6 OBSERVATIONS ON THE 
alternate arrangement of the fibre-cells without any formation of fibres. 
Mr. Wharton Jones’s drawing of alternately disposed fibre-cells in the small 
intestine has been alluded to in the note above. A portion of the outer and 
thicker part of the human sphincter pupillae proved also extremely rich in 
muscular fibre-cells. In the rabbit and guinea-pig the sphincter has much 
the same appearance as in man, whereas in the horse it forms a wide but very 
flat band. 
The dilating fibres of the iris present a very difficult subject of investigation. 
And here I must express my belief—a belief the result of repeated and 
very careful observations—that the fibres described by Mr. Bowman as probably 
the contractile fibres of the iris are in reality the outer cellular coats of the 
vessels. The outer coat is very abundant in the vessels of the iris, and indeed 
even in the blue eye towards the sphincter quite obscures the bore of many 
of the vessels, and prevents the recognition of their vascular character, which 
can only be determined by tracing them to their more external and more obvious 
vascular trunks. The distribution of these vessels, radiating between the 
sphincter and the circumference of the iris, and forming in the region of the 
sphincter a close and knotted plexus, corresponds accurately with Mr. Bowman’s 
description of the distribution of the fibres of the iris. His account of the tissue 
of these fibres, which he considers as probably contractile, harmonizes with the 
characters of the cellular tissue that clothes the vessels. This is peculiar ; con- 
sisting of very soft looking fibres, whose fasciculi often require the best aid of 
a first-rate glass to resolve them into their constituent elements; destitute 
apparently of yellow elastic fibres, as in the case of the cellular tissue of the 
uterus, but, like this, containing abundance of free nuclei, of roundish or 
elongated form. The fibres are completely gelatinized by acetic acid. Now 
such a tissue can hardly, in the present state of our knowledge, be regarded 
as contractile ; at any rate, if we can find any ordinary muscular tissue to 
account for the dilating action. On teasing out portions of the outer part of 
the human iris, I have found long delicate fasciculi, whose faint outline, absence 
of fibrous character, and possession of well-marked elongated nuclei parallel 
to the direction of the fasciculus, left no doubt in my mind that they were 
plain muscular tissue. 
So far my observations regarding the dilator agree with Kélliker’s, but 
whether or not these fasciculi are connected with the cellular coat of the vessels 
J have hitherto been unable to determine. 
Among the lower animals the albino rabbit and guinea-pig appeared but 
little suited for the elucidation of this point. I have been most successful with 
the eyes of a horse, where, from the thickness of the iris and the abundance of 
