Maximilian Salzmann.

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

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canal, the nasal border of the excavation is steep, indeed, even overhang-
ing, the temporal flat ; in an inferiorly oblique canal the upper border is
steep; in a nasally oblique the temporal border should be steep, but this
difference is not well enough marked to be of moment, because the
temporal border is usually so very low.

In the temporally oblique canal the border tissue is especially well
developed on the temporal side; in a wide canal the glial portion pre-
dominates, as if the glial tissue would fill out the superfluous space.

A light temporal obliquity of the nerve-canal is the most frequent
form; the nasal side then forms a more acute angle with the inner surface
of the chorioidea than does the temporal side. Marked obliquity (so
that the angle named is obtuse on the temporal side) does, indeed, occur
in emmetropic eyes as well, yet even then it is associated with a strik-



THE OPTIC NERVE 97

ingly long optic axis and so represents a transition to myopia. Wholly
symmetrical nerve-canals are in any case more rare than slightly oblique.
Properly speaking, nasally and inferiorly oblique canals are, indeed,
malformations, since such eyes are usually below par in function; yet
it is no more permissible to draw a sharp distinction between normal
and malformed eyes here than it is in myopia.

3. THE SIGNIFICANCE OF THE OPHTHALMOSCOPIC PICTURE IN RELATION
TO THE ANATOMY OF THIS REGION

That which one calls the disc or papilla in ophthalmoscopy (PL
VII, i) corresponds to the area of the lamina cribrosa visible in the inner
opening of the optic-nerve canal. In symmetric or only slightly oblique
canals the margin of the inner opening forms a border projecting (over-
hanging) on all sides. One can, therefore, never see the side walls of the
canal, and the inner opening often seems to be entirely filled out by the
lamina cribrosa.

The lamina cribrosa, in and of itself, appears clear white with washed-
out gray stipplings; the latter correspond to the translucent bundles of
optic-nerve fibers which pass through the cribriform plate. In normal
eyes the lamina cribrosa never lies completely bared. The layer which
covers it is only so thin over the floor of the physiologic excavation that
the white of the cribriform plate and also part of the stippling is visible.
In the marginal portions of the optic-nerve papilla there is such a thick
layer of nerve tissue in front of the lamina cribrosa that the stippling is
invisible and a uniform reddish color results. The latter is to be looked
upon as the actual color of the nerve-fiber mass and, on its part, is due
again to the blood contained in the capillaries. On the other hand, the
grayish admixture which one sees so often in this marginal portion of the
optic disc is solely a contrast in appearance in connection with the clear
floor of the excavation.

The red of the eyeground and the peculiar granulation comes mainly
from the pigment epithelium. The part played by the blood of the chori-
oidal vessels is almost nil in a smoothly red eyeground, according to
Marx (147); that is to say, in a fundus of such appearance the pigment
epithelium is so densely pigmented that one cannot see the chorioidal
vessels through it. When the chorioidal vessels are visible, as in less
pigmented epithelium, the blood in the vessels as well as the chorioidal
stroma has an influence upon the color of the fundus. The chorioidal
vessels appear as red stripes; the interspaces, when they are densely
pigmented, appear darker, blacker (tessellated fundus), when they are
weakly pigmented lighter, brownish to yellowish. The contours of the



98 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

chorioidal vessels are very indistinct, because the pigment epithelium
obscures these contours like a brown veil.

Only when the pigment epithelium contains absolutely no pigment at
all and the chorioidal stroma is, likewise, non-pigmented (albinotic
fundus) are the chorioidal vessels visible as plainly contoured red stripes
on a yellow-white background, for the color of the fundus is dependent
upon chorioidea alone.

If one excludes these cases which properly belong in the territory of
anomalies, one can say that the limits of the red color of the fundus and
that of the pigment epithelium coincide. In many cases the red of the
fundus does not reach clear up to the optic-nerve sheaths, but a narrow
white strip (connective tissue or scleral ring) is interposed between the
two. This ophthalmoscopic appearance may have various anatomic
bases; it may be due either to border tissue which is not wholly covered
by pigment epithelium, or it may be that the side wall of the scleral canal,
which becomes of a greater obliquity, is visible in a perspective fore-
shortening of the latter therefore, the lightest grade of distraction cres-
cent. The dark, black seam (chorioidal ring), by which the red fundus
is often separated from the optic disc or scleral ring, is due to a heavier
pigmentation of the epithelium, therefore bears its name incorrectly.

When the nerve-canal is straight, the trunk of the central artery is
not visible, or, properly speaking, visible only in optical cross-section,
for the artery courses in the line of vision of the observer; for the same
reason, its first branching makes an apparent angle of 180.

It can be closed in a temporally oblique nerve-canal, if the nasal
border of the excavation is unusually steep or, indeed, overhanging, i.e.,
when the reddish-gray color of the nasal part of the optic disc is sharply
set off from the white of the excavation and especially when the first
division of the arteria centralis retinae forms a temporally open angle,
for in an oblique nerve-canal the plane of the branching is inclined away
from the line of vision. The trunk of the central artery is not, however,
visible as a rule, because a very thick mass of nerve-fibers covers up the
nasal part of the optic disc.

The image is different in a nasally oblique canal. The first of the
central arteries then presents an angle open nasally (reversed vessel
distribution), and the trunk of the central artery is wholly and plainly
visible over a long stretch, because the nasal wall of the excavation on
which the artery lies is now visible throughout almost its whole extent.
The vessel entrance, i.e., the place where the central vessels come forth
out of the lamina cribrosa, then appears strongly displaced toward the
temporal side, for we project everything into the plane of the inner open-
ing of the nerve-canal, although it really lies at the deeper level.



THE OPTIC NERVE 99

b) The Medulated Section of the Optic Nerve
(PL VI, 2)

This begins immediately behind the lamina cribrosa, therefore, about
o . 5 mm behind the inner surface of the chorioidea. Its thickness in-
creases up to the level of the outer surface of the sclera and then remains
constant. Since the cross-section does not vary essentially from the form
of a circle, the orbital section of the optic nerve forms a cylindrical strand
3 to 3 . 5 mm thick.

This strand is surrounded by connective-tissue sheaths, which are
united with the sclera on one hand and with the brain membranes on the
other, and are named on account of their analogy to the latter. With
the addition of these sheaths the thickness of the optic nerve increases
to 4 to 4 . 5 mm.

Directly at the lamina cribrosa a thin connective-tissue layer is given
off as an immediate covering for the optic nerve (the inner or pial sheath,
P) ; it clings firmly to the outer surface and extends into the supporting
tissue framework (the septa). The outer layers of the sclera, which are
not united to the lamina cribrosa, turn back with a part of their fiber-
bundles and form a second coat of some o . 5 mm thickness (the outer
or dural sheath, D} ; it forms a hollow tube united to the inner sheath by
only a few trabeculae. Between these two there lies a space, the inter-
vaginal space; this begins at the root of the pial sheath and opens
centrally into the cavity of the skull.

The intervaginal space is divided into two spaces by a delicate mem-
brane, the arachnoidal sheath (Ar). The outer space, between the dural
and arachnoidal sheaths, is called the subdural space (sd) and is a
narrow cleft-like space, broken only by the trabeculae which course from
the dural to the pial sheath. The inner space, between the arachnoidal
and the pial sheath, is called the subarachnoidal space (sar) and contains
a richly subdivided system of finer and grosser trabeculae (subarach-
noidal trabeculae) constituting the union between the arachnoidal and
the pial sheaths; this space is wider than the subdural space, or capable
of appreciable widening in any case.

The central supporting tissue strand courses in the axis of the optic
nerve for a distance of 7 to 12 mm from the eyeball. Thereupon it
turns downward at almost a right angle and leaves the optic nerve. In
the medullary section it has a plain connective-tissue hull, which goes
over into the pial sheath at the exit point and so is to be looked upon as a
continuation (invagination) of the pial sheath. In this way, too, the
orbital section of the optic nerve can be further divided into a (centrally)
vascular and a (centrally) avascular portion.



ioo ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL
I. THE SHEATHS OF THE OPTIC NERVE

The outer and inner sheaths have exactly the same structure; they
consist of tough fibrous tissue, in the moderately tortuous bundles of
which the collagenous fibrillae are mixed with numerous elastic fibers.
These are larger than are those of the sclera, often branched and provided
with membranous expansions at the branching points. The arrangement
of the fibers on the surfaces turned toward the intervaginal space is
predominantly a circular one, on the surface turned away from the
intervaginal space, mainly a longitudinal one. The cells are usually flat
connective-tissue cells with membranous bodies, and have, therefore, a
certain similarity to endothelia; they lie on the surface of the connective-
tissue bundle. So far as one can judge from ordinary stains, there is no
essential difference between them and the cells of the sclera.

Near the bulb the dural sheath often splits into several layers sepa-
rated by open spaces. I have not been able to convince myself that an
endothelium is present in these spaces ; yet one sees flat cells on the walls
of the spaces here and there, though these may be only ordinary con-
nective-tissue cells. The dural sheath shows an especially noticeable
splitting where a posterior ciliary artery courses through it (see chap. xv).

According to authorities, a space (the supravaginal space) lies outside
the dural sheath; this is a continuation of the Tenon's space and is
bordered outside by an extension of Tenon's fascia. But this supra-
vaginal space is no more clearly a space than is Tenon's space, and is
rather only a very loose layer of connective tissue similiar to subcon-
junctival tissue, in which fluid can easily broaden out. On the other
hand, the dural sheath is sharply delineated and provided with a plain
endothelium on its inner surface. A good many blood-vessels and nerves
are present, the former principally on the outer surface of the dural sheath.

The pial sheath shows identically the same structure, only it is much
thinner and has a circular fibrillation on its outer surface. The trabeculae
of union between the dural and pial sheaths (PL IV, 5, Vb) are pretty thick
cylindrical strands made up of bundles of longitudinally coursing collage-
nous fibrillae reinforced by large elastic fibers, and, like all the surfaces
bounding and passing through the intervaginal space, covered by plain
endothelium. These trabeculae run through the intervaginal space very
obliquely, so that only cross or oblique cuts of these trabeculae are seen
in sections ; they are, therefore, easily distinguished from the elastic fibers
and the subarachnoidal trabeculae by their size. The vessels, too, are
carried by these trabeculae to the inner sheath.

The rest of the structures filling out the intervaginal space usually
show another make-up; collagenous tissue is, indeed, the substratum



THE OPTIC NERVE 101

here also, but elastic fibers fail, and the cells of the connective tissues are
replaced by endothelium.

The actual arachnoidal sheath (Ar) is a continuous membrane of
some 10 mu thickness. The following layers can be made out: an endo-
thelial covering (outer endothelium, aE) lies on the outside (that turned
toward the dural sheath); its cells appear spindle-form on cross-section,
i.e., the oval nucleus is surrounded by some protoplasm. One very
frequently sees proliferations of this endothelium, even in eyes which are
otherwise normal; it then appears to have several layers for stretches;
indeed, even spherical pearls, made up of rounded, concentrically strati-
fied endothelium, may form, and then concentric concrements develop
from a degenerative process.

The outer endothelium is succeeded by a very delicate layer of non-
nucleated connective tissue made up of small stellate expansions whose
processes build a network (PI. IV, 6, a). Some clearly demonstrable
tiny bundles with a tortuous course go inward from the centers of these
little stars. Several of these little bundles, which consist solely of
collagenous fibrillae, course farther on parallel to one another, and these
groups cross and interweave; in this way there arises a second, heavier
layer of connective tissue (i] inside the stellate expansions, made up of
bundles.

The inner endothelium follows this layer; this seems to be exactly like
the outer endothelium, yet it has no tendency to proliferation. The
inner endothelium forms the inner surface of the arachnoidal sheath
proper, i.e., the one turned toward the pial sheath (PI. IV, 5, iE).

The subarachnoidal trabeculae (PI. IV, 5, sb) forms out of the
second (inner) connective tissue ; groups of little bundles of fibrillae unite
into a larger bundle and leave the arachnoidal sheath, covered with a
continuation of the inner endothelium. These primitive subarachnoidal
trabeculae, therefore, consist only of non-nucleated strands of collagenous
fibrillae covered by a plain endothelial membrane made up of a thin layer
of protoplasm strewn with oval, somewhat prominent nuclei.

The primitive trabeculae unite into a meshwork and in this way
permeate the whole subarachnoidal space. The nearer one approaches
the pial sheath, the larger the trabeculae become (SB), and within these
larger trabeculae are a few cells and elastic fibers as well.

Finally, these trabeculae go over into the pial sheath; their endo-
thelium passes into the (outer) endothelial covering of the pial sheath,
and their fibers into the outer circular fiber layer of the connective tissue.

The trabeculae of union, which traverse the entire intervaginal space,
do not enter the arachnoidal sheath, but the latter is invaginated inward



102 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

along these trabeculae and envelops them still for a long stretch. The
cross-section of such a trabeculae of union (Vb}, therefore, shows a core
of firm connective tissue with elastic fibers surrounded by a delicate layer
of exactly the same structure as the arachnoidal sheath.

In general, the subdural and subarachnoidal spaces are entirely sepa-
rate from one another, for the arachnoidal sheath is continuous and free
from dehiscences.

The following is to be said concerning the union of the sheaths of the
optic nerve with the sclera. When it is stated that the outer half or
two-thirds of the sclera goes over into the dural sheath, it is meant that the
intervaginal space reaches to this depth. Only a part of the scleral
fiber-bundles go over into the dural sheath; these bundles in the sclera
course directly up to the optic nerve and then bend about in a sharp
bow into the longitudinal fiber course of the dural sheath; they are, there-
fore, visible throughout their whole course. Numerous cross-sections of
scleral fiber-bundles also appear at the insertion of the dural sheath ; these
are bundles which do not enter the dural sheath and are deflected in flat
curves out around the optic nerve.

These deflected bundles may intermingle extensively with these
which go over into the dural sheath; the root of the dural sheath is then
broad and deep. In other eyes these deflected bundles are systematically
compressed into a layer, and then the dural sheath is sharply set off
from the sclera. It can come about, for example, that the outer third of
the sclera does not go over into the dural sheath at all, and thus appears
to be a continuation of the middle third (consult Elschnig, 52).

The distance between the root of the dural sheath and the optic nerve
is also subject to variation. The greater this distance, the wider the
anterior end of the intervaginal space seems. When, therefore, the end
of the dural sheath retracts after enucleation, as it does regularly to a
greater or less extent, it sinks into the widened end of the intervaginal
space, and this then shows an angular bending about into the course of
the scleral fibers.

The intervaginal space is always wider at its anterior end than in the
orbital part of the optic nerve, because just behind the lamina cribrosa
the optic nerve does not yet possess its full thickness. The inner scleral
layers, united only with the pial sheath and the lamina cribrosa, always
close off the intervaginal space in front, and to a certain extent form the
anterior wall. The form of this anterior end of the intervaginal space is
subject to many variations and often is not alike on the two sides of the
optic nerve, yet one cannot establish fixed types. The oblique direction
of the optic-nerve canal is, as a rule, associated with a more marked



THE OPTIC NERVE 103

widening of the intervaginal space ; one finds such a widening on the nasal
side in a temporally oblique canal, especially.

The arachnoidal -sheath usually ends in the angle which the dural
sheath makes with the anterior wall of the intervaginal space, at times
even farther back, for it goes over into the dural sheath.

The pial sheath usually shows a thickening at its scleral end, so that
the angle between it and the anterior wall of the intervaginal space is
rounded out. The most anterior part of the pial sheath consists of con-
nective-tissue bundles, which, in the main, have a circular course; since,
too, the neighboring portions of the sclera are almost exclusively made up
of such fiber-bundles, a delimitation of the pial sheath from the sclera is
not possible. The longitudinal fibers can be followed some distance
farther forward, but they do not form so compact a mass that one can
speak of a continuation of the pial sheath up to the lamina vitrea chori-
oideae. When, therefore, some authors speak of the border tissue as a
sheath extension, they have chosen a subjective conception, or a form of
presentation and description, in order to make the complicated anatomic
relations more demonstrable; it is not an extension in the strict sense of
the word.

2. THE OPTIC-NERVE TRUNK

This consists of a large number of rounded nerve-fiber bundles sepa-
rated from one another partly by glial tissue and partly by connective
tissue trabeculae. This separation is, however, an incomplete one. The
individual nerve-fiber bundles exchange fibers here and there.

The connective tissue trabeculae (septa) carry the vessels to the
nerve-tissue and form a closed system (septal system) united on one
side with the pial sheath and on the other side with the central con-
nective-tissue strand, in so far as this is present; as described above, it
goes over into the lamina cribrosa in front. The septa do not, however,
inclose the individual nerve-fiber bundles on all sides, but only unite
groups of bundles, and, indeed, they do not form a continuous septal wall
between such bundles, but only along stretches ; otherwise the framework
is made up of oblique and cross trabeculae. The grouping of the bundles
into septa changes in every cross-section; the partitioning walls formed
by the glial tissue mount up over the septa and to a certain extent form
its continuation and bridge over its interspaces.

The make-up of the septal system can most easily be studied in longi-
tudinal sections after staining by Van Gieson (PL VI, 4). The longitudi-
nal section of the optic nerve gives a surface view of the septa in some
places, a longitudinal section of the septa in others.



104 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

In the surface view, the septa (SJ sometimes appear as expanded
plates, sometimes as narrow bundles of connective tissue carrying blood-
vessels of varying caliber. They bridge over a few of the nerve-fiber
bundles, then bend out of the plane of the section, so that one cannot
follow them any farther. On the longitudinal section each septum (S 2 )
appears as a narrow strip of connective tissue, or as a row of cross-
sections of rounded connective-tissue bundles broken by larger inter-
spaces. The interspaces are filled out by rows of glial cells (G7). After
a short distance the cross-section of connective tissue stops altogether
and only the rows of glial cells continue; cross-sections of connective
tissue appear thereafter in one or another place. The glial cells, there-
fore, appear arranged in regular longitudinal stripes, and when one
follows such a stripe he sooner or later comes upon a septum.

In this description I have an ideal longitudinal section in mind; it
is to be remembered in the study of the preparation, however, that even
after a careful choice of the direction of the section, it is only rarely possible
to maintain the exact longitudinal direction of the septa for long stretches,
because the nerve normally shows an S-shaped curve, and the end farthest
away from the bulb is slightly bent. The accurate production of cross-
sections is easier.

In such a section (PL VI, 2) the septal system appears more con-
tinuous and better delimited and subdivided into irregular polygonal
fields with rounded angles. Yet even these are not set off on all sides,
and many septa appear to be discontinued after they have pressed some
distance into the nerve-fiber mass. The largest septa are called the
primary, the weaker and more incomplete ones the secondary septa.
Their incompleteness is, however, only apparent: they are trabeculae
which course obliquely to the plane of section, and, therefore, fall into the
plane of section only in part.

Upon cross-section, as well, the "incomplete" septa are continued by
empty glial tissue, that is, glial tissue containing no nerve-fibers. When
one stains the cross-section of the nerve by Weigert's method (PI. VI, 6),
the nerve-fiber bundles appear much sharper and more completely sepa-
rated from one another, because after this staining the connective tissue
and the glia take on the same light-brown nuance.

Aside from these glial continuations of the "incomplete" septa,
glial cells are found in the nerve-fiber mass, apparently in irregular
arrangement. But this lack of regularity is only an apparent one, for in
longitudinal section these glial cells correspond to regular rows which
accompany septal trabeculae, either farther above or below.

From this one comes to the conviction that the glial cells in the



THE OPTIC NERVE 105

medullated section of the nerve, as in the non-medullated, are found
only on the surface of the individual nerve-fiber bundles, i.e., where these
are not separated by septa. The pure glial septal walls arise as exten-
sions of the septa. But neither does the glial framework effect a
complete separation of the individual nerve-fiber bundles from each
other, for these fibers anastomose in places, and the cells lying inside
the nerve-fiber mass are nothing else than the ends of the glial frame-
work at the anastomosis of the nerve-fiber bundles. ,

The tissue of the septa is ordinary connective tissue; the collagenous
fibrillae form delicate bundles, which as a rule course crosswise or
obliquely, depending upon the direction of the particular trabecula;
however, they also course longitudinally in the case of the larger septa.
Elastic fibers are numerous and, in general, take the course of the col-
lagenous fibrillae. These sparse nuclei are slender and stain deeply.
The blood-vessels (PL VI, 3, g) lie throughout in the septal tissue and,
with few exceptions, are branches of the sheath vessels; the heaviest
part of the septal system likewise carries the largest vessels.

The glial tissue agrees wholly with that of the non-medullated section


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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 12 of 27)