Maximilian Salzmann.

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

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valleys by broad, short projections. In further development, the retinal border con-
stantly moves backward behind the processes, and the teeth become more and more
drawn out. These, too, on their part now undergo more or less regression and leave
behind more markedly pigmented stripes, the striae ciliares (cf. chap. ix).

This theory is not, indeed, entirely satisfactory, but has much more in its favor
than that of Schoen. It is exactly the opposite of this. Whereas Schoen has the ora
serrata develop in extrauterine life, O. Schultze considers it as congenital and admits,
indeed, the possibility of later regression.



CHAPTER VIII. THE OPTIC NERVE (NERVUS OPTICUS)

Besides the intraocular, only the orbital portion of this nerve, which
represents the union of the retina with the brain, will be described.

By the intraocular or bulbar portion of the optic nerve, one under-
stands the part lying within the bulb wall, that portion remaining behind
on the bulb after a properly made enucleation, i.e., when the optic nerve
is cut through at the level of the outer surface of the sclera. The orbital
or retrobulbar portion is that lying between the eyeball and the canalis
opticus.

The division into a non-medullated and a medullated section is better
than this division. The border between these two sections falls pretty
nearly at the outer surface of the lamina cribrosa, and still lies, therefore,
within the bulbar portion. This limit is, of course, not an absolutely
sharp one, for the medullary sheaths of nerve-fibers do not all cease at the
same place, but it has the advantage of being a natural limit, one which
bears the most important differences in structure.

a) The Non-medullated Portion of the Optic Nerve

(PL IV, 4)
I. MICROSCOPIC ANATOMY AND HISTOLOGY

This portion can be further subdivided into a retinal, chorioidal and
a scleral portion. These portions correspond to the like-named coats of
the bulb wall through which, one after another, the optic nerve passes.

When one follows the nerve-fiber layer in a centripetal direction in a
meridional section, one sees the nerve-bundles curve over the chorioidal
foramen in bows into the line of the optic-nerve axis. Since the nerve-
fiber layer and with it the entire thickness of the retina increases in
thickness toward the optic nerve, this transition area bulges a little
toward the interior of the eye (papilla nervi optici}. The prominence
is, however, insignificant and scarcely deserves the name papilla, espe-



THE OPTIC NERVE 89

daily since what one designates as such in ophthalmoscopy has nothing
whatever to do with the prominence.

A depression arises in the middle of the papilla from the bowing apart
of the nerve-fibers; sometimes this has the form of a funnel, sometimes
that of a crater. The first form, characterized by an ending in a sharp
angle, is called the vessel funnel; the latter form, showing a more or
less flat floor, is called the physiologic excavation (Ex). These terms are
used with respect to the variations in the ophthalmoscopic picture; an
essential anatomic difference, aside from the form of the depression, does
not exist. In general, the form and size of the depression is subject to
very great individual variation.

The retinal portion of the optic nerve really forms only a ring; the
temporal quadrant of this ring is lower and thinner than the other three
quadrants as a result of the lesser bulk of the nerve-fiber layer on this side
of the optic nerve. The physiologic excavation, therefore, does not lie
exactly in the middle of the optic nerve, but is shifted a little to the tem-
poral side. The larger blood-vessels mount up along the nasal wall of the
excavation to its depths; the temporal border is crossed only by very
fine vessels going to the fovea.

The blood-vessels of the retina come together in the upper and lower
quadrant of this ring and unite into upper and lower main branches
(arteriae or venae papillares superior et inferior). The union of the two
arteriae papillares into a trunk, (arteria centralis retinae) takes place
while the vessels are still in the level of the retina at an angle of about
90; still the plane of this angle is usually placed sagitally. The two
venae papillares, however, remain separated up to the beginning of the
scleral portion of the optic nerve and first unite here in it, i.e., in the
lamina cribrosa or on its inner surface. A very considerable part of
the arteria centralis retinae, therefore, lies inside the lamina cribrosa,
whereas the vena centralis retinae is just formed at it.

In the retinal part of the optic nerve the vessels lie wholly super-
ficial, i.e., they are not covered by nerve-fibers; on the other hand they
possess a thin glial covering on the vitreous side. This attains a special
thickness on the floor of the physiologic excavation and, to a certain
extent, forms the bulbar closure of the bundle, the central supporting
tissue strand, which accompanies the arteria and vena centralis in their
further course in the axis of the optic nerve. The glial covering of the
floor of the excavation has been designated as the central supporting
tissue meniscus (Me) in this sense by Kuhnt. Sometimes this tissue
extends still farther into the optic nerve along the central vessels
(Elschnig's intercalary tissue, 52).



go ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

An actual limitans interna, as denned on pp. 79-80, does not exist in
the region of the physiologic excavation. For this is the place where the
arteria centralis retinae goes over into the art. hyaloidea in fetal life, and
here lies the entrance of the canalis hyaloideus (about which contention
has again recently arisen; cf. chap. xi). Pathologic cases show that a
special delimitation from the vitreous fails here; for vascularized tissue
very easily grows out into the vitreous from this place in inflammations
(retinitis proliferans, cicatrices in the later stages of the septic endophal-
mitis).

The rest of the layers of the retina (from ganglion-cell to rod-and-cone
layer) end at the border of the optic nerve, the inner earlier than the
outer layers, corresponding to the bow-form course of the nerve-fibers.
Furthermore, the layers thin out toward the border and thereby compen-
sate for the increase in thickness in the nerve-fiber layer to a certain extent.
As a matter of fact, however, these layers do not reach clear up to the optic
nerve but a thinner or thicker non-stratified layer of tissue, the inter-
mediary tissue of Kuhnt (129) (im), is interposed between the border of
the retina and the most peripheral nerve-fiber bundles.

The pigment epithelium reaches up to the intermediary tissue at
the optic nerve, or nearly so; its border portions are, however, often not
uniformly developed and show abnormalities of pigmentation, i.e., an
excessive pigmentation or one defective, even to complete loss of color.

However, the pigment epithelium never extends as far as does the
glass membrane of the chorioidea. Of all the layers of the retina and
chorioidea, this is the only one which reaches clear up to the optic nerve;
indeed, its slightly forward curved end even covers it over somewhat.

The hole in the glass membrane of the chorioidea (foramen opticum
laminae vitreae chorioideae} forms the inner opening of the optic-nerve
canal (scleral, sclerotic, or entrance canal of the optic nerve) ; it is formed
by the union of the foramina optica chorioideae and sclerae and measures
some 0.5 mm in length. Its inner opening is about 1.5 mm wide;
the outer opening, measured at the level of the outer surface of the
lamina cribrosa or pial sheath, is usually much wider. Its cross-section
varies only a little or not at all from the form of a circle; the longitudinal
section, on the other hand, varies remarkably in individuals. The varieties
will be discussed more in detail later on.

The wall of the canal is formed by a white or whitish-colored fibrous
tissue, which is plainly set off from the chorioidal layers, but not from
adjacent sclera and, therefore, appears as a continuation of it like a
selvage surmounting the foramen opticum sclerae inwardly and extend-
ing clear to the glass membrane of the chorioidea (Gr).



THE OPTIC NERVE 9 i

This tissue has been called the connective-tissue ring, the scleral ring,
the sheath extension, and the border tissue. I prefer the latter (Elschnig's
term), because it does not commit one to anything. This border tissue
is much more strongly developed on the temporal than on the nasal side.
It separates the rest of the layers of the chorioidea (from the choriocapil-
laris to the suprachorioidea) from the optic nerve, so that none of the
layers come in direct contact with the optic nerve. This border tissue
shows its greatest thickness at the level of the inner surface of the sclera ;
outside this it thins rapidly and has completely disappeared before the
outer opening of the optic-nerve canal is reached. It does not go over
into the pial sheath of the optic nerve, for this has altogether another
histologic composition. The name "sheath extension" is not at all well
chosen, therefore.

The elements of the border tissue do, however, probably extend into
the framework of the optic nerve, and especially at the end of the inner
surface of the sclera. It builds a thick trabeculum in connection with the
elements of the sclera proper at the foramen opticum sclerae, and this
surrounds the individual optic-nerve fiber-bundles and separates them
from one another. In and of itself this framework has the appear-
ance of a sieve; it, therefore, bears the name cribriform plate (lamina
cribrosa, Lc}.

The optic-nerve canal shelters the chorioidal and scleral portions of the
non-medullated section of the optic nerve. The former is a solid strand,
even when the physiologic excavation is not very deep. This portion as
well as the scleral and the adjoining medullated section has, of course, the
form of a ring, if one takes only the nerve-fiber mass into consideration;
the lumen of this ring is wholly filled out by connective tissue and blood-
vessels. Commencing at least with the scleral portion of the optic nerve,
this tissue and these vessels form a round cord, the central supporting
tissue strand (cB), which courses exactly in the axis of the optic nerve.
It contains one large artery (the arteria centralis retinae} on the nasal side,
and, as a rule, only one large vein (the vena centralis retinae} on the
temporal side. The lamina cribrosa has a large central opening for the
passage of this strand, and is, moreover, united to it by means of tissue.

In the entire non-medullated section of the optic nerve the individual
nerve-fiber bundles remain strictly separated from one another; there
are no anastomoses. The optic-nerve trabeculum fills out its interspaces
and this, therefore, forms closed ensheathing walls between the nerve
bundles.

An exact longitudinal section through the non-medullated part of the
optic nerve, which, indeed, cannot always be obtained on account of the



9 2 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

form of the scleral canal, shows, therefore, a regular longitudinal striation
caused by the alternation of non-nucleated nerve-fiber bundles and nuclear-
rich separating walls. Seen in longitudinal section, these are called the
nuclear columns (Ks). The cross-section, on the other hand, shows the
ensheathing walls as a continuous network of rounded meshes.

The framework is weakly developed in the retinal portion and at the
level of the inner opening of the optic-nerve canal; it thickens, however,
in the chorioidal portion, as the optic-nerve canal widens and the nerve
bundles spread apart. The fibrillation becomes more and more plainly
transverse as one approaches the lamina cribrosa; the trabeculum of the
chorioidal portion is, therefore, called the lamina chorioidalis (Lch) by
many, while our lamina cribrosa is given the name lamina scleralis.

This latter is nothing else than the framework of the scleral portion;
it is differentiated from the remaining portions of the optic-nerve frame-
work histologically, however, and is less developed than these. It does
not, therefore, deserve to be described as a special structure.

It is useless to discuss whether the lamina cribrosa is a continuation of the inner
scleral layers or a modification of the septal system of the medullated section of the
optic nerve. Each view has as much for as against it. The same is true of the ques-
tion whether the border tissue belongs to the chorioidea or to the sclera or is a continua-
tion of the pial sheath.

The fact is that here various supporting tissue structures meet and merge. The
supporting tissue is continuous, as everywhere else in the body, and we only separate
various portions from one another for purposes of description.

Taken as a whole, the lamina cribrosa shows a certain concavity
inward, more outspoken in the more overhanging nasal side of the optic-
nerve canal. Its fibers, therefore, show an oblique course at the border,
i.e., they are not parallel to the inner surface of the chorioidea; and when
one makes a cross-section of the optic nerve at the level of the lamina
cribrosa, the center of the section shows layers of the lamina cribrosa lying
farther inward than do the marginal portions of the lamina cribrosa.

The thickness of the lamina can be placed at o . 2 to o . 3 mm; it cannot
be given accurately, because the cribriform plate is not very sharply
demarkated inwardly or outwardly. In any case the lamina cribrosa
completely or nearly completely fills out the foramen opticum sclerae, and
the border of its outer surface lies pretty nearly at the level of the root of
the pial sheath or the bulbar end of the intervaginal space (/). The
center of the outer surface, however, lies considerably behind this level.

The fibrillation of the cribriform plate is tense and directed transversely,
i.e., wholly, or almost, at right angles to the course of the nerve bundles;
its trabeculum is much thicker than the framework of the chorioidal



THE OPTIC NERVE 93

portion of the optic nerve. For this reason it is very easily differentiated
from the adjoining portions of the optic-nerve framework.

The peculiarities of its structure come out still more clearly on the
surface-section, i.e., on cross-section of the nerve at the level of the
lamina cribrosa (PI. VI, i). The trabeculae are broad, the angles are
rounded off, the fibers course straight, the nuclei are numerous. Above
everything else, however, each nerve-fiber bundle is strictly separated
from its neighbor; no interruptions of the framework are present.



Histologically, the non-medullated section of the optic nerve agrees
most closely with the nerve-fiber layer of the retina. Its varicose non-
medullated fibers are arranged in plainly separated bundles ; immeasur-
ably fine glial fibers support the nerve-fibers and interlace with them.
Golgi preparations show that part of the glial fibers course longitudinally
or obliquely, but the main mass courses transversely, i.e., perpendicular
to the direction of the nerve-fibers between which they run. The direc-
tion of this portion of the glial fibrillation is. moreover, to be made out
without recourse to special stains, for these fibers appear in part as fine
cross-lines crossing the longitudinal striation (brought out by the nerve-
fibers), in part as little points (cross-sections) between the nerve-fibers.

Glial cells appear only here and there inside the nerve-fiber bundles,
in large numbers, on the other hand, between the bundles, so that the
framework of the optic nerve seems rich in cells everywhere. The indi-
dividual glial cells have a rounded or oval nucleus of 6 to 9 mu in length and
an irregular stellate protoplasmic cell-body. The glial fibers are in close
relation to the cells through whose protoplasm they course. However,
as Weigert's neuroglia stain shows, they are different from the proto-
plasm and constitute independent fibers lying at one and the same time
intra- and extracellular, grouped about the nuclear center. The Golgi
stain does not differentiate cells and fibers, and by this method the glial
cells, therefore, show up as longish, deep black masses. A large number
of extremely fine processes go out in the most varied directions (so-called
spider cells).

Kuhnt's intermediary tissue consists of pure glial tissue, i.e., it con-
tains no nerve-fibers; the same is true of the covering of the physiologic
excavations and the central supporting tissue meniscus. The fibrillation
is circular in the intermediary tissue and in the lateral walls of the excava-
tion; in the meniscus the fibers interlace in various directions (Jacoby,
109).

The gliae also appear as a constituent of the border of the optic nerve.



94 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

The border tissue is more or less richly permeated with glial fibers or
consists almost exclusively of glia (Elschnig, 52), at least in the portions
immediately bordering on the optic nerve. In my own experience this
latter does not, indeed, seem to be the usual occurrence. As a rule the
foundation of the border tissue is a dense collagenous tissue such as is
found in the sclera; a sharp separation of the two is not, therefore, pos-
sible. The fibrillation is mainly circular, the bundles are finer than in the
sclera proper. A rich mass of elastic fibers is intermingled ; near the optic
nerve and at the level of the inner surface of the sclera the amount of
elastic fibers attains its maximum; this place, therefore, takes on an
especially dark color after elective staining (orcein, etc.). Finally, pig-
ment cells of the chorioidea (chromatophores) enter into the composition
of the border tissue in varying amount. In addition, there is still the
glial tissue shown by Jacoby. It is true that such elements require a
special elective stain (Weigert's glia stain) for their demonstration, which
unfortunately often miscarries. Still one can recognize the presence of
glial fibers, to a certain extent without any such stain, although not every-
thing in the border tissue which stains yellow by Van Gieson's stain takes
the orcein stain.

The lamina cribrosa is similarly constituted (PI. VI, i); glial fibers
and cells with elongated, transversely placed nuclei, elastic fibers, collage-
nous fibers and numerous capillaries make up the cribriform plate. The
latter arise from the Hallerian or vascular circle of Zinn (see p. 25),
along with elements of the sclera (collagenous and elastic fibers), and enter
the cribriform plate in a meridional direction. The majority of the struc-
tural elements seem, however, to come out of the border tissue, for surface-
sections show them bending obliquely out of their circular course toward
the axis of the eye.

The indistinct demarkation of the lamina cribrosa from the neighbor-
ing portions of the framework of the optic nerve has been already reported.
The histologic structure of the framework of the optic nerve makes this
clear. As one proceeds inward (in the centrifugal direction) one notes
that the collagenous and elastic elements become more and more sparse,
so that finally there remains only a fine glial tissue with plain transverse
fibrillations and elongated nuclei; the elastic fibers persist longest in the
marginal portion of the optic-nerve framework. Thus, one always finds
a few fine elastic fibers radiating out of the border tissue into the frame-
work of the optic nerve at the level of the chorioidea, but they are limited
to the immediate neighborhood of the border tissue. The framework
of the chorioidal portion of the optic nerve as a whole consists entirely of
glia, and only the more outspoken transverse course and the greater size



THE OPTIC NERVE 95

and richness of the fibrillation brings to mind and seems to justify the
name lamina chofioidalis in the lamina cribrosa.

When one proceeds outward (in the centripetal direction), on the
other hand, one notes that the glia constantly decreases and the collage-
nous fibrilla increases. In this way the cribriform plate goes over into the
septal system of the medullated section of the optic nerve. This system
contains only collagenous and elastic fibrillae.

Blood-vessels are found in all portions of the optic nerve, their mass
and origin, only, vary; but they are found only in the framework (between
the bundles), not inside the nerve-fiber bundles. The retinal and chori-
oidal section of the optic nerve, however, contains no mesodermal ele-
ments in its framework, aside from the vessels and their adventitia.
These first appear in the lamina cribrosa, the elastic elements first and
then the collagenous fibrillae. The lamina cribrosa, therefore, forms a
sort of transition structure between the purely ectodermal (glial) frame-
work of the retinal and chorioidal section and the partly mesodermal,
partly glial framework of the medullated section. The lamina cribrosa,
as well as the border tissue, is, however, especially characterized by the
intricate interweaving of the ectodermal and mesodermal elements,
whereas in the medullated portion of the optic nerve, as well as in the
central supporting tissue strand and its branches (the retinal vessels), the
ectodermal and mesodermal elements are sharply separated and plainly
set off from each other.

According to Krueckmann (124), who repeatedly quotes Held, this sharp separa-
tion is due to the fact that the glial fibers abut upon the mesodermal tissue by conical
formed ends "end feet," and a border membrane similar to the limitans perivascularis,
described in the nerve-fiber layer of the retina (p. 78), arises, by the confluence of these
little extremities. As a result of this, according to Krueckmann, the glial portion of
the border tissue is separated from the mesodermal portion, as everywhere else, by
limitans of this nature.

A splitting up of the lamina vitrea at the margin of the optic nerve, once described
by Heine (91) does not exist. The membrane named ends abruptly at the optic nerve;
it is, however, so firmly fastened to the framework of the nerve that this follows every
pull upon the lamina vitrea. The tensely spanned glial fibers of the optic-nerve frame-
work in myopic eyes then appear to be derived from the edge of the lamina vitrea.

2. VARIETIES OF THE NON-MEDULLATED SECTION OF THE

OPTIC NERVE

The variations found in this region have been thoroughly studied by
Elschnig (52) and accurately analyzed in their relations to the ophthalmo-
scopic picture, as well. I will here only very briefly sketch the form of
the varieties found.

In as far as they relate to ophthalmoscopy, one can really look upon



9 6 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

all of them as varieties of the optic-nerve canal; the formation of the
other parts depends for the most part upon the structure of the optic-
nerve canal. The nerve-canal varies with respect to its width, its form,
and its direction.

Numerical estimates of the width of the nerve-canal must be made as if
of a certain level; the inner opening is adapted for this purpose, because
it is well characterized and can be sharply brought into view, on the one
hand, and conveys most of interest in the relation to the ophthalmo-
scopic image of the papilla, on the other hand. Its horizontal diameter
varies between i . 26 and i . 6 mm (a mean of 1.5) according to my
measurements. Its form varies only a little from that of a circle; at
times the vertical diameter is a trifle greater and the opening, therefore,
weakly elliptical.

The form of the nerve-canal varies from that of a cone to a cylinder.
The sides of this basic form can be straight or bayed out or in.

By the direction of the nerve-canal we understand its axis, character-
ized anatomically by the central vessels. This direction may be perpen-
dicular to the inner surface of the chorioidea, i.e., the nerve-canal may form
a straight cone or cylinder, or be inclined. In the latter case the center
of the inner opening of the canal seems displaced away from the outer
opening (the lamina cribrosa) ; the nerve-canal is oblique, and, as the case
may be, temporally oblique, nasally oblique, or inferiorly oblique, accord-
ing to the direction in which the inner opening is displaced. The central
vessels, of course, take part in this obliquity.

The size of the excavation depends principally upon the width of the
optic-nerve canal; in a narrow, cylindrical canal a vessel funnel only, not
a genuine excavation, is formed. The form of the excavation is deter-
mined by the direction of the optic-nerve canal. In a temporally oblique


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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 11 of 27)