STRUCTURE OF THE lEIS. 



6or> 



ing near to its inner margin ; and at the ciliary margin also they arch 

 romid and take a somewhat circuhar direction. 



In the substance of ;he iris anteriorly and throughout its thickness 

 are variously-sho-ped ramified pigment cells hke those in the choroid. 

 The pigment contained in them is yellow, or of ligiitcr or darker shades 

 of brown, according to the colour of the eye. At the posterior surface 

 is a covering of dark pigment, the nvca of authors ; this is continuous 

 with the (retinal) pigmentary layer lining the clioroid and the ciliary 

 ]3rocesses, and here consists of several strata of small roundish cells 

 filled with dark pigment. The colour of the iris depends on the pig- 

 ment ; in the different shades of blue eye it arises from the black 

 pigment of the posterior surfoce appearing more or less through the 

 texture, which is only slightly coloured or is colourless ; but in the 

 black, brown, and grey eye, the colour is due to the pigment scattered 

 through the iris substance. 



Pupillary membrane (membiana pupillaris). — In fcetal life a delicate trans- 

 parent membrane thus named closes the pupil, and comj^letes the cui'tain of the 

 iris. The pupillary membrane contains minute vessels, continuous with those of 

 the iris and of the capsule of the crystalline lens ; they are aiTanged in loops, 

 which converge towards each other, but do not quite meet at the centre of the 

 pupil (iig. 412). At aljout the seventh or eighth month of foetal life these vessels 

 gradually disappear ; and, in in-oportion as the vascularity diminishes, the mem- 

 brane itself is absorbed fi'om near the centre towards the circumference. At the 

 period of birth, often a few shreds, sometimes a larger portion, and occasionally 

 the whole membrane is found persistent. 



Vessels and nerves of the 

 iris. — The long cUiarij arteries, 

 two in number, pierce the 

 sclerotic a little before, and one 

 on each side of, the optic nerve. 

 Having gained the interval 

 between the sclerotic and 

 choroid coats, they extend hori- 

 zontally forwards (fig. 407, i) 

 through the loose connective 

 tissue to the ciliary muscle. In 

 this course they lie nearly in 

 the horizontal plane of the axis 

 of the eyeball, the outer vessel 

 being however a little above, and 

 the inner one a little below the 

 level of that line. A short space 

 behind the fixed margin of the 

 iris each vessel (fig. 414, 1, 1) 

 divides into an upper and a 

 lower branch, and these, anasto- 

 mosing with the corresponding vessels on the opposite side and with 

 the anterior ciliary, form a vascular ring {circulus major) in the ciliary 

 muscle. From this circle smaller branches arise to supply the muscle; 

 whilst others (fig. 414, 5) converge towards the pupil, and there, freelv 

 communicating by transverse offsets from one to another, form a second 

 circle of anastomosis {circulus minor), from which capillaries are con- 

 tinued inwardly and end in small veins. 



Fig. 412. — PL-pi:.;..vi;Y ..ie.mbrane of New- 

 born Kitten, Lnjected (Kolhker, from 

 a preparation by Thiersch). Magnified. 



