236 
Histology of the Eye. 
[Monthly Microscopical 
Journal, Nov. 1, 1869. 
The Iris corresponds to the diaphragm in the cornea. Stretched 
across the anterior chamber, it stops out the most peripheral rays, 
which, in its absence, would pass through the edge of the lens, and 
in this way it lessens spherical aberration ; then, by varying the 
size of the pupil, it regulates the quantity of light admitted to the 
retina ; and, finally, it is an accessory of the apparatus of accommo- 
dation, although not in man an actual factor. 
The iris is essentially a muscular organ. The contraction and 
dilatation of the pupil are due to muscular irritability, and not to 
vascular erectility. Their continuance after the heart has ceased 
to beat, and even after the head has been severed from the body, 
are facts which place this beyond discussion. 
In mammalia, the muscular tissue is of the unstriped kind; 
while in birds and reptiles it is striped. One of the most useful 
chemical agents for demonstrating it is the chloride of palladium. 
The iris should be placed in a solution of this, containing from 
one-fourth to one-eighth per cent., until it acquires a deep straw 
tint. The palladium chloride hardens the tissue, without making 
it so granular and opaque as chromic acid does ; and it beautifully 
preserves the nuclei. With this reagent, its demonstration is easy 
and certain in the eyes of white rabbits, where it is unobscured by 
pigment which conceals it in human eyes. 
The cells, which are not easily individually isolated, are long 
spindles containing a rod-like nucleus. They resemble closely the cells 
of the larger organic muscles. The cells cohere in small flat bands, 
and these again combine in larger bundles. In man, I beheve also in 
Fig. 7. 
Iris of White Rabbit, prepared with Chloride of Palladium, to show the disposition 
of the Muscular Tissue. 
mammalia generally, in birds, and in reptiles, the muscular bundles 
are disposed in two sets, which have a radial and a circular direction, 
and constitute a sphincter and a dilator muscle of the pupil. (Fig. 7.) 
