688 THE ORGANS OF SENSE. 
peripheral border, or margo ciliaris, is directly continuous with the ciliary body, and, 
through the medium of the ligamentum pectinatum iridis, with the posterior elastic 
lamina of the cornea. Its free edge, or margo pupillaris, forms the circumference of 
the pupil, and rests upon, but is not attached to the anterior surface of the capsule 
of the lens. 
The distinctive colour of the eye, im different individuals, depends on the arrangement 
of the pigment in the iris; in the blue eye this is limited to the posterior surface of the 
iris, but in the brown or black eye it is also scattered throughout its stroma. In the albino 
the pigment is entirely absent, and the red appearance of the eye in such a case is pro- 
duced by the network of blood-vessels in the iris. 
The pupil is closed, during the greater part of foetal life, by a thin transparent 
vascular membrane, the membrana pupillaris, continuous with the pupillary margin of 
the iris. Its vessels, derived’ partly from the vessels of the iris and partly from those of 
the capsule of the lens, converge towards the middle of the membrane, near which they 
form loops so as to leave the central part non-vascular. About the seventh month the 
vessels begin to be obliterated from the centre towards the circumference, and this is 
followed by a thinning and absorption of the membrane, which becomes perforated by the 
aperture of the pupil. This perforation gradually enlarges, and at birth the membrane 
has entirely disappeared, although in exceptional cases it persists. 
On the anterior surface (facies anterior) of the iris is a layer of flattened 
endothelium, placed on a basement membrane, and ‘continuous with that which 
lines the spaces of Fontana and covers the back of the cornea. Depressions or 
crypts are here and there seen in which the endothelium and basement membrane 
are absent, and are, by some, regarded as stomata, through which the lymphatics of 
the iris communicate with the cavity of the anterior chamber. Its posterior surface 
(facies posterior) is covered by a basement membrane, on which are placed two layers 
of columnar, pigmented epithelium, continuous with the pars ciliaris retinee and 
termed the pars iridica retine. The proper tissue of the iris, or stroma iridis, con- 
sists of delicate connective tissue and elastic fibres, with pigmented cells, blood- 
vessels, nerves, and non-striped muscle. 
The blood-vessels of the iris (Fig. 498) are derived from the long and anterior 
ciliary arteries. The long ciliary arteries, two in number, pierce the sclera on the 
inner and outer side of the 
optic nerve respectively, and 
extend forwards between the 
sclera and chorioid towards 
Aperture of the ciliary margin of the iris. 
U7 pupil, Here each divides into an 
upper and a lower branch, 
and the resulting four 
tone branches anastomose in the 
prey form of a circle, termed the 
‘ circulus arteriosus major 
Cireums This circle is joined by a 
mnor varying number of anterior 
Cireulus cillary arteries derived from- 
ee the lachrymal and muscular 
branches of the ophthalmic 
artery, and, after supplying 
the cilary muscle, sends 
converging branches inwards 
towards the aperture of the 
pupil, around which a second 
circle, the cireculus arteriosus 
minor, is formed. The veins 
proceed towards its ciliary 
margin, and communicate with the veins of the ciliary processes and with the 
canal of Schlemm. The convergence of the blood-vessels towards the aperture of 
the pupil gives to the anterior surface of the iris a striated appearance. 
Anterior ciliary arteries 
Long 
ciliary, t 
artery 
Anterior ciliary arteries 
Fra. 498. —BLoop-VESsELS OF IRIS AND ANTERIOR PART OF 
CHORIOID, viewed from the front (Arnold). 
