Maximilian Salzmann.

The anatomy and histology of the human eyeball in the normal state, its development and senescence ; online

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tion into a pigmented epithelium, which is uniform over all parts of the
iris and goes over onto this without any line of demarkation. In this
way the border between the ciliary epithelium, and the pigment epithelium
of the iris does not fall at the root of the iris but somewhat behind it.
This is quite without significance for the function of the eye, and in some
other respects ; its significance lies solely in that it demonstrates ad oculos
that the pigment epithelium of the iris belongs to the tunica interna,
that is to say, to the inner layer of the optic vesicle, and not to the outer,
as one would be led to believe from its pigment content.

That which was said about the border portions of the retina holds true in an
analogous way for the ciliary epithelium; it is often extremely difficult to draw the
line between normal relations on the one hand and artefacts and pathologic conditions
on the other hand. It is certain that the ciliary epithelium has a greater smoothness
and regularity in young eyes than in old. Certain it is, too, that the lens usually
shrinks considerably in the hardening fluid (about i mm in the equatorial diameter),
and that the shrinking causes a greater tension upon the zonula fibers and thereby
draws upon the ciliary epithelial cells. Under such circumstances special care is
necessary in forming judgment concerning variations in the ciliary epithelium.

8. THE MEMBRANA LIMITANS INTERNA CILIARIS

(Inner glass membrane of the pars ciliaris retinae, 184)
(PI. yfil, 2, 4, 5, Li)

A structureless, connected layer is to be made out on the inner surface
of the ciliary epithelium, at least in the adult eye; it is plainly differen-
tiated from the protoplasm of the ciliary epithelial cells by its staining
reaction and homogeneous structure. This membrana limitans interna
is probably best looked upon as a cuticular formation of the ciliary epithe-
lium and corresponds in its position to the membrana limitans interna
retinae. In eyes fixed while still warm, it adjoins the protoplasm of the
ciliary epithelium without any intervening space; in cadaver-eyes, how-
ever, it is detached from this, and the inner space seems to be filled out
with a clear coagulated fluid.

This detachment does not take place along the whole extent of the
membrane but in sections, and forms relatively short, often very regular
bows or arcades (PI. VII, 2). In the places where two arcades meet this
membrane sinks down in between the epithelial cells in the form of a
ridge, and is thereby more firmly fixed.



124 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

Such ridges (PI. VII, 5, /) are most plainly developed in the anterior
part of the orbiculus ciliaris and have here a meridional direction, as com-
parison of meridional and transverse sections, and especially teased prepa-
rations, show. The ridges do not course entirely straight, however, but
zig-zag, and often give off short lateral branches at the ends. Yet a
regular net-formed union does not come about, as in the reticulum of
Heinrich Mueller to which this ridge system has similarity in its relation
to the epithelium. Neither the ridges of the membrana limitans interna
ciliaris nor those of the cuticular lamella project as far as the outer
border of this layer.

On the free surface the membrane is very thin, often scarcely measur-
able; the ridge may, however, attain considerable thickness. In young
individuals, of course, the cross-section of such a ridge appears narrow
and has parallel sides, and can be very easily overlooked. In older people,
however, the cross-section appears distended like a flask and not infre-
quently shows lobulated branchings. These thickenings of the ridges,
apparently increasing with years, call forth irregularities and even foldings
of the ciliary epithelium. Each ridge can be considered a fold or duplica-
tion of the membrana limitans interna ciliaris whose two leaves lie closely
opposed and are fused to one another; often one sees a delicate separating
line and at times fine zonula fibers lying therein (cf. chap. xii).

Surface preparations of this membrane can only be obtained from fresh eyes,
and even then very many cells still cling to the sides of the ridges, so that only a few
places give a pure picture of the ridges in such a preparation. I have sketched this
appearance (loc. cit., 184; Text Fig. 10). Transverse sections through the orbiculus
are the most instructive; meridional sections cut the ridges longitudinally, give only a
very indistinct picture and, therefore, easily permit misinterpretations.

Ridges are likewise present in the ciliary valleys, yet their arrangement
is less regular. On the crests of the ciliary processes they go over into
a circular direction, i.e., they course obliquely over the crests and thereby
give rise to an appearance recalling that of the large intestine and its
haustra.

The membrana limitans interna ciliaris is easily demonstrable in all
these places, and, moreover, can be prepared anatomically. In the most
posterior portions of the orbiculus, however, it is otherwise; I am in
doubt whether or not a special membrane is present here. The ciliary
epithelium certainly appears very plainly set off from the vitreous in
section, but more than a contour between the two I cannot see. It is
not represented by the anatomic preparation which I once considered
proof of a connection between the membrana limitans interna ciliaris
and the membrana limitans interna retinae (loc. cit., 184; Text Fig. 9),



THE IRIS 125

because this contained a part of the vitreous base as well. According to
Addario (3), it is the superimposed, imbricated, inward prolongations
of the ciliary epithelial cells, which give rise to the appearance of a
membrane. The ends of these processes would go over into vitreous
fibrillae; such a consideration excludes the conception of a border mem-
brane. Wolfrum, on the other hand, holds that there is a delicate basal
membrane in this situation.

The limitans interna retinae is also at times very indistinct in the
neighboring border portions of the retina. I have described a very
peculiar proliferation of the Mueller supporting fibers into the vitreous
in a case of probable congenital keratoconus (185). As it later developed,
Iwanoff had already described and depicted similar changes in the year
1865 (113), and since then I have found this condition indicated in some
other eyes. The proliferation does not appear to have a pathologic
significance; it only indicates that the delimitation of the supporting
tissue of the peripheral portions of the retina from that of the vitreous
is a defective one, and the developmental history of the vitreous gives the
key to this.

The limitans interna retinae, like the limitans interna ciliaris, has,
therefore, the tendency to fade away toward the most posterior zone
of the orbiculus ciliaris. Whether it actually does so or not or is only
there reduced to a membrane so thin that it vanishes, I leave unsettled.
Probably this is not uniform in all eyes and at all ages.

In front the limitans interna ciliaris can be followed as far as the
ciliary epithelium remains unpigmented, in any case.

Wolfrum (242) will only admit a limitans in those places where no
zonula fibers are given off. This is only a logical sequence of his views
concerning the relations of the zonula fibers to the ciliary epithelial cells
(cf. chap. xii).

CHAPTER X. THE IRIS

Severed from its connections, the iris forms a circular plate closely
resembling a diaphragm, such as one uses in optical instruments; yet,
in fact, it is not broadened out into a plane, but forms the mantling
surface of the base of a low trunkated cone, because the lens presses the
central portion forward.

The outer (peripheral) margin of this plate, that united to the ciliary
body, is called the ciliary border (margo ciliaris} or iris root, the circular
opening in the middle is called the pupil, and the limiting margin the pupil-
lary border (margo pupillaris}. The pupil does not lie exactly in the
center but slightly to the nasal side.



i 2 6 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

The diameter of the ciliary border (considered as a circle) is the same
as that of the cornea (some 12 mm) and is unchanging; the width of the
pupil changes very considerably during life, and likewise the breadth of
the iris (considered as a ring). In the thickest portions, the iris measures
some o . 4 to o . 6 mm, depending, naturally, upon the state of contraction of
the pupil. Since, too, the iris thins out appreciably toward both borders,
one can only differentiate two surfaces, an anterior and a posterior.

With the exception of the most extreme periphery, the anterior surface of the iris
can easily be seen, even during life. This is, indeed, actually covered by sclera only
above and below, but, since the cornea magnifies the image of the iris, the outermost
periphery is necessarily invisible even upon this ground. One can, therefore, see the
anterior surface of the iris in its whole extent only in an anatomic preparation. This
can easily be made by tearing the iris away from the ciliary body, for just as occasion-
ally after injuries in life, the dead iris tears away from the ciliary body because this
place is the weakest of the whole anterior part of the uveal tract.

An iris prepared free in this way can be spread out flat and studied out under
moderate magnification just as it is. Strong magnification cannot, moreover, be made
use of, partly because of the thickness of the iris, partly because of the necessity of
study by reflected light.

In the study of the iris from in front (PI. VIII, i) with the pupil
moderately wide, one sees a fine, black-brown seam (Ps) at the pupil-
border; this breaks up into a chain of little beads under the loop. As
readily understood, this seam is best seen in a blue iris, and especially
when the lens behind it is clouded.

Out of the often very complicated markings of the anterior surface of
the iris there comes first a zig-zag line, located about i . 5 mm fom the pupil
when it is of middle width. This angular line (Z), usually called the
small circle (or ruffle, Krueckmann, 125), separates the anterior surface
into two zones : an inner, the pupillary zone, annulus iridis minor Merkel
(Pz), and an outer, the ciliary zone, annulus iridis major Merkel (Cz).
The two zones vary in structure and also at times in their color.

The name small circle, the circulus iridis minor, really denotes the anastomosing
circle lying in this situation in the vessel system of the iris. But that which one sees
on the anterior surface of the iris is not the anastomosing circle itself but only contains
it. If one wishes to learn the real significance of this border line, those cases must be
drawn upon in which remnants of a persistent fetal pupillary membrane are present.
One then recognizes that such threads go out from the angular line, bridge over the
pupillary zone, and so extend to the lens capsule. The zig-zag line is, therefore, the
insertion point of the fetal pupillary membrane (Lohmann, 144), or rather its physio-
logic remnant.

The pupillary zone as a whole slopes pretty uniformly away from the
pupil-border. It carries numerous ridges and trabeculae, which go out
from the angular line, and in its neighborhood form a net-form marking,



THE IRIS 127

more radially striated toward the pupillary border. Larger, sharply
limited cavities, the pupillary crypts (k a ), arise by a wide separation of
the individual trabeculae which go to make up the net, usually just at
the angular line; net-like trabeculae are again united on the floor, or
visible only as wavy radial stripes.

As a rule, a crypt appears darker than the neighborhood, partly be-
cause the floor is thrown into a shadow by the steep borders, partly
because a thinner layer of connective tissue is present and the darker color
of the back surface of the iris shimmers through.

In general, the number, size, and position of the crypts is subject to
considerable variation. At times isolated crypts lie on the other side of
the angular line in the ciliary zone.

When the pupillary zone has an especially delicate structure, one
can at times make out the sphincter pupilae as a whitish band of about
i mm in width shimmering through from below, immediately adjoining
the pupil-border.

As a rule, the ciliary zone shows a more uniform structure. A mark-
ing made by fine, radially wavy stripes dominates here ; the blood-vessels
of the iris produce this. The striation is seen best in the middle of the
ciliary zone. In the neighborhood of the angular line it is often entirely
covered over as a result of the more marked development of the anterior
connective-tissue layers, and this part is, moreover, also the thickest part
of the whole iris. The inner half of the ciliary zone is pretty smooth;
in the outer half, on the other hand, one sees the so-called contraction
furrows (f) coursing obliquely over the meridional striations; these are
sharply demarkated circular furrows, running concentric with the ciliary
border, deepened when the iris is narrowed, and almost eradicated when
the iris is broadened. An individual contraction furrow extends over a
greater sector of the iris, but never over the whole iris. The furrows are
fixed structures like the folds of the skin in the hollow of the hand, and
are to be made out even when obscure (as slight deepenings or by another
color).

In the very neighborhood of the ciliary border crypts again appear
(peripheral, or ciliary crypts, & x ). They are much smaller and shallower
than the pupillary ones and form little groups or rows between which
the larger branches of the iris vessels and of the nerves course. So the
outermost periphery of the iris (the marginal zone; Fuchs, 67) again has
a net-form structure and a darker coloring.

This zone is usually not visible in the living eye; but anatomic
preparations show that it extends to the very ciliary border, and this is
thereby broken up into a number of teeth which run from the anterior



128 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

surface of the ciliary body to the meshwork of the iris angle. When
such a tooth is separated wholly from the substrata and bridges over the
iris angle to the meshwork, it becomes an iris process. All these extensions
of the iris tissue go over into the uveal meshwork (cf. p. 48).

As is well known, the color of the anterior surface of the iris varies
greatly, yet in general two types can be distinguished, the light (mostly
blue-gray), and the brown iris. The two types are not, however, sepa-
rated alone by their color, but they differ as well in the relief of the
anterior surface; in the blue iris many more details of the deeper layers
can be recognized; the radial fibrillation comes out much more markedly,
the crypts are more numerous and larger ; the contraction furrows, on the
other hand, are only to be seen by oblique illumination. The brown iris
shows a grosser relief; the ciliary zone is smoother, the crypts are more
sparse and smaller, the contraction furrows, on the other hand, are more
plain and are visible as clear lines even in a partly obliterated state. When
the pupil is narrow, one can also see radial furrows well in the smooth
part of the ciliary zone.

The posterior surface of the iris (PL VIII, 2) has an almost uniformly
dark brown almost black color in all cases, and appears smooth to the
naked eye. On moderate magnification and a strong focal light, such as
sunlight, a fine relief of radial and circular folds comes out.

The radial folds form two systems independent of one another. The
one consists of numerous shallow little furrows (rF), goes out from the
pupil-border, and gradually loses itself about i mm away. This furrow
system (Schwalbe's contraction folds) bends about the pupil-border onto
the anterior surface with the pigment epithelium and produces the
crenations of the pigment seam visible from in front, for this notching is
nothing more than the optical cross-section (the profile) of the furrow
system.

The second system contains fewer but deeper furrows, called struc-
tural furrows (structural folds, Schwalbe), because they are present in the
vessel layer as well. They begin about i . 5 mm from the pupil-border, are
narrow and deep at first, then broaden out and flatten down toward the
ciliary border (SF). Alongside them are still shorter, shallower furrows.
The number of well-developed structural furrows is significantly less than
the number of ciliary valleys; there are two or three valleys to one struc-
tural furrow. The circular furrows (cF) are considerably finer than the
radial. They are found only in the region of the structural-furrow sys-
tem, and cross these in very regular arrangement.

At the very ciliary border the pigment ceases somewhat earlier than
does the vessel layer, for the torn surface (the iris root) is turned somewhat



THE IRIS 129

backward. The torn edge of the pigment covering also shows a wavy
margin, because the ciliary processes are to some extent grown to the
back surface of the iris.

The preparation of sections in the three main directions is demanded :
for an accurate understanding of the anatomy and histology of the iris.
These directions and the pictures corresponding to them are :

(1) The meridional or radial section (Taf. I), i.e., the section in the
direction of the iris radius and perpendicular to the surface of the iris,
shows a very irregular limitation in front. The various crypts are
recognized as interruptions of the most anterior layer of the tissue (k ly & 2 ),
the contraction furrows as sharp angular indentations of this layer (/) ;
the maximum thickness in the region or neighborhood of the angular
line, the gradual slope of the anterior surface toward the pretty sharp
border of the pupil, the bending of the posterior pigment covering, and
the ending of this layer are seen at the pupil-border. One sees the
sphincter pupillae stretching out from this border into the stroma of the
iris, the firm connective tissue which supports it from behind and unites
it with the posterior surface of the iris; one recognizes the more uniform
thickness of the ciliary zone and the thinning toward the root of the iris.

When the section falls between two structural furrows, the posterior
margin of the iris, as a whole, appears straight as a line; in other cases,
it shows a step slightly ciliaryward to the peripheral margin of the sphincter
pupillae. In the ciliary portion the circular furrows appear as regular
crenations of the pigment covering, while this appears smooth toward
the pupil-border because the furrows lying here course in the direction
of the section.

(2) The section at right angles to the radius may be called the trans-
verse section. Yet such a section is strictly a transverse one at only a
single point owing to the radial course of most of the structural elements.
The farther one goes from this point the more oblique are the elements
encountered, and the less clear is the picture. Naturally, the picture
varies according to the part of the iris through which the section goes ; the
lateral portion of such a section always contains the more ciliaryward
lying portions.

In general, the transverse section shows a smoother course along the
anterior border of the iris; on the other hand, the structural furrows on
the back surface come out very plainly; the posterior limitation is a
wavy line.

(3) The surface-section, i.e., the section parallel to the surface expanse
of the iris, presents apparently the least instructive picture, partly because
the individual layers of the iris are much too lacking in evenness for the



130 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

section not to vary into either the next higher or lower layer. Neverthe-
less, the surface-section is of great significance for the histology of the iris.



Like the ciliary body, the iris contains portions of the tunica vasculosa,
the stratum pigmenti, and the tunica interna (cf. p. 14) ; yet there is such
an intergrowth of these coats in the territory of the iris that a division
of the layers of the iris on this basis cannot be carried out.

The arrangement of the layers of the iris is regular only in the middle
part of the ciliary zone; therefore the following survey holds true only
for this portion, and the beginner will do well to take up the study of the
microscopic anatomy of the iris with this portion..

The part of the iris belonging to the tunica vasculosa is called the pars
uvealis iridis. It consists mainly of a continuation of the vessel layer of
the ciliary body the vessel layer of the iris (PL VIII, 3, G). On the
anterior surface, this layer is characterized by the disappearance of the
vessels and the preponderance of the cells of the anterior border layer
(vG); on the side of the anterior chamber this is closed off by an endo-
thelium which is connected with the endothelium of the trabeculae of
the iris angle and thereby with that of the posterior surface of the cornea.

The layers belonging to the stratum pigmenti and the tunica interna,
therefore, the layers of ectodermal origin, have many times been placed
together under the name pars retinalis iridis, or pars iridica retinae.
Here, to prevent misunderstanding, they will be designated as the ecto-
dermal layers of the posterior surface of the iris. (Compare what was
said on p. 13 concerning the conception of the retina.)

The continuation of the stratum pigmenti grows into an epithelium
muscle, the musculus dilatator pupillae (Dt) in the region of the iris;
it appears to divide again into two layers, namely, a non-nucleated,
membrane-like layer in front (posterior border lamella) and a layer of
nucleated, pigmented, spindle-form elements behind it (layer of the
pigmented spindle cells). The continuation of the tunica interna is a
simple epithelium of densely pigmented cells, the pigment epithelium of
the iris (P).

So, from in front backward the iris may be divided into the following
layers :

ii. Endothelium
2. Anterior border layer
3. Vessel layer r \ T> u j i 11

\ a) Posterior border lamella

b) Ectodermal layers of the \ 4. Dilatator pupillae i \ ,. A A\ 11

/ B) Pigmented spindle cells
back surface of the ins ( 5. Pigment epithelium

The sphincter pupillae and the clump cells are also, indeed, of ectoder-
mal origin, although anatomically they are imbedded in the vessel layer
of the iris and will be considered after the vessel layer.



THE IRIS 131

a) The Uveal Portion of the Iris (Pars uvealis iridis}

I. THE ENDOTHELIUM OF THE IRIS

In general, it is true that it has been settled that the anterior surface of the iris is
covered by an endothelium, but the descriptions of this layer differ greatly. Aside
from the possibility of an imbricated covering of endothelial cells (Arnold, 8), which
in and of itself is very improbable, the endothelium is sometimes described as similar
to that of Descemet's membrane although made up of smaller cells (Iwanoff, 115),
sometimes as a weakly granular membrane possessed of a moderate number of nuclei
which can be isolated by maceration in Mueller's fluid (von Michel, 155). Moreover,
it is demonstrable only in young individuals, and, in general, with much greater difficulty
in the human than in animals (Koganei, 119).

The endothelium of the iris is, indeed, an object very difficult of his-
tologic demonstration. The endothelium practically cannot be demon-
strated in sections, because the anterior border layer with its numerous
cells and nuclei lies immediately under it. Only when these cells are very
densely pigmented, as in dark brown irides, can one recognize nuclei on
the surface here and there; these are surrounded by an unpigmented
protoplasm. Such cells are probably to be looked upon as endothelial
cells, but one must not confound them with wandering cells; they are
found much too sparsely to give the impression of a connected cell-layer.

The classical method for the demonstration of cell borders (by silver
nitrate) brings out a very complicated system of lines in the normal iris
of older people (PL VIII, 4); here, larger cell-like fields alternate with
numerous very small slender and often poorly demarkated fields. One
does not get the impression that the borders form a single layer of flat
cells. Other elements than the endothelial cells of the anterior border
layer are possibly drawn into the formation of particular portions of the
anterior iris surface.

2. THE ANTERIOR BORDER LAYER
(The anterior stroma leaf; Krueckmann, 125)


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