CONTRACTILE TiSSUE.OF THE IRIS = 
part of the radiating fibres, which, he says, are joined and knotted in a plexi- 
form manner round the pupil. It is scarcely needful to observe that such 
a statement from such an authority could not but go far to impugn Professor 
Kolliker’s assertion respecting the existence of a sphincter pupillae. 
My experience, I must confess, accords with that of KOolliker, viz. that the 
sphincter is readily seen, while the dilator is that whose investigation alone 
presents very serious difficulty. In the first iris that I examined with a view 
to the distribution of the muscular tissue, I was struck, after removing the 
uveal pigment, with the appearance of a band on the posterior surface of the 
iris, near the pupil and parallel to its margin, quite evident to the naked eye, 
elastic and highly extensible. This proved to be the thickest part of the sphincter 
pupillae. I have examined six human irides with reference to the distribution 
of the muscular tissue, but in none have I had any difficulty in recognizing 
the sphincter, which I have also found equally distinct in some of the lower 
animals, viz. in the rabbit, the guinea-pig, and the horse. In man I find it 
about 1-30th of an inch in width, thickest towards its outer part, where it lies 
nearer the posterior surface of the iris than the anterior, and thinning off towards 
the pupil, where it forms a sharp margin, covered apparently on its anterior 
aspect only by some vessels and nervous threads and a delicate epitheliated 
membrane, which is thrown into beautiful folds when the pupil is contracted. 
The fibres of the sphincter are not absolutely parallel, and this deviation is 
probably produced in part by the dilating fasciculi sweeping in at various parts 
in a curved manner, and becoming blended with the sphincter. The reason 
for this supposition will appear hereafter. By teasing out under the microscope 
a portion of the actual pupillary margin, I found the sphincter to consist at 
this part of apparently unmixed muscular fibre-cells, without any connecting 
cellular tissue. Fig. 13 is a camera-lucida outline of the edge of a portion of the 
sphincter so prepared, which edge is seen to be formed of projecting fibre- 
cells, and similar appearances may be seen with great readiness under a high 
power, after stroking the pupillary margin with the point of a needle. Indeed, 
the great facility with which the tissue may be thus broken up appears opposed 
to the idea of the fibre-cells being united end to end into fibres, as the descrip- 
tions formerly given of unstriped muscle would lead one to suppose. The 
ends appear to separate as readily as the edges and surfaces, and it would rather 
seem as if the fibre-cells of a fasciculus were placed with their long axis in one 
direction, cohering generally to one another, but without the formation of 
longer fibres than each cell itself constitutes. I may here mention incidentally 
that in the circular coat of the aorta of the sheep, where the muscular tissue 
is disposed in thin layers among the elastic tissue, I have observed a distinctly 
