CONTRACTILE TISSUE OF ‘THE IRIS 7 
pigment (for the eyes were black ones), I anticipated but little result from my 
examination. Having removed the uveal pigment from behind, I found that I 
was also able to strip off from the anterior surface a tough membrane, a portion 
of which, put under the microscope, appeared to be made up of peculiar short 
felt-like fibres, which were gelatinized by acetic acid. At and near the pupillary 
margin this membrane comes off in a continuous layer, leaving a delicate 
reticular structure, which contains the muscular tissue. It also contains vessels, 
as I proved by injection, and a black network, which consists of fine fibres, 
yellow, and highly refracting, more or less encrusted with pigment. I am un- 
certain whether or not this be a network of divided nerve-tubes with adhering 
pigment ; in some spots the pigmental crust was absent from a considerable 
length of the fibres. The sphincter pupillae is beautifully seen as a broad flat 
band, of extremely well-marked, unmixed, muscular fibre-cells ; but crossing 
this at right angles are found, here and there, other flat bands of fibre-cells, 
which are in so thin a layer that without isolation the width of the individual 
cells can be seen, and they are evidently of similar dimensions to those of the 
sphincter. On addition of acetic acid their nuclei are also seen to be exactly 
like those of the sphincter. These bands divide in their course towards the 
pupil into several fasciculi, some of which cross over the sphincter at right 
angles till very near to its pupillary margin, and then seem to blend with the 
sphincter by making a slight curve. Most of the fasciculi, however, arch away 
earlier from their first course and join the sphincter in more or less oblique 
lines. The bands from which these fasciculi diverge may be traced away from 
the pupil for some distance, continuing their course at right angles to the 
sphincter till they are obscured by other tissues. Hence I think the inference 
may fairly be drawn that these are the insertions of the dilating muscular 
bundles. In the horse, then, the dilating fasciculi appear to consist of 
precisely the same tissue as the sphincter, and to blend with it in their 
insertion. The flat bands of muscular tissue above spoken of seemed to 
have no special relation to the vessels, some of which were filled with 
injection. In the outer part of the iris of the same horse I found a delicate 
muscular fasciculus lying near but not intimately connected with one of the 
radiating vessels of this part. In the human iris I have seen a muscular fasciculus, 
as it appeared from the nuclei it contained, crossing the sphincter at right 
angles for a short distance ; this observation, so far as it goes, seems to Imply 
that the same mode of insertion of the dilator occurs in man as in the horse. 
The fibre-cells of the dilator appear to be held together much more closely 
than those of the sphincter, at least in the outer part of the iris; for I have 
never been able to define the individual fibre-cells in a perfectly satisfactory 
