Maximilian Salzmann.

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

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The optic nerve (0), with its sheaths, forms a triangular rounded
cord some 5 mm in diameter. It is inserted to the nasal side, so that
the center of its insertion surface lies some 3 mm mesial to and i mm
below the posterior pole.

On both sides of the optic nerve (nasal and temporal to it) the arteriae
ciliares posteriores longae are visible as bluish stripes. They hold pretty
closely to the line of the horizontal meridian and their line of union, there-
fore, goes slightly above the axis of the optic nerve. The point of
entrance of the arteria ciliaris posterior longa medialis (Am) lies some
3.6 mm from the optic nerve, somewhat nearer the optic nerve than does
that of the a. c. p. I. lateralis (At), 3 .9 mm away.

The vortex veins (venae vorticosae) are usually four in number and
lie grouped in two pairs (an upper and lower). The points of exit of the
upper pair lie on the respective sides of the vertical meridian, displaced
somewhat nasally, and 7 mm (the superior nasal vein, F x ) to 8 mm (the
superior temporal vein, F 2 ) behind the equator. The latter lies very
close to the insertion of the m. obliquus superior. The lower pair (F 3 , F 4 )
show a similar relationship to the vertical meridian, but lie, however,
somewhat farther forward (5.5 to 6 mm behind the equator).

The decision as to whether an eyeball is the right or left one is made by the aid of
the back segment, because in the anterior segment one does not notice difference
enough between things in the upper and lower portions.

First one determines the horizontal meridian: anteriorly, by the long axis of the
cornea, posteriorly, by the position of the optic nerve and the long posterior ciliary
arteries. Then one searches for the lines of insertion of the obliques. The one
reaching closer to the optic nerve, lying snugly on a long posterior ciliary artery, pro-
vided with a short tendon (or none at all), is the obliquus inferior and belongs on the
temporal side. The other, lying farther from the optic nerve and the ciliary artery,
provided with a longer tendon of insertion, belongs above. The bulb is, therefore, to
be so oriented that the piece lying between the two insertion lines corresponds to the
upper temporal quadrant; thereby the correct position, and, moreover, the side to which
the eyeball belongs, is found.



io ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

III. General View of the Structure of the Eyeball

(Division into Coats and Zones)

For the study of the grosser anatomy it is recommended to divide
the eyeball, fresh or preserved in a very weak solution of formalin, into
two.

The division may be made in a meridional or equatorial direction.
Since the lens is easily dislocated in meridional cutting, it is well to
freeze the eye before cutting it. The equatorial section can be made
without any special preparation; one carries the section as far as it will
go with a thin sharp knife and completes it with scissors.

It is at once seen that the eyeball consists of a firm wall and a softer,
transparent contents.

We will first consider the inner surface of the wall of the eyeball in
an equatorially halved eye.

The posterior segment of a wholly fresh eye presents about the same
picture as with the ophthalmoscope except that the eyeground does not
appear red but brown. One recognizes the entrance of the optic nerve
as a circular or slightly oval disc of white color (PL VII, i). The
retinal vessels are empty or the veins filled with broken columns of blood ;
therefore, one sees only the larger branches. The region of the fovea
centralis comes out temporal to the optic nerve entrance by its darker
color. Toward the equator the markings of the vortices are seen (PL

HI, 5)-

The retina is wholly transparent in a fresh state, but a cadaverous
clouding soon occurs. This usually appears first in the region of the fovea
and at the same time this region usually becomes detached and folded.
Simultaneously the yellow fleck (macula lutea) comes forth in this region.
In older cadaverous eyes the retina is completely opaque and usually
detached, and the eyeground, therefore, shows a gray instead of a brown
color.

In the anterior segment (PL II, i) one notes first in the cut surface
the jagged border of the retina (or a serrata, Os}', it is much better seen
in the cadaverous than in the fresh eye, because of the clouding of the
retina. The zone lying in front of the ora serrata is considerably darker
than that behind it, has no irregularities visible to the naked eye, and is
known as the orbiculus ciliaris (Or). This zone is 4 mm wide on the
average. Toward the lens it is succeeded by a crown of whitish, plainly
prominent, radiating stripes: these stripes are the ciliary processes and
the whole zone (1.5 to 2 mm wide) is called the corona ciliaris (Cor}.
Upon this follows the uniformly dark-brown back surface of the iris.
The lens (L} must be first removed if one wishes to bring this into plain



GENERAL VIEW OF THE STRUCTURE OF THE EYEBALL n

view. The lens is a circular, disc-form structure, some 9 mm in diameter.
Between it and the corona ciliaris is a narrow interval (scarcely o . 5 mm
broad), the circumlental space (C7), bridged over by the fiber-bundles
of the zonula ciliaris'.

In this way one can make out three main zones in the wall of the
eyeball.

The posterior main zone extends from the optic-nerve entrance to
the ora serrata retinae; the middle zone comprises the orbiculus and
corona ciliaris, the anterior zone includes the iris on the inner, the cornea
on the outer, surface of the eyeball.

The wall of the eye also shows a number of main layers (tunics);
these are especially easy to make out in the posterior main zone and
can be made into anatomic preparations.

Outside, there is a thick, firm coat, whose posterior white segment
we know by the name sclera, and whose anterior transparent portion we
have already come to know as the cornea; for the two together, i.e.,
the whole coat, the old name tunica fibrosa (Text Fig. i, S, C) is best
adapted. Then follows a more delicate, brown, vessel-rich coat, the
tunica vasculosa s. uvea (Text Fig. i, Ch, Cc, /). On the inner surface
of this coat there lies an extremely thin coat consisting microscopically
of a single-celled layer, which, likewise, is strongly pigmented, the stratum
pigmenti (Text Fig. i, P}. Finally, inside this there lies the retina
(Text Fig. i, R). This is a part of the tunica interna; it is transparent
during life except for its blood-vessels, and after death is lightly clouded
and mostly detached.

The walls of the eyeball are, therefore, made up of four principal
layers; from without inward there are: (i) tunica fibrosa, (2) tunica
vasculosa s. uvea, (3) stratum pigmenti, (4) tunica interna.

The above-described main zones form from unequal development
(differentiation) of the tunica interna and the stratum pigmenti. In the
tunica vasculosa the zone borders are not sharp and in the tunica fibrosa
only partly expressed.

For the purpose of the more detailed description we will take up the
individual parts of the wall of the eyeball from without inward and from
behind forward.

a) The Wall of the Eyeball
i . The tunica fibrosa

This tough fibrous capsule of the eyeball is closed on all sides; it
lends form and size to the eyeball, its firmness protects the delicate inner
portions from insult. At the optic-nerve entrance its outer layers go over
into the optic-nerve sheaths (Text Fig. i, D}. Its inner layers show



12 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

a round hole (foramen opticum sclerae). This is incompletely closed by a
sieve-like perforated plate (lamina cribrosa, Text Fig. i, Lc). Other-
wise the continuity of the tunica fibrosa is broken only by fine canals
(emissaria), which contain vessels and nerves going to the tunica vasculosa.
One can make out' only two zones in the tunica fibrosa. The one
corresponds to the middle and posterior zones of the bulb; here the
tunic is white and opaque (sclera, Text Fig. i, S). The other zone
corresponds to the anterior zone of the bulb; here the coat is trans-
parent and shining (cornea, Text Fig. i, C). This part is, moreover, an
integral part of the optical system.

2. The tunica vasculosa s. uvea

This is the main organ of nutrition of the eyeball and the bearer
of- the intraocular musculature. It consists mainly of blood-vessels;
the connective tissue system is poorly developed and colored brown by
the richly branched pigment cells (chromatophores).

The tunica vasculosa lies snugly against the sclera, yet it is only grown
to it at two places. These places have the forms of rings and may be
designated from their position as the posterior and anterior insertion rings.

The posterior insertion ring lies about the optic nerve; the two tunics
are here directly bound to one another. The anterior insertion ring
lies on the border of the middle and anterior zones of the bulb (the
corneoscleral border). The union of the two coats is partly direct, partly
by means of a peculiar meshwork.

Between these two rings there extends a capillary space (the peri-
chorioidal space), crossed only by a few blood-vessels and nerves, and
bridged by extremely delicate tissue lamellae (suprachorioidea) . One
can, therefore, demonstrate the entire expense of the tunica vasculosa
with relative ease.

The posterior insertion ring is the most difficult to detach; this region must eventu-
ally be cut away. The anterior insertion ring can be very easily detached by means of
blunt instruments; one then only needs to cut the efferent and afferent blood-vessels and
the ciliary nerves and the tunica vasculosa is entirely separated from the tunica fibrosa.

One also recognizes only two zones on the outer surface of the tunica
vasculosa, for here the border between the posterior and the middle zone
is not visible; both appear uniformly brown, and, when the preparation
lies in water, provided with fine floating brown shreds (suprachorioidal
lamellae). Some meridional white bands stand out against the brown
background : these are the ciliary nerves coursing forward along the outer
surface of the tunica vasculosa.

A broader strand is present in the horizontal meridian, nasal as well
as temporal; more accurately studied, this is seen to be made up of three



GENERAL VIEW OF THE STRUCTURE OF THE EYEBALL 13

stripes: the central one is an arteria ciliaris posterior longa; the two by
its side are nerves.

Forward, all these meridional bands sink into the ciliary muscle;
the muscle itself is inserted directly into the anterior insertion ring as a
whitish girdle about 3 mm broad. For the description of the anterior
zone of the tunica vasculosa, see the iris (chap. x).

The tunica vasculosa shows two round holes: one behind for the
entrance of the optic nerve (foramen opticum chorioideae) , corresponding
in its position exactly to foramen opticum sclerae, and one anterior for
the entrance of light (pupil).

3. The stratum pigmenti and

4. The tunica interna

These are present in exactly the same expanse as the tunica vasculosa
and, in general, intimately united to it. In their differentiation these
three coats are so dependent upon one another that it is best to continue
their description as zones rather than as coats.

a) POSTERIOR ZONE

The tunica vasculosa here forms the chorioidea (Text Fig. i, Ch).
This serves mainly as the nutritional source for the outer layers of the
retina and for this reason possesses a distribution of blood-vessels of
its own.

The stratum pigmenti is made up of a smooth, regular, easily detach-
able cell-layer (pigment epithelium of the chroioidea, Text Fig. i, P}.

The stratum pigmenti belongs developmentally to the tunica interna (cf. chap, xvi),
but anatomically it is classified with the tunica vasculosa; especially in cadaver-eye does
the pigment epithelium of the chorioidea appear as a covering of the latter, whereas
only the corresponding portion of the tunica interna (the retina) is detached. But the
addition of "the chorioidea" to this term should never be used to mean more than the
zone in this case.

The tunica interna is highly differentiated in this zone and developed
into a regular stratified coat of nerve tissue (retina, Text Fig. i, R),
which functionates as the organ for the reception of visual impressions.

The term retina is used by authors in many different senses. We use it in the
narrower sense, i.e., to designate that membrane which receives the visual impressions.
By many authors the retina is synonymous, however, with the tissues arising out of the
optic vesicle, and the term, therefore, is used in the wider sense of the word. In this
sense the stratum pigmenti is also included and the posterior zone is then designated as
the pars optica retinae. I will not dispute the justification for such a conception of the
term retina, but I think that it is more to the purpose to use the word retina in the
narrower sense with the beginner, for it is almost always used in this narrower sense
in ophthalmology and pathology.



i 4 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

/3) MIDDLE ZONE

Since one cannot separate the stratum pigmenti and tunica interna
from the tunica vasculosa macroscopically in this zone, one designates
the totality of all three coats, in so far as they belong to the middle zone,
as the ciliary body (corpus ciliare, Text Fig. i, Cc).

According to the division of its inner surface, the ciliary body is
divided into two adjacent zones, about which we have learned above
(p. 10); the posterior one is the orbiculus ciliaris, or flat portion (pars
plana, Or], the other the corona ciliaris, or folded portion (pars plicata,
Cor).

The ciliary body is the organ for the nourishment of the vitreous and
lens, secretes the aqueous, and performs the act of accommodation.

The main mass of the ciliary body belongs to the tunica vasculosa
and is designated as the pars uvealis corporis ciliaris. Like the chorioidea,
it is rich in blood-vessels, yet these show another distribution. Above
all, however, it is characterized by the ciliary muscle (Me).

The stratum pigmenti adheres firmly to the inner surface of the pars
uvealis c.c., and, as in the chorioidea, is a single layer of pigmented cells
although of slightly different characteristics than there. It should be
designated as the pigment epithelium of the ciliary body.

The tunica interna is reduced to a simple layer of unpigmented cells
(non-pigmented ciliary epithelium, or ciliary epithelium for short).

Those who use the expression retina in the wider sense call the pigment epithelium ,
together with the non-pigmented ciliary epithelium, the pars ciliaris retinae.

T) ANTERIOR ZONE

The totality of all three coats (tunica vasculosa, stratum pigmenti,
and tunica interna), so far as they belong to the anterior zone, bear the
name iris (Text Fig. i, /). Its most important function is that of a
diaphragm, and for this reason it is suspended entirely free in the interior
of the eye, i.e., lies separated from the tunica fibrosa by a wide space,
the anterior chamber (Vk), and is provided with a round opening of
changeable width (pupil). The tunica vasculosa ends at this opening,
and the stratum pigmenti goes over into the tunica interna (transition point
or border of the optic cup; cf. chap. xvi).

The tunica vasculosa portion of the iris is called the pars uvealis
iridis; the stratum pigmenti is differentiated into an epithelial muscle
(dilatator pupillae) ; the tunica interna, as in the ciliary body, has a pure
epithelial character; it takes on here, however, the function of making the
interior of the eye dark and appears, therefore, as the pigment epithelium
of the iris.



GENERAL VIEW OF THE STRUCTURE OF THE EYEBALL



When the term retina is used in the wide sense, the dilatator pupillae and pigment
epithelium of the iris form the pars retinalis iridis, s. pars iridica retinae.





ANTERIOR ZONE


MIDDLE ZONE POSTERIOR ZONE




Cornea


Sclera


i. Tunica fibrosa








Anterior chamber


Perichorioidal space (suprachorioidea)






Pars uvealis




Pars uvealis


2. Tunica vasculosa




iridis




Chorioidea






Sphincter pup.




corporis ciliaris






Dilatator




%

a










3. Stratum pigmenti


2
'C


pupillae
(+ Sphincter
pup.)


'oT

rt

s


orpus cili


Pigment
Epithelium of the
Ciliary Body


V

a

_c

g


Pigment
Epithelium of the
Chorioidea


1
fi








cfl


O




.'




5








'-3






J5












Tl






r_.




a.






Pigment


g




Unpigmented


o

If:


Retina


o

en


4. Tunica interna




Epithelium of
the Iris


rt

fe




Ciliary
Epithelium


rt
^


(sensu strictiori)


3

OH,



In the above chart I have sought to give a survey of the division of
the eyeball into coats and zones with the idea of making the connection
of the various parts, as well as the somewhat confused nomenclature, more
easily understood.

Only the following need be said in advance about the developmental
history of the eyeball : coats 3 and 4 come from the primary optic vesicle
and the tissues belonging to these coats are, therefore, of ectodermal
origin; layers i and 2 arise from mesoderm; only the blood-vessels
(mesoderm) of the retina and the epithelium of the cornea (ectoderm)
constitute exceptions to the above rule ; the same is true of the deposition
of ectodermal elements (sphincter pupillae) in the iris mesoderm.

b) The Contents of the Eyeball

The vitreous (corpus vitreum, Text Fig. i, G) forms the main mass of
and completely fills out the interior space encompassed by the posterior
zone, and that surrounded by the middle zone, partially. Behind and
to the sides it lies against the inner surface of the retina; in front it pre-
sents a concavity (Jossa patellaris) in which the lens (L) lies. This is
mainly held in its place behind the pupil by the zonula ciliaris (Z), a fiber
system given off from the inner surface of the ciliary body.

The space yet remaining is 'filled out by aqueous, and is divided by the
iris into a posterior (Hk) and an anterior chamber (Vk).



1 6 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

IV. Asymmetry of the Eyeball

When looked at from the outside the asymmetry of the eyeball is at
once apparent in the unequal remoteness of the lines of insertion of the
eye muscles, in the oblique position of the anatomatic equator, and
especially in the entrance of the optic nerve to one side.

The interior of the eye also shows this asymmetry in many places.
In general, it may be said that all intervals are smaller on the nasal side,
larger on the temporal side.

Pupil and lens are slightly displaced to the nasal side; in the pupil
this can be seen even in the living eye; the shifting of the lens is recog-
nized by the fact that the circumlental space (PL II, i, C7) is narrower
on the nasal than on the temporal side.

The form of the ciliary muscle (Text Fig. i) approaches more nearly
the type of that in hyperopia on the nasal side, in myopia on the
temporal side (cf. chap, ix), and, corresponding to this, there are slight
differences in the formation of the iris angle and the position of the
canals of Schlemm in relation to the root of the iris. Most striking,
however, is the difference in the length of the ciliary muscle (nasal
about 5 mm, temporal 6 mm). As a result the retina reaches farther
forward on the nasal than on the temporal side (PL II, i).

The emissaria as a rule are longer and lie farther back on the temporal
than on the nasal side.

Moreover, the upper and lower halves are not symmetrical, yet the
differences are for the most part slight and, therefore, not conspicuous.



B. SPECIAL ANATOMY AND HISTOLOGY OF THE

EYEBALL

CHAPTER I. SCLERA (SCLEROTICA)

The outer surface of the sclera has a dull-white color. A very
delicate fine connective tissue clings to it and forms a union with Tenon's
capsule that layer immediately surrounding the eyeball in the orbit.
Only rarely does the outer surface of the sclera contain any pigment;
it rather comes to view in the form of slate-gray flecks about the canals
of the anterior ciliary vessels (congenital melanosis of the sclera).

The inner surface of the sclera appears much smoother and in front it
has an almost silky luster; posteriorly, it takes on an increasingly brown
color and a duller appearance, on account of the greater number of the
suprachorioidal lamellae which cling to it and the appearance of pigment
cells in the sclera itself. The cut surface shows a tendonous reflex in
places.

The thickness of the sclera varies greatly with the individual; in
youth and in the female sex it is, in general, thinner. The maximal
scleral thickness (Text Fig. i) is at the posterior pole (i mm and over;
according to Stilling [213], it varies between 0.5 and 1.6 mm). From
here the thickness gradually decreases to o . 4-0 . 6 mm toward the equa-
tor. The minimal thickness is immediately beneath the recti mus-
cles close to the lines of insertion of their tendons (0.3 mm). The ten-
dons show a varying thickness at the insertion lines but in many cases
they are as thick as the sclera itself (0.3 mm) so that immediately in
front of the insertion a thickness of 0.6 mm is attained and held to the
margin of the cornea.

At its margin the tendon texture is usually quite sharply set off from
that of the sclera. One observes the following varieties of position and
direction in this border line: (i) The border runs at right angles to the
surface of the sclera just at the place where the interval between the
sclera and tendon disappears; the sclera forms a step. (2) The border
runs obliquely from in front and without, backward and inward, i.e.,
the innermost layers of the tendon (those lying nearest to the sclera)
lie farther backward; the outer ones go farther forward over into the
scleral tissue. (3) The 'tendon, as a whole, courses obliquely inward
into the sclera and disappears farther forward in the scleral tissue: the
tendon takes root, so to say, in the sclera.

17



1 8 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

All the vessels and nerves supplying the uvea pass through the sclera;
they are only loosely imbedded in special canals. Following an older con-
ception of the lymph circulation, one designates these canals as emissaria.

The emissaria of the short posterior ciliary arteries course in very
different ways, sometimes straight, sometimes obliquely, sometimes bent
at an angle; the inner ends often lie nearer the optic-nerve entrance than
do the outer ones.

The emissaria of the long posterior ciliary arteries, of the ciliary nerves
and of the vortex veins course very obliquely from without and behind,
inward and forward. The outer (posterior) portion of such an emis-
sarium courses wholly flat, almost parallel to the outer surface of the
sclera (PI. Ill, 3) and the outer opening of the emissarium is bordered
in front by a sharp semilunar margin and passes over onto the outer sur-
face of the sclera behind in a furrow. The inner end of the emissaria
is more steep, has the sharp semilunar border behind, and in front passes
over into an albeit short and only slightly prominent furrow. In longi-
tudinal section the emissarium, therefore, forms a flat bow with its
concavity inward.

The long posterior ciliary arteries are accompanied by large nerves;
a cross-section of a given emissarium (PL II, 2) shows, therefore, two
rounded canals, close together and separated only by a thin vaginal wall
of scleral tissue. The artery (A) lies in the one canal, the nerve (N) in
the other; both are fastened to the wall of the emissarium by loose con-
nective tissue. According to Fuchs (65), the length of this emissarium
is 3 to 7 mm. Its direction is strictly meridional and horizontal. The
rest of the ciliary nerves show a wholly similar relationship.

The emissaria of the vortex veins (PI. II, 3) vary more from the
meridional direction; their inner ends are farther removed from the verti-
cal meridian than their outer. (For the position of the latter see p. 9.)
The length of the emissaria is usually 3 mm; the upper temporal, only,
may attain a length of 4.6 mm (Fuchs, 65). The cross-section of the
emissarium shows a broad and low lumen (PL II, 4); the thin vein
wall lies with its long side against the sclera and no space intervenes; on
the short side, only, is some loose connective tissue interposed.

The emissaria of the anterior ciliary arteries course much less
obliquely, are often almost perpendicular to the surface, and often
extremely wide. The intrascleral nerve loops described by Axenfeld (10)



Online LibraryMaximilian SalzmannThe anatomy and histology of the human eyeball in the normal state, its development and senescence ; → online text (page 2 of 27)