal of Micr. Science," vol. i. p. 8, October, 1852. TRS.] 

 note, p.730. TBS.] 



