364 SPECIAL HISTOLOGY. 



terlaced, constitute the principal bulk of the stroma of this 

 tunic, and, towards its surface, form a more homogeneous 

 layer. It contains a large number of elongated nuclei, which, 

 at any rate in part, are situated in fusiform cells, similar to 

 those of the choroid, only smaller ; and also a few rigid, pale 

 fibres, which, as prolongations of the ligamentum pectinatum of 

 the iris, or of the " membrane of Demours," are continued 

 over a part of the anterior surface ; lastly, the smooth muscular 

 fibres of the iris, presenting exactly the same characters as 

 those of the choroid [ciliary muscle]. In Man, these fibres 

 constitute a very distinct occlusor muscle of the pupil [sphincter 

 pupillce), in the form of a smooth ring, |"' 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 recognised 

 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'" long. Besides this larger muscular 

 ring, I find, close to the annulus iridis minor, another very 

 narrow ring, nearer the anterior surface of the iris, not more 

 than i'" in breadth. Briicke traces the dilatator 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 fasciculi, 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 the pigmentum nigrum of the iris 

 (fig. 295 n), is a stratum, 00089"' 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 delicate, but sharply denned line, which has been described 



1 [Compare Mr. Lister's 'Observations on the contractile tissue of the Iris/ 

 ' Quarterly Journal of Micr. Science/ vol. i, p. 8, October, 1852. — Eds.] 



