Maximilian Salzmann.

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

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can be observed here and there in these emissaria. There are variations
in the course of the ordinary ciliary nerves, and/ according to Fritz (64) ,
are most often found under the musculus rectus superior, but do not by
any means occur in all eyes.



THE SCLERA 19

The affected ciliary nerve is especially thick, courses in a normal way
from behind throughout the perichorioidal space, then presses into the
emissarium to the outer surface of the sclera, bends sharply about there,
and goes back again through the emissarium into the interior of the eye
and into the ciliary body, where it divides up like the others. The caps
of the loop are made especially thick by a deposition of nuclear-rich
connective tissue and may even contain ganglion cells (Fritz, 64).

Good drawings of such nerve loops have been published by Naito (164), Groenouw
(78), and especially by Meller (150). Although Axenfeld and Naito make mention of
no blood-vessels, I hold, with Fritz, that anterior ciliary arteries are constantly or
very frequently associated with nerve loops.

When the nerve loops are not so beautifully cut (longitudinally) as in the reported
drawings, they often give beginners very considerable difficulty in recognizing them as
nerve loops One must think of this possibility when one finds the continuity of the
sclera sharply broken in the region of the posterior border of the ciliary muscle and
this place filled out by a clear, nuclear-rich tissue with fibers of another course. The
demonstration of medullary sheaths in this tissue makes the diagnosis.

The structures going through all of the emissaria (blood-vessels or
nerves) are for the most part bound to the wall of the emissarium by
loose tissue only. Outwardly, this tissue is a continuation of the loose
connective tissue which unites the Tenon's capsule to the bulb (PI. II,
3, 7Y); inwardly, it is a continuation of the suprachorioidea and, there-
fore, characterized by pigmentation. These tissues go over into one
another in the middle of the emissarium; often there is here a firmer
fixation to the wall, especially along the vortex veins. Yet many times
pigment cells (chromatophores) can be followed throughout the entire
length of the emissaria out onto the surface of the sclera.

Corresponding to the situation of this tissue, the spaces found on each
side of the sclera (perichorioidal and Tenon's spaces) extend into the
emissaria and, indeed, at the ends of the canals these spaces between the
walls of the canal and the vessel or nerve become very plain and visible
without any special help. With respect to the middle of the canal, views
vary. According to the older conception of Schwalbe (194), the peri-
chorioidal and Tenon's space are lymph spaces and the emissaria are to
be looked upon as communications between the two. It was, however,
admitted, even by the supporters of this view (Fuchs, 65), that it was
difficult or impossible to inject the emissaria. A later investigator
(Langer, 136) denies any communication between Tenon's and the peri-
chorioidal space, and conceives of the spaces found, in his opinion, at the
ends of the emissaria, as well as the perichorioidal and Tenon's spaces as
a whole, as articular spaces. The oblique direction of most of the emissaria,



20 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL



in any case, has the effect of preventing tearing and bending in the motion
of the eyeball or in the shifting of the chorioidea in the act of accom-
modation.

In any case, simple anatomic study shows every emissarium to be a locus minoris
resistentiae, and this comes fundamentally into consideration in the matter of the
extension of malignant tumors.

The union of the sclera with the optic nerve and its sheaths will be
more accurately discussed in the consideration of the optic-nerve entrance
(chap. viii).

The inner surface of the sclera presents a shallow furrow close to its
anterior limit (scleral furrow, or sulcus sclerae internus, Text Fig. 3,

Sw-iH). The back margin of this
furrow projects a little forward and
somewhat toward the interior of the
eye, and is known as the scleral spur
or better as the scleral roll (PI. Ill,
i, Sw). The ciliary body is inserted
into it (here one finds the anterior
insertion ring of the uvea). The
anterior border of the furrow slopes
very gradually over onto the inner
surface of the cornea; the breadth of
the furrow is about 0.75 mm. On
the floor of the furrow, close to




Ae



TEXT FIG. 3. Tunica fibrosa, anterior half.



its posterior margin, lies



Viewed from within. Magnification 2.

Ae equatorial cut surface, Ac white stripes cana j (p L JJT Sch \ the rest Q f

corresponding to the cQurse of the long posterior

ciliary arteries, Sw scleral roll (back margin of the depression IS filled OUt by the

the scleral furrow), iH inner corneal margin meshwork of the iris angle. With
(unbroken line), aH outer corneal margin ,, , . , , < , i

(dotted line). respect to the histology of this por-

tion, see chap. iii.

On the meridional section the corneal margin of the sclera in the
fresh cadaver-eye appears to the naked eye to be a pretty sharp line,
because the white of the sclera stands out well from the transparent
cornea. But even when magnified by the loupe the sharpness of this
border is lost in part and entirely so in the fully prepared and stained
section, cleared and mounted in balsam; the sclera takes on a somewhat
denser stain than does the cornea, but these differences merge imper-
ceptibly into one another. One must, therefore, use fresh material to
study the position and course of the corneoscleral margin.

In horizontal sections the border line is then seen to run from without
inward toward the ciliary body in a direction about parallel to the optic



THE SCLERA 21

axis. However, it then bends about a little along the inner surface to-
ward the axis (PL I; the corneoscleral border is indicated by a punctate
line).' The floor of the scleral furrow is therefore formed of scleral tissue,
as, indeed, preparations of the inner surface also show, although the
corneoscleral margin is soon collinear with its posterior border.

When, on the other hand, one studies a vertical section, a very oblique
course of the corneoscleral border is shown, so that its outer end lies i mm
or more nearer the optic axis than does the inner one. (Concerning the
effect which this has upon the form of the cornea, consult chap, ii.)

The corneoscleral border, therefore, meets the outer surface of the
sclera at an acute angle (less than 90). This is the main reason why
in life the white of the sclera does not appear sharply set off from the
transparent cornea; a very narrow transition zone, which is, neverthe-
less, always demonstrable with the loupe, is present. Moreover, the
circumstance that the conjunctiva gradually goes over onto the trans-
parent cornea contributes to this obscuration of the corneal margin.

Above and below, therefore, this extension of the sclera over onto the
cornea is much more outspoken than along the nasal and temporal margins ;
indistinctness of the border is correspondingly more marked. Owing to the
fact that the upper margin of the cornea is the one of preference for opera-
tive procedures, these relations become of great importance for operative
technique.

The sclera as a whole shows ja, very uniform structure, but one can
always make out certain modifications of the structure toward the outer
surface and in this by no means strict sense of the word one can
speak of various layers. These layers from without inward, are: (i) the
episcleral tissue; (2) the sclera proper; (3) the lamina fusca sderae.

i. The Episcleral Tissue

This is of a looser structure than the sclera proper, its bundles are
more delicate, more tortuous, and course in varying directions. Out-
wardly, it extends over into the loose tissue which fills out Tenon's space.
Toward the inner side its bundles are firmer, thicker, and the mattressing
is more close. In this way it graduates over into the tissue of the sclera
proper. It is especially characterized by relatively numerous vessels, and
so is easily differentiated from the vesselless tissue which fills out Tenon's
space, as well as from the tissue of the sclera, which is scanty in vessels.
Therefore one makes out the episcleral tissue best in much inflamed eyes,
where the individual tiny vessels are filled full of blood.

Behind the insertion line of the recti muscles, the episcleral tissue
forms only a very thin layer with a very loose net of vessels. In front



22 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

of this insertion line it is much more strongly developed and rich in
vessels, since the eye muscle-tendons also add a similar tissue a thicker
layer, with larger blood-vessels (per imy slum) . This tissue continues,
equally thick, forward into the sulcus sclerae externus, and here goes over
into the limbus corneae (PL I, Es). A large number of thick elastic
tissue fibers are mixed with this tissue.

The vessels in the back part of the episcleral tissue are branches of the
posterior ciliary artery, those in the fore part of the tissue, of the anterior.
These branches form a network in the usual way, that is to say, one
artery is accompanied by two veins ; the meshes of this net are very wide
in the back part and first become increasingly narrow in front of the recti
muscles and toward the border of the cornea. A thicker capillary and
venous net exists only in this anterior zone of the sclera; a marked filling
of this net produces the so-called ciliary injection.

2. The Sclera Proper

This is a dense fibrous tissue in which the bundles cross in the most
varied directions and often divide at sharp angles. At least one obtains
this impression in surface-sections. The cross-section (at right angles to
the surface) (PI. II, 5) shows, on the other hand, long band-like stripes
(apparently representing bundles which have been cut in their long
axis), and short oval or lance-formed fields (apparently cross-sections
of bundles), and, if one bases his studies of the course of the fibers in
the sclera solely upon such cross-sections, one is very easily led to the
view that there are two main directions of the scleral bundles, meridional
and equatorial.

Indeed, there are places in which equatorial bundles are solely or
mainly present. The immediate neighborhood of the optic nerve is such
a place. A second is the scleral roll and its immediate neighborhood.
However, in the remaining portions of the scleral the bundles possess a
most varying direction.

I think that one obtains the best conception of the structure of the
sclera from a consideration of the inner surface. Here, even though one
is not able to follow the course of the individual bundles, one can recog-
nize the prevailing direction from the silky reflex of the inner surface, or
bundles of a certain direction can be made out, to some extent.

The one can recognize an equatorial fibrillation (concentric to
the border of the cornea) just along the posterior edge of the scleral
furrow; farther back the fibers form more marked loops with their
convexities turned backward, and in this way they gradually go over
into meridional bundles. If one conceives of these loops as being given
off from all points of the corneoscleral border, one realizes that a crossing



THE SCLERA 23

with a prevailing circular course in front and a meridional one behind
comes about.

In the body of the sclera one cannot, of course, follow the bundles in
this way, but it is very easy to believe that a similar principle rules here,
i.e., all the bundles form loops, and whether they appear as meridional,
oblique, or equatorial bundles depends solely upon the way in which
the loop is sectioned. I do not trust myself to state anything more
definitely concerning the scleral fibrillation, despite the thorough studies
which Ischreyt (in) has made concerning them.

The bundles are quite delicate in the anterior section and have sharp
borders; in the posterior section one sees considerably larger bundles
with subdivisions which do not have exactly the same direction and are
not sharply separated from one another. In short, the structure of the
sclera, which is difficult enough to make out anywhere, becomes more and
more complicated as one proceeds backward.

On surface view the individual scleral fiber-bundle shows a fine parallel
striation. The dimension of the bundle in the direction of this striation
is spoken of as the length of the bundle, the dimension at right angles to
and striation and parallel to the surface of the sclera, as the breadth,
and /the dimension at right angles to the surface of the sclera as the
thickness of the bundle.

We must look upon the length of the bundle as an unconditionally
great one; in any case one cannot measure it microscopically, because
the bundles are always cut off. In thickness, the bundle measures 10 to
1 6 mu in the anterior segment (when pure cross-sections are studied);
the breadth varies according to the locality. In general, I hold this to
be 100 to 140 mu in the anterior segment, and the relation of the thick-
ness to the breadth to be something like i to 10 or 12; the cross-
section, therefore, has a somewhat elongated lance-form. The circular
bundles of the scleral roll are much narrower (30 to 50 mu), and the
relation of the thickness to the breadth is i to 3. The cross-section
is oval or has the form of a myrtle leaf. In the posterior section one is
so often in doubt where the border of the bundle lies that one cannot
accurately measure it.

The bundles, almost throughout, course parallel to the surface; even
when the intermixing of the bundles shows slight variations from this
direction, it is scarcely noticeable. Only along the emissaria are bundles
found which have a course that is oblique, or, indeed, at right angles
to the surface; it is the narrow sides of the individual canals which are
flanked by such bundles.

The funiculus scleroticae described by Hanover (87) (according to him, a remnant
of the fetal cleft) appears to be only such a bundle flanking a posterior ciliary artery.



2 4 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

The individual bundle consists of fine collagenous fibrillae, coursing
parallel to each other and to the length of the bundle; the fine striation
visible in surface-sections comes from this and gives the direction to the
entire bundle. The longitudinal section shows either a similar striation
or a more homogeneous appearance (PI. II, 5, /). Cross-sections do not
show the individual fibrillae but only groups of them; the entire cross-
section of the bundle is divided into smaller angular fields in this way,
giving it a peculiar appearance which I call the cross-section marking for
short (PL II, 5, q).

Aside from the collagenous fibers, ordinary elastic ones, i.e., fibers
staining with orcein, are visible. They occupy the periphery of the
bundle and only a few extend into the interior between the groups of
fibrillae. As a whole the elastic fibers course parallel to the direction of
the bundles. They are very much more delicate than in other tissues,
and, therefore, are to be made out in longitudinal section and in surface
preparations only by strong staining. They are more easily seen in
cross-section.

Between the bundles, especially in the angles formed by the crossing
of the bundles, lie the fixed cells of the scleral tissue; they are also visible
in the posterior portions, on account of the incomplete separation of the
bundles from one another.

After Held's stain these cells come out in surface-sections as membra-
nous structures, with an extremely thin and only very faintly stained body
going over into finer and broader extensions. The fine extensions seem
to course in the direction of the collagenous fibrillae, the broader ones
across these. The cells are united with one another by means of these
processes; they form a syncytium, yet I think that this is not as com-
pletely closed on all sides as in the cornea. The cell-nuclei are very
irregular in form owing to the position of the cell; for the most part,
however, they are longish, have a fine chromatin mesh and i to 3 very
small nucleoli. The long-strung-out spindle-form cells, which are not
infrequently seen in surface-sections, likewise appear to be provided with
very long nuclei lying mainly in the angles between the bundles; these
may in part be only side views of flat cells.

With certain stains the scleral bundles seem to me to possess border
membranes in the anterior segment; for example, beside the elastic
fibers the orcein stain shows an extremely fine, weak, brownish layer
surrounding the bundle. These membranes are connected with the
cells in any case; according to Pes (171), they connect the elastic fibers
with the cell-processes.

The blood-vessels of the sclera fall into two groups. The one makes



THE SCLERA 25

use of the sclera only for passage, and subdivides into capillaries in other
tissues; the vessels of the uvea belong in this group. The points of
entrance for these have already been described as the emissaria. There
are, however, a large number of smaller vessels in the sclera, which do not,
properly, supply this membrane itself, e.g., in the neighborhood of the
optic nerve. Here several branches of the short posterior ciliary arteries
form an anastomotic ring about the optic nerve (the circle of Zinn or
Haller's vascular circle, or the circulus arteriosus nervi optici (Leber),
which, as the name suggests, serves for the supply of the optic nerve,
especially the lamina cribrosa. Moreover, in the most anterior portion of
the sclera just behind and outside Schlemm's canal relatively numerous
vessels are present, for the anterior ciliary veins pass through here.

How insignificant the second group is, i.e., how few vessels the sclera
possesses in and of itself, comes out best in the region of the equator:
here and there a capillary lumen is visible, but in a cursory inspection this
tissue appears entirely devoid of vessels.

The nerves of the sclera are branches of the ciliary nerves ; they branch
off in the perichorioidal space and broaden out (especially in the inner
two-thirds of the sclera) and form a trabeculum similar to that in the
stroma of the cornea.

According to Smirnow (209) and Agababow (6), there are three
sorts of endings in the scleral tissue: free sensory endings, trophic ones
in connective-tissue cells, vasomotor ones in vessels. Finally, Agababow
describes a rich nerve-fiber net in the lamina fusca. In any case, most
of these findings are not marked in human eyes; only sensory endings
have been proven for the human (by Smirnow).

In general, the anterior part of the sclera is, furthermore, perforated
by numerous finer nerves which supply the cornea (Fritz, 64).

The histologic relations of the tendon insertions are best studied in
sections which go at right angles to the line of insertion, i.e., meridional
sections in the case of the recti muscles. The tendon itself (PI. I, Mr)
consists of a mass of exactly longitudinal coursing bundles of collagenous
fibrillae supported by thick elastic fibers. Aside from the difference in the
size of these fibers, it is made up of the same tissue as the sclera; the
texture only is different: in the tendon all the bundles are parallel, hence
the peculiar silky reflex; in the sclera itself the bundles are much crossed,
hence the dull-white appearance. The tendon in this way goes directly
over into the tissue of the sclera; the bundles of the tendon spread apart
and interweave with the cross and oblique bundles. The tendon-bundles
go over into the direction of the scleral bundles concerned and only the
cessation of the thick elastic fibers marks the border line.



2 6 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

3. Lamina fusca sclerae

Near the inner surface the connective-tissue bundles become smaller
and flatter, the elastic fibers more numerous and thicker; branched pig-
ment cells (chromatophores) appear here and there in the interspaces
of the tissue. Finally, along the inner surface itself the whole is closed
off by an endothelial layer. This insignificant modification of the scleral
tissue on its inner surface is the lamina fusca. 1

The name of the lamina fusca is given to it by the brown color which
the pigment cells lend to the inner surface of the sclera. It cannot be
isolated, for it goes continuously over into the tissue of the sclera itself.
What one brushes away is only the lamellae of the suprachorioidea clinging
to the lamina fusca.

In a histologic sense the lamina fusca forms the transition over into
suprachorioidea. The new tissue elements appearing in it are identical
with those of the suprachorioidea and will be more accurately described
under that heading.

The pigmentation of the lamina fusca is not uniform; anteriorly,
immediately behind the scleral roll (Text Fig. 3, Sw), it is as good as
colorless, and, moreover, farther back the brown color only appears in
flecks. Farther back it usually becomes more intense along the larger
ciliary nerves; the pigmentation is weaker along the larger nerves and
in particular corresponding to those bundles which, with their accompany-
ing nerves, form the long posterior ciliary arteries; for this reason two
broad white streaks appear especially prominent in the horizontal
meridian along the inner surface of the sclera (Text Fig. 3, Ac}.



CHAPTER II. THE CORNEA

The cornea has the form of a strongly curved meniscus, to which in
and of itself one must ascribe a weak refracting power, because it is thinner
in the center (0.8 mm) than it is at the edge (i mm, or slightly more).

The measurement of the thickness of the cornea is difficult, because the stroma
swells up easily in water; sections hardened in Mueller's fluid also usually show the
cornea in a swollen condition. The swelling is mainly inward; a marked tumifaction
develops at the edge of Descemet's membrane (see PL I), and the entire inner
surface becomes wavy. In this respect formalin-alcohol hardening conserves the
cornea better.



1 The meaning of this term is not the same to all authors; by it many understand that part of the
suprachorioidal lamellae which remains clinging to the sclera after detachment of the uveal tract, others
probably, the whole suprachorioidea as well. It is perhaps best to allow this expression to drop entirely
into disuse.



THE CORNEA 27

The radius of curvature of the anterior surface is 7.84 mm on the
average (computed from the 43.03 D. of average corneal refraction given
by Steiger, 211).

This radius of curvature only holds true, however, for the central
third of the cornea, the so-called optical zone, which is curved almost
exactly like a sphere ; the peripheral parts are notably flattened and more
so upon the nasal than upon the temporal side. Aside from this there
usually exists a certain degree of meridional asymmetry, i.e., the vertical
meridian is somewhat more strongly curved than the horizontal one.

We are very little informed concerned the curvature of the posterior
surface. Merkel and Kallius (151) hold it to have the curvature of a
sphere and the central portion to be concentric with the anterior surface.
According to this, its radius of curvature would be something like 7 mm.
Tscherning (227) has measured it ophthalmometrically in the living and
has found very great differences in the curvature of the anterior and
posterior surfaces. He holds the radius of curvature for the posterior
surface to be 6 . 2 2 mm.

Viewed from the front, the cornea appears weakly elliptical with the
longer axis horizontal. According to Priestley Smith (174), the hori-
zontal diameter of the cornea varies between the extremes of 10.5 and
13.5 mm, but usually is between n and 12 mm. The average for eyes
of all ages is n .6 mm: for males n .65, for females n . 54. The vertical
diameter is some i mm less.

Looked at from behind (within), the cornea appears completely
circular with a diameter close to the long diameter of the anterior surface.
This is due to the form of the corneoscleral border discussed above.
These relations come out very beautifully if one looks at the anterior
half of the tunica fibrosa after the removal of the other membranes and the
contents of the bulb (Text Fig. 3) ; one then sees the oval contour of the
anterior surface (aH) outlined in the circular contour of the posterior
surface (i H) .

A plane going through the outer visible border of the cornea is called
the base of the cornea, and the distance between it and the center of



Online LibraryMaximilian SalzmannThe anatomy and histology of the human eyeball in the normal state, its development and senescence ; → online text (page 3 of 27)