Maximilian Salzmann.

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

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This is only a modification of the iris stroma, yet it differs from this
in its greater density, and is of special significance because it has to do
with the color of the iris.

It is principally made up of cells between which there are only a very
few collagenous fibrillae and numerous nerve-endings, but no blood-vessels.

The cells are chromatophores, like the stroma cells, and usually possess
only two or three processes (sometimes more, sometimes less). These
processes are often arranged in little bundles, especially in the zone of the
contraction furrows, so that a porous appearance is produced in the
anterior border layer (PI. VIII, 5). A net-form union probably exists
between them. Through repeated superimposition of such cells and



132 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

through the crossing of the processes there arises a very thick plexus, very
difficult of solution, and which gradually goes over into the iris stroma
behind (inward) by a gradual spacing up of the cell framework and an
increasing preponderance of collagenous interstitial substance.

In the middle portions of the iris, where it is quite smooth, the plexus
is very uniformly developed in all directions; at the border of the crypts,
however, the processes are arranged more parallel to this border.

The pigment, the mass of which varies a great deal, individually, con-
sists of finer and grosser granules. Concerning the endings of the nerve-
fibers, which are demonstrable only by the methyl-blue method, see below

(P- 134).

The thickness of the anterior border layer varies a great deal in the
different portions of the iris. It fails entirely at the entrances of the
crypts; only in the places where a pupillary crypt is large and extends
obliquely into the stroma has the wall of the crypt facing forward a
thick border layer. Furthermore it is very much thinner on the floor of
the contraction furrows than in the neighborhood; on this account the
contraction furrow can never be completely obliterated. It is thickest
on the border of the pupillary and the ciliary zones and thereby obscures
the structure of the vessel layer.

The anterior border layer gives the color to the iris, not alone through
its pigment content but also its density; blue irides have a delicate border
layer and almost unpigmented cells; brown irides have a thick border
layer and very heavily pigmented cells.

A complete absence of pigment in the border layer and in the stroma
probably occurs only in the newborn and in very young children. These,
therefore, at times actually have blue eyes, for the blue color is only due
to the fact that a clouded but colorless medium (border layer and stroma)
lies in front of a dark background (pigment epithelium). A complete
absence of pigment scarcely occurs in the adult, and the blue of such eyes
appears dulled, runs into gray, or more rarely into a greenish hue.

This difference also comes out when the color in different sectors of the iris varies.
A different color in the two eyes of the same individual (one eye blue, the other brown),
heterochromia iridis, can up to a certain extent be physiologic and is due to the above-
reported state of the border layer; it is, however, also often pathologic, especially when
the individual is, in general, of the brunette type. In these cases the blue eyes very
easily get iridocyclitis and cataract, and it has been shown anatomically that in such
cases the blue color of the iris is only the expression of a chronic disease (atrophy of
the iris; Fuchs, 68).

Albinism is a developmental anomaly and, therefore, does not come into considera-
tion here. In passing, it may only be mentioned that it is due to the lack of color of
the pigment epithelium. The stroma pigment does not need to fail entirely in such eyes.



THE IRIS 133

3. THE VESSEL LAYER

This forms the main mass of the iris. It contains the numerous larger
blood-vessels and nerve-plexuses, held together by a very loose delicate
stroma.

The blood-vessels of the iris enter the iris root between the peripheric
crypts (PL VIII, i) in larger bundles and branch into finer branches as
soon as they have passed the zone of these crypts ; these pass through the
ciliary zone in a radial direction in several layers. They give the ciliary
zone its meridional striation. The vessels usually show a corkscrew-like
winding, for only in this way can they adapt themselves to the changing
states of contraction in the iris tissue. The narrower the pupil the
broader the iris, and the blood-vessels must be just so much the more
stretched out, and then the meridional section shows longitudinal sections
of the vessels almost exclusively. On the other hand, the wider the pupil,
the narrower the iris, and just so much closer are the windings of the
vessels upon the meridional section; each vessel is then broken up into a
series of cross-sections.

In the pupillary portion this course is somewhat changed partly in the
formation of anastomoses (smaller circle), partly in the supply of the
sphincter pupillae; many circular vessels are also to be found here.

All of the vessels of the iris are characterized by a thick adventitia;
this consists of a finely fibrillated and therefore almost hyalin-appearing
collagenous tissue, the thickness -of which often exceeds the diameter of
the vessel lumen. The arteries possess a thin muscularis and a very
weak intima, which many times does not stain by orcein; the veins
have perivascular sheaths (perithelium) bordering immediately upon the
endothelium.

According to Leber (138), the arteries of the iris give off a pretty wide-
meshed capillary net in the ciliary zone going through the entire thickness
of the vessel layer; a special capillary net is found neither on the anterior
nor on the posterior surface. The capillary net first becomes narrower at
the sphincter pupillae. The last ends of the arteries bow over into the
veins.

The nerves of the iris likewise advance through the root of the iris
in larger trunks and then build a plexus in front of the larger vessels
(Pause, 169); often these branches consist of only a few fibers, which,
aside from the meridional course, also run obliquely and crosswise.
But the nerves can only be followed over longer stretches and, therefore,
identified with certainty in teased preparations. Only in such preparations
does one get an idea of the richness of the iris in nerves. Nerves are
practically not to be recognized in cut sections.



i 3 4 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

The nerves in the larger trunks and branches are partly medullated,
partly non-medullated. The individual branches possess a thick con-
nective-tissue hull (neurilemma) , which in this respect is like the adventitia
of the blood-vessels. Moreover, the relations of the neighboring stroma
cells to the nerves is the same as in the case of the blood-vessels.

The nerve-fibers end partly in the stroma (sensory fibers), partly in
the vessels (sympathetic fibers), partly in the sphincter pupillae, partly
in the dilatator (motor fibers). Yet there is only very little known con-
cerning the nature of the ending in man, for the study encounters
enormous difficulties as a whole in the human iris on account of its
thickness and richness in pigment.

According to Meyer (153), a sensory net lying immediately under the endothelium
goes out of the wide-meshed plexus of the iris nerves in rabbits; furthermore, the motor
fibers for the sphincter, which form a net made up of long-strung-out meshes between
the muscle-fibers and the vasomotor fibers of the iris vessels and form two plexuses
in each artery, one in the adventitia, the other in the muscularis, are also given of! by
this same plexus. Retzius (i 79) also demonstrated numerous nerve-fibers in the posterior
border lamella. Muench (162), finally, postulates a union of the nerve-fibers and the
stroma cells (cf. p. 135).

Opinions concerning the presence of ganglion cells in the iris are
divided. Meyer depicts two cells in a human iris which look exactly
like ganglion cells, yet no union with nerve-fibers can be made out; I
myself have, upon one occasion, but only upon one occasion, seen an
unquestionable ganglion cell in the iris. The views of Muench concerning
this matter will be given consideration later on (p. 135).

The spaces between the blood-vessels and the nerves are filled out by
the iris stroma proper. This is an extremely delicate, loose, collagenous
tissue containing the pigmented stroma cells (chromatophores), non-
pigmented stroma cells, clump cells, and, finally, sparse wandering cells.

The collagenous intervening tissue (PL VII 6, b) consists of very
delicate and discrete fibrillae of such fineness that only intensive staining,
e.g., Mallory's hematoxylin, give a clear picture. The fibrillae are not
arranged in bundles. Their course in the middle parts of the iris and in the
depths of the vessel layer is meridional. Toward the ciliary border they
go over into a fiber plexus and also in the pupillary zone they change the
course of their fibers. The intermediary substance is most markedly
developed behind the sphincter pupillae (cf. p. 137).

Elastic fibers, i.e., fibers which stain with orcein, fail almost entirely
in the iris ; a few isolated fibers of this nature are found, but only in the
peripheral portions; these are apparently radiations of the elastic frame-
work at the insertion of the ciliary muscle into the scleral roll. In



THE IRIS 135

connection with closely compressed collagenous fibrillae they course into
the iris in a meridional direction.

According to de Lieto Vollaro (143), elastic fibers are also found in the
tissue behind the sphincter, yet so far I have not been able to see these
fibers. In any case, the elastic tissue of the iris is very much less in
amount than in the other portions of the uveal tract. This is all the more
striking because when wounded the iris shows an especial tendency to gap.

The chromatophores are grouped principally about the vessels and
the nerves the adventitia (and especially the neurilemma) of which
they invest with their processes. The interstices proper between the
vessels are permeated by a very loose framework of stroma cells. This
framework is somewhat thicker in the pupillary zone, especially in the
neighborhood of the sphincter and at the ciliary border of the dilatator
lamella.

Each chromatophore shows a small oval body, which stains well,
and an oval nucleus not surrounded by pigment ; the processes are slender
and long (up to 100 mu) and few in number. They unite with those of
their neighbors into a plexus. The pigment is finely granular, and for
the most part much paler than in the chromatophores of the chorioidea.

With respect to the direction of the cell-processes one can only say
that no particular direction rules in the anterior part of the vessel layer,
and that a meridional course comes out in the depths. Especially in the
very deepest stroma layer immediately in front of the dilatator one en-
counters very much elongated, bipolar chromatophores with a meridional
direction. Finally, one sees divergent stroma cells radiating out from the
thickened places in the dilatator lamella at the ciliary border toward
the anterior iris surface.

Aside from the chromatophores, non-pigmented stroma cells are also
found. These likewise possess processes, yet these are much more deli-
cate and fine. This kind of cells is held to be nerve-cells by Muench
(162).

According to this author, there are no transitions between the two
kinds of stroma cells. In man the distinction is often difficult when the
chromatophores are little pigmented yet their processes are larger than
are those of the unpigmented stroma cells. In the much more heavily
pigmented iris of apes the difference is much more plain.

Muench (160) held, furthermore, as reported above (p. 50), that the
chromatophores are muscle cells and states that they are united with
nerve-fibers; the latter form a network, the nodal points of which are the
above-described unpigmented stroma cells, and either press into the body
of the chromatophore or are attached to it by means of conical insertions.



136 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

Aside from the chromatophores, characterized by their processes,
larger pigment cells are found in the neighborhood of the sphincter
pupillae and occasionally also in the neighborhood of the ciliary border.
They are without processes and therefore rounded; their pigment is made
up of large, round, and very dark granules They have long been known
by the name clump cells (Koganei, 119) (PI. VIII, 3, 10, K). Their
true nature has, however, only recently been made clear by Elschnig
and Lauber (55); they are cells displaced out of the ectodermal layers
of the posterior surface. This is shown not only by the structure of the
pigment but also by its density (it covers the cell-nucleus), and finally by
the circumstance that these cells are just as intensely pigmented in blue
irides as in brown. These cells have renounced their epithelial nature in
only one matter; they have lost the tendency to form closed bands and
lie wholly isolated in the stroma. These cells are also to be found in the
sphincter pupillae and in the connective tissue behind this muscle. The
most conclusive proof of their nature is, however, found in those cases
in which there is a defect in the sphincter and the cells radiate out from
the posterior surface through this into the iris stroma (cf. loc. cit., 55; PI.
XIX, Fig. i).

The number of wandering cells is very small, at least in the normal
iris: these are small, round, sharply contoured cells with homogeneous
or weakly granular protoplasm and a small, heavily stained, round or
lobulated nucleus. Other forms are probably pathologic, even when no
other diseased changes are found in the iris, for the iris is very easily
affected in diseases of the other tissues of the eye or body.

In general, the iris stroma is set off from the anterior chamber by the
anterior border layer and the endothelium. This delimitation, however,
fails in the crypts; the crypts are places in which the aqueous bathes the
stroma of the vessel layer. On histologic examination, the peripheral
crypts appear as simple defects of the anterior border layer and of the
endothelium and in this way a deeper layer of the stroma is exposed; the
form of these crypts is, therefore, that of pit-like deepenings.

The larger pupillary crypts are hollowed out of the stroma of the
iris, on the other hand; they often stretch out toward the periphery in
such a way that the peripheral border of the crypt appears undermined.
Many times the entrance to such a crypt is bridged over by free trabeculae
(PL I shows a cross-section of a trabecula at k 2 ). Viewed from the front,
the floor of such a crypt is not entirely obscured by a border layer and this
is only much weaker developed here than on the anterior surface of the
iris. According to Fuchs (67), who first studied the histologic relations



THE IRIS 137

in these crypts more accurately, the endothelial covering is interrupted
at the crypts and the spaces in the tissue of the iris stroma communicate
freely with the anterior chamber. At the same time it is not possible
to inject the tissue spaces of the iris from the chamber.

The vessel layer undergoes a special modification in the pupillary zone
of the iris by the interposition of the structure which contracts the pupil
(m. sphincter pupillae). From a developmental standpoint this muscle
belongs to the ectodermal layers of the iris, it is true, yet throughout
its development it is so completely imbedded in the vessel layer that it
can only be treated in connection with the vessel layer in the anatomic
description of the iris.

The sphincter pupillae (PL VIII, 3, Sph) forms an annular band some
0.9 mm broad, of which the inner (pupillary) border is entirely closed
off by the border of the pigment epithelium (Ps). I doubt very much
whether, in general, a true connective tissue limitation is present.

The sphincter is made up of bundles which cross each other at very
narrow angles and form a framework similar to that in the ciliary muscle.
The direction of the bundles is purely circular on the surface of the muscle
(concentric with the pupil-border) and parallel with the surface of the
pupil. The bundles are thick, the intervening tissue sparse. Toward
the back surface the framework is somewhat more loose, the bundles more
slender, the intervening tissue richer, the variations from the strictly
circular course more marked, and, moreover, bundles are found which
course obliquely toward the dilatator lamella or the pigment epithelium
(PL VIII, 10). The intervening tissue of this part and the connective
tissue lying behind the sphincter is especially rich in collagenous fibrillae
and, therefore, shows a much denser structure than the rest of the iris
stroma; according to de Lieto Vollaro, elastic fibers are also found
in it.

As a result, the sphincter shows a well-marked limitation in front
on meridional sections but not behind toward the supporting connective
tissue. Both layers thicken gradually from the pupil-border toward the
ciliary border of the sphincter and attain there a thickness of o . i to
0.17 mm.

The bundles of the sphincter consist of smooth muscle-fibers. Possibly
they have a shorter, more oval, therefore, less rod-form nucleus, but
otherwise they agree completely with those of the usual form. Their
protoplasm stains pretty heavily with eosin, ammonia-carmin, indigo-
carmin, takes on a yellowish to orange-yellow nuance by Van Gieson's
stain, and shows sharp, plain contours, for each fiber is surrounded by



138 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

a delicate sheath of connective tissue; myoglia fibrillae are not found in
the sphincter fibers (Forsmark, 59).

The connective tissue behind the muscle apparently serves to support it and effects
a firmer union between the dilatator pupillae and sphincter, on the one side, and with the
pupil-border, on the other.

Concerning the union between the sphincter and the dilatator pupillae,
see the latter.

b) Ectodermal Layers of the Back Surface of the Iris
4. OUTER LEAF: Dilatator pupillae

The recent embryologic investigations of Grynfeltt (80), in animals,
of Heerfordt (88), von Szili, Jr. (216), and Herzog (95), in man, have first
made clear the nature and the origin of the dilatator pupillae to us:
the dilatator pupillae, like the sphincter, is an epithelial muscle, i.e.,
its fibers develop out of epithelial cells, and, indeed, out of those
of the outer layer of the optic vesicle. But while a complete transition of
the epithelial cell into a muscle cell occurs in the sphincter pupillae,
this takes place in only a part of the cells (its basis) in the dilatator
pupillae; the head of the cell has an epithelial character and maintains
pigmentation.

Therefore, when typically developed, the dilatator element appears
as a spindle-form cell with an oval nucleus and a moderately pigmented
protoplasm, extended at each end into an unpigmented fiber-like process
(PL VIII, 7). Since these processes correspond to the cell-basis, they
lie at another level (farther forward) than the nucleated head of the cell,
and in a cursory look at the section the dilatator pupillae, therefore,
appears to be made up of two layers: one, a non-nucleated membranous
layer in front (posterior border lamella or border membrane of Fuchs,
Bruch's or Henle's membrane) and the other, a layer of nucleated pig-
mented spindle cells (anterior pigment layer of Fuchs, anterior epithelium
of Gruenhagen and others).

This description contains much which is not at once to be seen in the
preparation but can only be deduced from the whole outcome of
the investigation. We have, indeed, made considerable progress in the
recognition of the anatomic make-up of the posterior layers of the iris
by means of the various methods of bleaching the section, yet owing to
the fact that it is scarcely ever possible to isolate the dilatator element
completely and intact, phantasy still plays a certain part. In order now
to meet the criticism that I have been guided more by phantasy than
by the actual circumstances I add a purely anatomic description of the
dilatator pupillae.



THE IRIS 139

The posterior border lamella (PI. VIII, 8, 9, hG) is a layer of
some 4 mu thickness, which does not always show a sharp delimitation,
behind at least. On meridional section (PL VIII, 8) it appears almost
homogeneous or indistinctly striated longitudinally; on surface-section it
shows a plain though fine, straight meridional striation; on transverse
section (PL VIII, 9) it breaks up in very small, rounded or angular fields
united into little groups, but forming a continuous layer as a whole;
it shows the staining reaction of protoplasm: ammonia-carmin stains it
especially densely, Van Gieson stains it orange-yellow ; orcein- and resorcin-
acid fuchsin stains are negative. From all of this it follows that this
layer is made up of straight protoplasmic fibers coursing meridionally
and forming a pretty uniform layer not plainly separated into bundles.
When typically developed, cell-nuclei are not found in this layer.

Aside from the above-described protoplasmic fibers, the posterior border lamella
contains a second kind of fibers, especially characterized by the fact that they stain
intensely and electively with iron-hematoxylin. These fibers were first observed by
Widmark (237), later more accurately described by Forsmark (59) and at the same time
recognized as identical with the constituent portion of the smooth muscle tissue, called
myoglia by Benda (19).

The myoglia fibers, according to Benda, lie intra- and extracellular; the same
relationship, therefore, exists as in the cells and fibers of the neuroglia. So far as can
be recognized in the drawings of Widmark and Forsmark at hand, the myoglia fibers of
the dilatator lie almost exclusively between the protoplasmic fibers or on their surface;
in part, too, they press down in between the pigmented spindle cells.

Upon meridional section the layer of pigmented spindle cells (PL
VIII, 8, 9, sp) is only indistinctly set off against the posterior border
lamella, toward the pigment epithelium, however, very sharply. The
borders between the cells can scarcely be made out at all. These borders
are seen best in depigmented preparations as fine lines running obliquely
toward the posterior border lamella and going over into the longitudinal
striations of it. Possibly, however, these lines are caused by the myoglia
fibers. The nuclei of the spindle cells are long and placed parallel to the
border lamella. The protoplasm contains a moderate amount of pigment
not covering the nucleus; its granules are throughout of the same size,
form, and color as those of the pigment epithelium. The thickness of the
entire layer is 8 mu.

The spindle-form suggested in the meridional section comes out with
entire clearness in the surface view (PL VIII, 7). Here the cells show a
regular spindle form, they are some 7 mu broad, and approach 60 mu in
length; the nucleus shows the same form as on meridional section, i.e., a
breadth of 4 to 6 mu and a length of something like 14 mu. The axes of
the spindle cells and likewise those of the nucleus have a strictly meridional



i 4 o ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

direction. 1 The pigment is located principally in the tapering portion
of the spindle and in the unstained preparation the whole layer seems to
be made up of narrow triangular flecks of pigment.

Finally, upon the transverse section (PI. VIII, 9) the cells appear
small and almost quadrilateral, the nuclei likewise are small and rounded;
the lateral borders of the cell are quite as plain as the posterior border of
the whole layer. A nucleus is not visible in every cell in very thin sections
of this kind, for many spindles are encountered in the tapering portion, and
the cross-sections of these are smaller and lower than the portions which
contain nuclei. Despite these and other smaller irregularities the trans-
verse section shows that the layer of pigmented spindle cells is a single
cell-layer; in other sections one cannot recognize this with such certainty.

That the posterior border lamella and the pigmented spindle cells
belong together is indicated in various ways: (i) The two layers are not
clearly set off from one another. (2) The direction of the fibers of the
posterior border lamella and of those of the spindle cells is exactly the
same. (3) It is not possible to separate the two layers from each other.


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Online LibraryMaximilian SalzmannThe anatomy and histology of the human eyeball in the normal state, its development and senescence ; → online text (page 16 of 27)