8 OBSERVATIONS ON THE CONTRACTILE TISSUE OF THEARIS 
manner in the dilator, though I have often teased out portions of the outer 
part of the iris. The dilating muscular tissue is also probably less abundant 
than the muscular tissue of the sphincter; and this, if the fact, will help to 
account for the comparative difficulty in discovering it. I may here mention 
that both in the cat and in the rabbit, soon after death, dilatation of the pupils 
being present, exposure of one iris to the air caused it to contract at once, 
while the pupil continued dilated in the other eye, which was untouched. I do 
not know if this fact has been observed before, but it is interesting in two ways 
—first, as showing that the muscular tissue of the iris, like other muscular 
tissue, is obedient to the stimulus of exposure ; and, second, as proving either 
that the sphincter is in these animals a decidedly more powerful muscle than 
the dilator, which is equally exposed to the stimulus; or else that the fibres 
of these two muscles have different endowments, as has been shown by 
Mr. Wharton Jones to be the case with the muscular tissue of the arteries and 
veins of the bat’s wing; where, although the veins are muscular, and even 
contract rhythmically, yet the arteries alone exhibit tonic contraction when 
irritated by mechanical stimulus. 
A rich network of extremely fine fibres, seen readily in the blue human iris 
viewed from the anterior aspect, appears to represent the nerves of the organ. 
The fibres are of a yellowish colour, and are possessed of pretty high refractive 
power ; they present, if really nervous, a good illustration of the division and 
anastomosis of ultimate nerve-fibres ; the smallest divisions visible under 
a high power are seen only as fine lines. 
I have not seen any nerves in the human iris presenting the double contour ; 
but in the iris of a cat, so fresh that the tissue contracted under the needles 
as I teased it out, the double contour of the nerve-tubes was already very strongly 
marked, showing the existence in this animal of the white substance of Schwann 
in these nerves. The double contour surrounded the ends of the nerve-fibres 
which I supposed to have been broken by the teasing process. This last fact 
seemed to confirm the general belief that the double contour is a post mortem 
effect, which, however, was in this instance a very rapid one. 
I believe that a further investigation of the fresh blue iris in man, and 
of the horse’s iris, would supply the means of finally settling the question of 
the distribution of the dilator pupillae. 
My engagements do not allow me to carry the inquiry further at present ; 
and my apology for offering the results of an incomplete investigation is, that 
a contribution tending, in however small a degree, to extend our acquaintance 
with so important an organ as the eye, or to verify observations that may be 
thought doubtful, may probably be of interest to the physiologist. 
