Maximilian Salzmann.

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

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true for the portion lying in front of Schlemm's canal, in greater measure
because the lamellae increase by new branchings. Whereas anteriorly,
behind the border of Descemet's membrane, possibly 3 or 4 lamellae lie
over one another, their number mounts up to 15 to 20 at the posterior
end (in front of the scleral roll).

On the outside (toward Schlemm's canal) the trabeculae lose their
dominant circular direction, the branches become more stellate, the meshes
more rounded and smaller, the trabeculae more delicate (Asayama, 9).

The individual trabecula (PL II, 10) consists of four elements. Its
foundation is formed by a thick non-nucleated bundle of collagenous
fibrillae; the fibrillae run parallel to the long axis of the trabecula, there-
fore, mainly circular (6). This connective tissue is supported along its
surface by relatively thick elastic fibers (/); they course in nearly the



46 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

same direction as the collagenous fibrillae and are, therefore, cut obliquely
in meridional sections and appear as points, which do not disappear upon
change of focus but, at best, change their position. They are not plainly
visible by ordinary staining yet their presence brings about a very much
sharper defination of the tissue, a darker contour.

A glass membrane (g) succeeds the elastic fibers ; this is thicker along
the surfaces than upon the edges of the trabecula. It possesses all the
tinctoral and morphologic peculiarities of Descemet's membrane and can
be followed into it. The trabeculae do not arise simply by a splitting up
of Descemet's membrane, as depicted by the older authors, but are rather
entirely covered over by a continuation of Descemet's membrane.

The whole is finally covered by an endothelium (e) which, as usual,
shows no cell borders, so one can recognize the individual cells only
through the prominent oval nuclei. The cell-body usually forms an
extremely thin membrane, at least along the sides of the individual trabe-
culae, which one cannot differentiate from the contour of the membrane;
the cells contain protoplasm (are thicker) only in the angles and corners
of the mesh work and partially fill out the angle. The spaces in the mesh-
work, which from the course of the fibers would otherwise be angular,
are thereby rounded out. If the section goes close to such an angle it
encounters only the rounded endothelial cell although the converging
trabeculae still appear separated, and one obtains the impression that
the endothelium forms a bridge between the individual trabeculae (to
be seen in most of the places marked e).

The endothelium of the meshwork is also only a continuation of the
endothelium of the cornea. It covers over all the trabeculae and so
clothes all the spaces of the meshwork, but is not united in any way with
the endothelium of Schlemm's canal. No cell-nuclei other than those
of the endothelium are found in the scleral meshwork; the entire struc-
ture contains neither blood-vessels nor nerves.

Two stains are especially to be recommended for the study of histologic structure
of the trabeculae: the orcein stain and that of Van Gieson. The first brings out the
elastic fibers clearly; the glass membrane stains also slightly though far less heavily
than does Descemet's membrane. Van Gieson's stain colors all connective tissue an
intense red, the glass membrane rose-red to orange-yellow, the protoplasmic body of
the endothelium a pale yellow. Finally, absolutely meridional sections are indis-
pensable; one then gets pure cross-sections of the trabeculae and clear pictures, and
the sections do not need to be as thin by half.

It should be an easy thing in this way to distinguish endothelium from glass
membrane two layers which are not differentiated with the necessary precision in all
contributions concerning the meshwork. Moreover, the endothelium is perishable;
it is thrown off in eyes with advanced cadaverous appearances, and it likewise may
disappear under pathologic conditions.



THE STRUCTURES OF THE SCLERAL FURROW 47

The glass membrane, however, is indestructible, and even in the severest pathologic
changes the cross-section of the trabecula (aside from the endothelium) maintains its
characteristic appearance.

Posteriorly, the main mass of the meshwork goes over into the scleral
roll, as stated above. This consists of a large number of wide and
narrow connective-tissue bundles with the exact appearance of scleral
fiber-bundles and pursuing an absolutely circular course (for dimensions
see p. 23). Peripherally they are supported by large elastic fibers;
these fibers are even a trifle larger than those in the meshwork. A con-
nective tissue lies between the circular bundles in the form of a mattress-
work and completely fills out the interspaces; this tissue is incompletely
separated into bundles in which the fibrillae have a more oblique course.
The trabeculae go directly over into this tissue, and as a rule one can see
two rows of elastic fibers in every such stripe; these are apparently the
continuations of the layers of elastic fibers present on the two surfaces of
a trabecula. Anteriorly and inward the scleral roll has no sharp limits;
the circular bundles become smaller and so finally disappear in each
direction.

Schwalbe (194) spoke of the scleral roll as the posterior border ring
of the meshwork (ligamentum pectinatum in his terminology), and this
name has, indeed, a. certain justification, for the scleral roll varies con-
siderably in its structure from that of scleral tissue. On the other hand
the histologic peculiarities are not limited to the scleral roll but extend
to the parts of the sclera lying outside and behind it as well. Here, too,
the circular fibrillation predominates, the bundles are narrow, and the
large elastic fibers are present. Gradually this formation goes over into
ordinary scleral tissue (H. Virchow).

The size of the scleral roll shows individual variations, but it never
includes the whole thickness of the meshwork. A limited number of
trabeculae (lamellae) are always excluded, and these course past the
inner border of the scleral roll and proceed directly to the fore surface
of the ciliary body and are lost there in the intermuscular connective
tissue.

The uveal meshwork (H. Virchow, 234), ligamentum pectinatum of
Seefelder and Wolfrum, is the inner portion, that going to the iris root;
it springs in part from the inner surface of the scleral meshwork a slight
distance from the border of Descemet's membrane, in part, although
probably to a lesser extent, from the border of the latter (PL III, 2, i).

Moreover, in its further course the uveal meshwork is pressed against
the scleral and, therefore, usually passes by and around the sinus angle in
a bow, i.e., goes along the anterior surface of the ciliary body over to the



48 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

root of the iris (PL III, i). As already noted, one sees very little of
this portion on a meridional section; only a surface preparation clears up
its formation (PL III, 3). The trabeculae which make up the uveal
meshwork are not flattened down, as in the scleral meshwork, but rounded
like wire, provided here and there with roll-like thickenings, so that they
appear turned out as by a lathe; in any case expansions come out only
at the nodal points of the meshwork.

These trabeculae form a very loose reticulum made up of wide
polygonal meshes with a tendency to stretch out in a meridional
direction.

The histologic make-up of these trabeculae is the same as that of
the trabeculae of the scleral meshwork, except that the elastic fibers fail,
and the differentiation of the central strand of connective tissue from the
glass membrane is less complete, its contours are softer as the artist says.
Thereby the trabeculae belonging to the uveal meshwork are easily dis-
tinguished from those of the scleral, even in ordinary staining (PI. II,
10, i). The transition to iris tissue (PL III, 3, 7) is completed as
follows : the endothelium goes over into that of the anterior surface of the
iris; the glass membrane vanishes, the central connective-tissue strand
becomes fibrillated and so merges into the connective-tissue stroma of the
iris. The pigmented cells of the anterior surface of the iris (chromato-
phores) often dispose themselves along these trabeculae, even as far as the
scleral meshwork.

In addition, the so-called iris processes appear here and there, yet
they are not to be found in all eyes. They are cord-like structures pro-
jecting from the anterior surface of the iris at the ciliary border and
consisting of the same elements as iris tissue; they are considerably
thicker than the trabeculae of the iris angle and pigmented, when the
iris itself is. These processes more or less tortuously bridge over the
iris angle and unite with the uveal meshwork. For further details of
their relationship to the relief of the anterior surface of the iris, see chap. x.

CHAPTER IV. THE PERICHORIOIDAL SPACE AND THE
SUPRACHORIOIDEA

The perichorioidal space is an extremely narrow cleft lying between
the inner surface of the sclera and the outer surface of the uvea; it has
almost as great an expanse as the sclera itself. Its forward limit is formed
by the insertion of the ciliary muscle into the scleral roll; behind, toward
the optic nerve, the space becomes less plain to the eye and probably
ceases altogether some distance in front of the nerve, especially on the



THE PERICHORIOIDAL SPACE AND THE SUPRACHORIOIDEA 49

temporal side in the region of thefovea centralis. Extensions of this space
go into the emissaria (see p. 19).

The lumen of the perichorioidal space is probably nil in life, i.e., the
two border walls and the lamellae lying between them adjoin each
other directjy. In the hardened eye, however, one often finds this space
distended, especially when Mueller's fluid is used for fixation.

The two coats cling firmly together whenever blood-vessels go from
the chorioidea to the sclera, or vice versa, as in the localities of the vortex
veins and the short posterior arteries. Pathologic detachments of the
chorioidea, therefore, usually stop at the vortex veins or are traversed
by furrows at these places.

As a section through the entire eye shows best, the whole perichori-
oidal space is traversed by delicate lamellae going from the uvea to the
sclera, i.e., coursing from in front and within, outward and backward,
but in such an oblique direction that when in situ they appear to lie
parallel to the bordering walls. These lamellae fuse together here and
there, and from place to place contain large round openings. The whole
perichorioidal space is in this way subdivided into smaller portions,
which, however, communicate through the openings.

The number of lamellae lying over one another in any given place is
something like 6 or 8. In the posterior part of the perichorioidal space
the lamellae are shorter, the unions between the uvea and sclera, there-
fore, more frequent ; fewer lamellae lie over one another. Forward, they
are longer, therefore the union is more loose and the lamellae apparently
more numerous. In the region of the ciliary muscle the lamellae gradu-
ally disappear between the muscle-bundles, so that in this zone the number
of the lamellae steadily decreases from behind forward and the most
anterior reaches of the perichorioidal space immediately behind the
scleral roll seem entirely empty.

Nothing is easier to make than a surface preparation or a teased specimen of the
suprachorioidea ; one needs only to tear off one of the delicate brown fragments which
always cling to the outer surface of the uveal tract in greater or lesser number with a
fine forceps. One may mount it in glycerin or water and study it without further
preparation, or stain as desired and mount it in Canada balsam. Only in such a
preparation can one study the arrangement of the suprachorioidal lamellae; the
meridional section only brings out the lamellae as extremely fine lines, in which nothing
more than nuclei and chromatophores can be made out.

Each suprachorioidal lamella (PL IV, 2) has an endothelial coat as a
basis; this is an entirely transparent, structureless, extremely fine mem-
brane with only here and there an oval, or somewhat irregular, very flat
nucleus and 1-2 fine nucleoli (e). This membrane is supported by a rich
plexus of elastic fibers (/"). These fibers stain in the usual way, notably



50 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

heavier than those of the sclera, but always much more delicately than
those of ordinary connective tissue. They are straight or weakly bowed ;
for the most part they form a plexus, i.e., the fibers cross in various
directions and the angular branchings and insertions here and there seem
to form a reticulum. No particular direction predominates; only at the
margins of openings in the lamellae do the fibers press together and form
a sort of ring.

The chromatophores (cti) form the third structural element essential
to the suprachorioidea; these are flat, branched cells whose bodies as well
as processes are densely filled with fine brown pigment; the nucleus is
oval or irregular, and likewise flat. In places where elastic fibers pass over
the cell the pigment fails, as a rule, and it looks as if the cell were cut in
pieces, especially in unstained preparations. The form of the cells varies
greatly: in the outer layers of the suprachorioidea the cells, as in the
lamina fusca sderae, are plump and have only a few short broad pro-
cesses. In the inner layers the processes are more slender, longer, and
plainly set off from the nucleated cell-body.

The chromatophores, which we meet here for the first time, are an important and
characteristic constituent part of the whole uveal tract. Much as they vary in their
form and in their content in pigment in the different parts, yet certain properties are
common to all forms: Their pigment consists of very fine round granules, finer and more
of a black brown (melanin) than the epithelial pigment. In comparison with other
forms of cells their processes are thick and are pigmented, as is the cell-body. The
number of these processes, their length and size vary within wide limits; there is an
unmistakable tendency to surface union in the form of nets, or a meshwork in space.
Many times they form, therefore, a syncytium. Yet one finds places enough where
the chromatophores are so sparse that there can be no thought of a reticulum.

Muench (160) believes he has found a cross-striation of the protoplasm of the
chromatophores, especially in the processes. He holds this cross-striation to be
mainly the expression of a very closely wound spiral. The muscular nature of the
chromatophores follows from the spiral structure, according to Muench. In addition
to this there is union with nerve-fibers.

I will not dispute these contentions, especially since Lauber (137) and Schock (191)
have corroborated them, yet I have not so far been able to see the cross-striations. I
cannot see what purpose so richly developed a net of muscular elements could have in
the chorioidea. According to this view, the chorioidea must possess a much greater
contractility than the iris. But so far as we know, the chorioidea has no active motion,
and, moreover, the experiments of Muench indicate at most only a certain elasticity,
nothing more. Moreover, the chorioidea should possess no motion, at least in its
back portions, because then any exact localization of visual impressions would be
impossible.

All these fixed structures of the suprachorioidea are so thin and flat
and so united to the endothelium which forms their groundwork, that the
cross-section of an individual suprachorioidal lamella often appears only



THE PERICHORIOIDAL SPACE AND THE SUPRACHORIOIDEA 51

as a simple fine contour. At best the cross-section has a measurable thick-
ness only when a nucleus or the body of a pigment cell is involved. It is
hardly, possible to say, therefore, on which side the endothelium and
on which side the other elements lie. To judge from the few places
especially favorable for observation, the endothelium lies sometimes on
the chorioidal, sometimes on the scleral surface side of the lamella, in
places, indeed, on both sides, probably as a result of the fusion of
neighboring lamellae.

Moreover, wandering cells are present in varying numbers. They
differentiate themselves from the endothelium, with which they may,
of course, be confounded by the beginner, by their dense-staining nucleus
and the plain non-pigmented but frequently granular protoplasmic
seam.

The entire suprachorioidea is without vessels, i.e., it possesses no
capillary system and, with the exception of the strands now to be described,
no collagenous connective tissue. Two large arteries, the a. ciliares
posteriores longae, do, indeed, course through the perichorioidal space,
but they give off no branches in the suprachorioidea. The finer structure
of these strands (see pp. 12-13) is as follows (PL III, 4). The artery (A)
is bordered on each side by a strand of collagenous tissue, which con-
tains a varying amount of smooth muscle-fibers (m). The latter are
connected with the ciliary muscle, into which the artery finally enters;
they have a longitudinal direction and are, therefore, only seen in cross-
sections (PL III, 4). Ciliary nerves are found on both sides of this con-
nective tissue strand; one of these is regularly larger (N^, the other
smaller (A 7 2 ). The latter branches off from the larger while it is still
within the emissarium. The larger nerve lies below the artery on the
nasal side, above on the temporal side. The nerves are flattened down
at right angles to the bulb wall and show an oval cross-section; the
whole strand is finally surrounded by suprachorioidal lamellae.

The remaining isolated ciliary nerves show the same form and investi-
ture. The fibers of the ciliary nerve have sheaths of Schwann like all
peripheral nerves; they are nearly all medullated but in varying degrees;
the neurolemma is extremely thin.

The ciliary nerves give off many finer branches, which break up into
still finer plexuses in the inner layers of the suprachorioidea and farther
on in the chorioidal stroma; many of these branches consist of only a few
or a single non-medullated fiber. In the nodal points of these plexuses,
and here and there in the course of the nerve branches, larger ganglion
cells are interposed (PL IV, 2, n). These ganglion cells are all multi-
polar, according to the statements of the authorities, and serve vasomotor



52 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

purposes. They have their endings in the blood-vessels of the chorioidea
(cf. chap. v).

The presence of smooth muscle-fibers in the suprachorioidea will be
considered in the description of its union with the ciliary muscle (chap. ix).

CHAPTER V. THE CHORIOIDEA (CHORIOIDES)

The chorioidea is a quite thin, soft, brownish membrane possessing a
certain degree of elasticity, and standing under a moderate tension during
life, or it shows a slight tendency to gape on a solution of its continuity.

Its outer surface is quite uniformly brown and dull, on account of the
suprachorioidal lamellae which cling to it; its inner surface is smooth and
under water shows a weak reflex the pigment epithelium must first be
teased away.

By transmitted light, and, for the most part, even when against the
light background of the sclera, one sees the larger vessels as clear streaks,
the interspaces as brown flecks. The intensity of this pigmentation
depends in general upon the complexion: brunettes have a more densely
pigmented chorioidea than do blondes. The vessel system is easiest made
out in the anterior portions of the chorioidea, because the chorioidea is in
general thinner here, the vessels larger and disposed more in a single layer.

That which first strikes the observer are the vortices, figures formed by
the confluence of the veins. Each such vortex (PL III, 5) includes,
however, not simply the veins of the chorioidea but also those of the
two other zones of the uvea (with few exceptions) ; these veins from in
front are quite straight, those from the sides and behind are more tortuous
and the lateral ones form bows with their convexities forward. The larger
stems arising out of the union of these vessels lie in front of the center
of the vortex, for the most part, and, therefore, converge in their further
course backward ; in this way the whole figure comes to have the appear-
ance of a sheaf or a fountain. The point where all the vessels unite is
widened into an ampulla of i . 5 to 2 mm (Fuchs, 65), and the somewhat
narrower vortex vein goes out of this and soon enters the sclera (see p. 18).

The position and number of the vortices is shown by the exit points
of the vortex veins, with which we are already familiar (p. 9). The
vortices lie about the length of the emissaria in front of and somewhat
more removed from the vertical meridian than do the points of exit of
the veins. So the vortices come to lie 2 . 5 to 3 . 5 mm behind the equator
and, like the veins, are grouped into two pairs, an upper and lower.
The distance between the members of a pair is about half as great as the
distance between the pairs (Fuchs, 65). Not infrequently the number of
the vortices is greater than that of the veins, i.e., instead of one vortex



THE CHORIOIDEA 53

there are two separated halves, and the corresponding veins unite within
the sclera.

The vessel system is more compact and the interspaces more obscured
by pigment in the region of the posterior pole ; to the naked eye this part
of the chorioidea, therefore, appears only flecked with brown. But with
the not inconsiderable magnification of the ophthalmoscope one can,
however, see the larger vessels here when the interspaces are either
abnormally heavily pigmented (tessellated fundus) or not pigmented at
all (albinotic fundus). In moderate pigmentation of the interspaces the
fundus is uniformly red.

From this it follows that the visibility of the chorioidal vessels is
dependent not only upon the pigmentation of the spaces between the
vessels but also upon the color of the pigment epithelium. By themselves
the chorioidal vessels, aside from the capillaries, would easily be seen
with the ophthalmoscope, as shown by those pathologic cases in which
the pigment epithelium and the choriocapillaris are destroyed but the
other layers of the chorioidea are intact. The pigment epithelium lies
over the vessel system of the chorioidea like a brown veil, and the darker
this veil is the more it obscures the details of the chorioidea.

In study by the ophthalmoscope one obtains the impression that
the larger vessels of the chorioidea form a richly divided network. This
is a misconception, however, as a more accurate anatomic study and espe-
cially injection experiments (Leber) show, and is brought about by a
repeated crossing of the vessels. Since we see the vessels very indistinctly
and do not see their thin walls at all, we look upon the crossings as
anastomoses. In reality these are much more rarely present than one
would think from the ophthalmoscopic picture; only the capillaries
form an actual net.

The thickness of the chorioidea is usually given as very slight; but,
when one remembers that the vessels are often empty after death and the
whole membrane, therefore, collapsed, one must come to the conviction
that the chorioidea is essentially thicker than it appears in most sections.
One must, therefore, consider areas in which the vessels are filled with
blood, then it develops that the region of the posterior pole has a thick-
ness of 0.22 mm (183). Wolf rum (240) also found a similar figure
0.3 mm for the thickest place. Toward the periphery the membrane
gradually decreases to one-half (o. i to o. 15 mm).

In the histolog?c study of the chorioidea I prefer the hardening in Mueller's fluid
to all others; the hardening in formalin, especially, is not at all adapted to finer study,
because the chorioidea is always thereby much compressed. Besides cut sections,
surface and teased preparations should be studied. To obtain these, one cuts a piece



54 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

of the chorioidea out of the hardened bulb, first brushes the pigment epithelium
away from the inner surface, and lays the piece, inner surface down, in a dish filled
with water or weak alcohol, fixes it with a finger of the left hand and begins to tease
at the outer surface with a fine forceps. First one removes suprachorioidal lamellae



Online LibraryMaximilian SalzmannThe anatomy and histology of the human eyeball in the normal state, its development and senescence ; → online text (page 6 of 27)