Maximilian Salzmann.

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

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mann, 124). The glia fibers develop out of the protoplasmic processes,
the glia cells out of the nucleated portion of the syncytium.

The primordium of the vena centralis is first visible at a length of 54 mm
(Seefelder, 201). The connective-tissue septa grow in with the blood-
vessels out of the neighboring mesoderm; the latter also furnishes the
optic-nerve sheaths.

The outer leaf of the optic cup (a) develops into the stratum pigmenti.

Pigmentation begins even very early (at a gr. 1. of 7 mm, Elze, 56),
and, indeed, first in the equatorial regions (Lauber, 137). Yet in this
respect there are great individual differences. At 9.75 mm gr. 1. (PI.
IX, 8) the whole outer leaf is, indeed, sparsely but uniformly pigmented,
yet still in several layers.

At 19 mm length (Dedekind, 37) the posterior part of the optic vesicle
has already become one layer, which still thickens, however, toward the
border of the cup. The single-layer portions consist chiefly of flat endo-
thelial-like cells; they attain their cylindrical form first after birth
(Seefelder, 208).

The inner leaf of the optic cup becomes differentiated into the retina
but also contributes to the formation of the primitive vitreous.

Even at the beginning of the invagination of the primary optic vesicle
(PL VIII, 7), the distal portion of its wall (i) is characterized by a greater
thickness, and a nuclear-free layer is found along the basal surface (that
turned toward the lumen) the marginal film of His.

This marginal film develops further into an increasingly plain syncyt-
ium, the primordium of the supporting tissue. At a length of 11.3 cm
a layer differentiates itself off from the heretofore undifferentiated thickly
disposed nuclei inward. The nuclei in this are not so thickly disposed the
primordium of the ganglion-cell layer; it soon becomes much thickened.

The formation of the ganglion cells begins, as does the differentiation
of the retina, in general, in the tempero-inferior quadrant, and this area
later becomes the region of the fovea. Differentiation progresses periph-
eralward from here.

The first nerve-fibers appear in the border film at a length of 13 to
14 mm; at 65 mm the dendrites of the ganglion cells and their diplosomes
and at the same time the inner nuclear layer differentiate themselves.



196 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

From this time on the thickness of the ganglion-cell layer again
decreases as a result of the increase of the retina in surface expanse; its
greater thickness is maintained only in the region of the differentiation
center (primordium of the area centralis).

A cell-layer first appears on the side of the retina which was originally
free (now lying on the pigment epithelium) at the end of the third month ;
this is the primordium of the outer nuclear layer. At the fifth month the
rods appear as small caps projecting over the membrana limitans externa;
the diplosomes lie in them, and at each cell a fine thread (the outer thread)
goes into the pigment epithelium from the diplosome.

The amacrin cells separate away from the nuclei of the Mueller's fibers
in the fifth month in the primordium of the area centralis, and there arises
an intervening layer (Chievitz ; transitory fiber layer, 32). This layer later
again disappears, but traces of it are often found in extrauterine life.

At the end of the sixth month that reduction of the cerebral layer
which leads to the formation of the fovea centralis begins in the middle
of the area centralis. The distance between the fovea and the papilla
is already as great as in the adult eye.

Although up to this time the development in the area centralis pre-
cedes that of the rest of the retina, it now falls behind, at least in respect
to the development of the neuroepithelium, and even at birth the fovea
is not yet completely developed (cf. chap. xvii).

The retinal vessels sprout out from the portion of the arteria hyaloidea
(later the art. centralis retinae] inclosed in the optic nerve at a length of
10 cm (Versari, 231), or at the beginning of the fourth month (Seefelder,
201), and at once press into the nerve-fiber layer of the retina, in which
they gradually broaden out farther. The vessel system is only com-
pletely developed in the eighth month. A membrana vasculosa retinae,
therefore, does not exist at any time in man.

We have left the lens primordium as a hollow epithelial vesicle with
somewhat elongated cells in the posterior wall (PI. IX, 8). By further
elongation of their axes these posterior cells grow out to lens fibers and so
fill out the lumen of the lens vesicle (at a length of 13 mm according to
Brueckner, 28). From there on the new formation of fibers is limited to
the equatorial portions of the vesicle. The youngest fibers at the periph-
ery are strongly concave toward the equator; this bowing is gradually
lost toward the center, where a purer sagittal direction of the fibers is
present (central fibers of Rabl, 175). Since all the fibers are nucleated,
the nuclear bow extends through the entire lens (PI. IX, 9).

At a length of 51 mm (collection of the I. Anatomic Institute of
Vienna) there is already manifest a tendency to concentric stratification,



THE EMBRYONAL AND FETAL DEVELOPMENT 197

i.e., the fibers undergoing development are, as heretofore, strongly con-
cave toward the equator; the middle fibers, however, show a convexity
toward the equator (transition fibers of Rabl), and only the central fibers
are strictly sagittal.

At 60 mm length (collection of the I. Anatomic Institute of Vienna) a
plain cortex of concentric layers of fibers with three-rayed lens stars is
already present. The form of the lens is still almost spherical. The
further growth then continues as in the adult, with only this difference,
that the number of undeveloped lens fibers is much greater and their
concavity more outspoken.

At the beginning (at the time of the invagination) the cells of the lens
primordium also contribute to the make-up of the primitive vitreous, and,
indeed, by the formation of conical basal processes (lens cone of von
Lenhossek, 140) ; this subsequently breaks up into fine fibers which unite
with similar fine filaments proceeding from the inner leaf of the optic cup.
The prevailing direction of the fibers is radial (embryonal supporting
tissue of von Szily).

The lens vesicle is soon closed off from the neighboring structures by
a cuticula (later the lens capsule), and thereby sacrifices any further
influence on the development of the vitreous.

The mesodermal process which presses in through the optic cleft
grows about the lens on all sides and forms the capsula perilenticularis
(Cirincione, 33). Vessels appear in this in the seventh week, and at the
same time the undifferentiated mesoderm disappears; in the ninth
week the capsula perilenticularis goes over into the tunica vasculosa
lentis (PI. IX, 9, TV), which consists only of vessels. Numerous branches
of the arteria hyaloidea sprout out to the sides and toward the front, and
the latter form the tunica vasculosa lentis, a net of vessels which wholly
surrounds the posterior half of the lens and unites with the primordium
of the outer vessel system along the entire border of the cup.

Up to a SS of 20 mm (second month) all the vessels have the same
caliber, according to Calderaro (29). A trunk (the arteria hyaloidea
proper) then differentiates itself, and courses from behind (as the branch
of the art. ophthalmica) through the pedicle of the optic vesicle and the
axis of the primitive vitreous. It gives off lateral branches in the vitreous
(vitreous vessels) and divides into several branches in the neighborhood
of the lens; these carry the blood to the tunica vasculosa lentis. The
drainage in front, at the border of the cup, proceeds into the outer vessel
system, and this place is called the isthmus (PL IX, 9, /), since the border
of the optic cup almost always lies close to the lens.

At 31 mm length (Seef elder, 203) a conical or rod-form structure



i 9 8 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

develops zaffo prepapillare (Calderaro, 29), glial mantle (Seefelder, 203)
at the place where the arteria hyaloidea passes out of the optic nerve
into the vitreous. According to Seefelder, it consists of glia cells, arranged
in two layers, and when fully developed may attain a length of 2 mm
(Calderaro). This glial mantle fills out the excavation arising at the
entrance of the optic nerve occasioned by the spreading apart of the
optic-nerve fibers (Seefelder, 200). Fibrillae are set into the surface of
the glial mantle; these course straight as a string to the lens and there
spread apart like a crater (central vitreous body of Retzius).

The disappearance of the inner vessel system begins at the fifth month
with the vessels of the vitreous proper; the circulation in the arteria
hyaloidea ceases in the sixth month, according to Calderaro ; in the seventh
month it is transformed into a filament, and this disappears also between
the eighth and ninth month. According to Seefelder, however, the arteria
hyaloidea carries blood much longer. With the disappearance of the
arteria hyaloidea and its glial mantle the excavation again opens.

That which later becomes the central canal appears to 'be identical
with the cavity of the central vitreous body of Retzius, if I have under-
stood Seefelder (203) correctly. However, according to the view of this
author, it also becomes filled out with vitreous tissue and Seefelder, like
Wolf rum, therefore, denies the existence of a central canal (p. 149).

The tunica vasculosa lentis undergoes regression at the same time as the
arteria hyaloidea. Only that portion of this artery lying in the optic
nerve persists, and from this time on supplies only the retinal vessel
system; it becomes the arteria centralis retinae.

After the development of the lens capsule the predominantly radial
fibers of the primitive vitreous are united rather with the retinal primor-
dium, and, indeed, with the marginal film. The formation of further
radial fibers proceeds from this (Wolf rum, 239) ; these are then united by
cross-anastomoses. Protoplasmic unions with the vitreous vessels, which,
in general, are pure endothelial tubes, do, however, also come about.
The cross-anastomoses at length acquire predominance in the fundus and
so go over gradually into the permanent structure.

Finally, the retina also becomes closed off from the vitreous by a
cuticulum (the subsequent membr. limitans internet) . However, the forma-
tion of radial fibers proceeds farther in the region of the pars coeca (see
p. 199), which has arisen meanwhile, so that the definitive vitreous is
mainly connected with the ciliary epithelium and appears to proceed out
of this (vitreous basis).

The formation of these zonula fibers from the cells of the pars coeca
goes on in a way similar to the development of the vitreous fibers. They



THE EMBRYONAL AND FETAL DEVELOPMENT 199

are only to be distinguished from the fibers of the vitreous by the fact
that they are larger and form no cross-anastomoses (Wolfrum, 239).

The views of the embryologists have undergone a significant change in the last
decades with respect to the genesis of the vitreous. Previously the vitreous was held
to be a mesodermal structure. Now, with a few exceptions, the trend of the views is
that the mesoderm forms only the vitreous vessels, that, however, the framework of
the vitreous is of ectodermal origin. In respect to the finer details the views are still
very much at variance; I, myself, have mainly followed the more intermediate views
of Koelliker, von Szily, and Wolfrum.

Even the fetal vitreous is very poor in cells (aside from the vitreous
vessels) ; these cells are explained in various ways, but, as it appears, the
later works are agreed that these cells form no essential part of the fetal
vitreous.

In the mesoderm surrounding the optic cup, a layer of capillary vessels
very early becomes differentiated off one lying immediately on the outer
leaf of the optic cup (the primitive choriocapillaris) . At a SS of 19 mm,
according to Dedekind (37), there is a thicker layer of mesoderm (the
primordium of the sclera) (PL IX, 9, 5) outside this capillary layer, and
in the posterior part of the chorioidal primordium a second layer of larger
vessels is already demonstrable. Likewise the vortex veins are laid out,
as well as the long posterior ciliary arteries, the temporal of which appears
as a direct extension of the arteria ophthalmica.

The primitive cornea (PL IX, ^ 8, H} differentiates itself in a similar
way, i.e., forms out of that layer of mesoderm which has interposed itself
between the ectoderm and the lens vesicle. This layer, which in the
seventh week is still completely undifferentiated, divides into an outer
(anterior), i.e., lying immediately under the ectoderm avascular, and into
an inner (posterior) vascular layer (Seef elder, 216).

The anterior, much thicker layer (PI. IX, 9, H] is the primordium
of the corneal stroma; the posterior layer (IP} is pretty thick at the border
of the cup, much thinner in the middle, and is best designated as the
lamina irido-pupillaris (Jeannulatos, no). The border of this portion is
formed by a layer of regularly arranged cells (primordium of the endo-
thelium) at a length of 26 mm, and this layer ends opposite the cup border
in a group of such cells (primordium of the scleral trabeculum).

The lamina irido-pupillaris is united at the isthmus (/) with the tunica
vasculosa lentis, the vessels of which bend about the border of the optic cup.

Toward the end of the third month the pars coeca of the optic cup is
laid out, i.e., the border of the optic cup grows out into an epithelial fold,
which in the course of further development always becomes more closely
articulated to the peripheral portion of lamina irido-pupillaris.



200 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

The pars coeca, like the optic cup itself, has two leaves, an outer and
an inner. The outer leaf is intensely pigmented and is soon disposed in
meridional folds (primordium of the ciliary processes), while the inner
unpigmented layer at first courses smoothly over these folds. The transi-
tion area of the pars coeca, or the forward displaced border of the optic
cup, is not folded. At this place a narrow space exists between the two
leaves (analogous to the lumen of the primary optic vesicle, or possibly
to the last remnant of this lumen, the ring sinus of Szili, 216).

At the beginning (Szili, 216; Lauber, 137) or toward the end of the
fourth month (Seefelder, 204) a club-like (or possibly better, roll-like)
thickening appears at the transition place the primordium of the
sphincter pupillae. Herewith begins the development of the iris proper.

The sphincter primordium consists of ectodermal cells, derivatives
of the transition area, in part also of the outer layer (Juselius, 112).
It is united wholly to the pars coeca (PL IX, 10, Spti) to begin with,
but imbeds itself, however, in further development, in the mesoderm.
Yet numerous connections with the pars coeca always remain.

According to Seefelder (204), the unfolding of the anterior chamber
begins in the fifth month, and, indeed, first in the region of the iris primor-
dium. From there it gradually advances toward the axis of the eye,
and is only completed in the sixth month. Still its peripheral limit is
then at the border of Descemet's membrane.

The iris angle then forms; the mesoderm between the primordium of
the scleral trabeculum and that of the iris becomes spaced apart and trans-
formed into a loose framework, the uveal framework of H. Virchow (234).
This framework, therefore, fills out the entire iris angle in the fetus;
later it disappears except for a slight remnant at the periphery of the
chamber bay.

A complete separation of the corneal primordium and its endothelium,
on the one side, from the lamina irido-pupillaris, on the other side, follows
in the development of the anterior chamber. The further development
of the anterior layer is quite simple (PL IX, 10) : the ectoderm becomes
the corneal epithelium (), the mesoderm becomes the corneal stroma
(C), and the above-reported cell-layer, the endothelium (J9); this latter
at first consists of small, quite high cells. The Descemet's membrane
arises as a cuticulum out of the endothelium, and is demonstrable from
the fourth month on. According to the newest investigations, a pre-
corneal vessel net does not exist (Hirsch, 103).

The lamina irido-pupillaris shows a thicker zone in the periphery
covered posteriorly by the pars coeca the primordium of the iris (/).
The vessels of the tunica vasculosa lentis (Tv) enter the lamina irido-
pupillaris at the inner margin of the iris primordium.



THE EMBRYONAL AND FETAL DEVELOPMENT 201

By far the greater remnant of the lamina irido-pupillaris is a delicate
membrane consisting mainly of vessels and called the pupillary membrane
(Pm).

With the further growth of the eye, the iris continues to broaden out
along with the contemporary growth of the pars coeca, and the impres-
sion is therefore given that the iris grows out of the chamber angle.
In fact the pupil always becomes larger with the age of the fetus; this
is only due to the fact that its increase in size does not keep pace with
that of the whole eyeball.

The iris primordium, therefore, consists of a mesodermal layer (part
of the lamina irido-pupillaris) and two ectodermal layers, i.e., the anterior
zone of the pars coeca. The mesoderm of the iris primordium at first
goes continuously over into the pupillary membrane, and the vessel
system of the iris forms a continuum with that of the pupillary membrane.
Furthermore, the vessel system of the iris takes up the drainage of the
tunica vasculosa lentis. The circulation in the pupillary membrane is,
therefore, independent of that of the tunica vasculosa lentis. Later, the
border between the iris and the pupillary membrane moves back onto
the anterior surface of the former, probably because of the greater develop-
ment of the sphincter pupillae and its neighborhood (Brueckner, 28).

The pars uvealis iridis develops out of the mesodermal layer of the
iris primordium; stroma cells differentiate themselves in it in the fourth
month, the anterior border layer in the seventh month (Lauber, 137).

At first, the outer leaf of the pars coeca consists of high cylindrical
cells ciliaryward, of low cylindrical cells in the region of the sphincter
primordium (Spti). On the border between the two forms of cells,
Michel's spur arises; and the clump cells arise through detachments from
this, and from the pigment spurs lying farther forward.

The ciliary zone of the outer leaf becomes differentiated into the
dilatator pupillae. According to Heerfordt (88), the bases of the cells
fuse to a diffusely pigmented lamella in the twenty-second week. In the
twenty-fourth to the twenty-eighth week, pigment disappears from this
lamella, and fine meridional fibrillae become visible in it. In the thirtieth
to the thirty-second week bundles of fibrillae become separated off from
one another the subsequent fibers of the posterior border lamella.
Meanwhile, the height of the cells gradually decreases.

The inner leaf of the pars coeca takes on pigmentation and, indeed,
progressively ciliaryward from the transition area. This pigmentation
has attained the iris root at a length of 19 cm (Juselius, 112).

The ring sinus disappears at the end of the seventh month, according
to Szili, yet the connection of the two leaves remains more loose at this



202 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

place than in the ciliary part of the iris; in posterior synechia the ring
sinus at times again opens.

The pupillary membrane persists longer than the tunica vasculosa
lentis (Brueckner, 28). Its resorption begins in the eighth month, and,
indeed, first in the center. The small iris circle and the pupillary crypts
form after the disappearance of the pupillary membrane, while the ciliary
crypts open after the completion of the regression of the uveal framework
in the ninth month.

According to Seef elder (204), the development of the ciliary body
sets in at the end of the third month by a folding of the outer leaf of the
pars coeca. The first muscle fibrillae are visible in the meridional por-
tion of the ciliary muscle toward the end of the fourth month ; the circular
fibers first appear at the end of the sixth month with the opening up of the
chamber bay.

The ciliary processes at first reach much farther over the iris (Taf.
IX, 10, PC) than they do in the developed eye. Moreover, to begin
with, the ciliary body consists only of the corona ciliaris, i.e., the border
of the retina lies at the posterior ends of the ciliary processes and sends
short extensions into the ciliary valleys (O. Schultze, 198). The primor-
dium of the ciliary muscle at first extends far behind the border of the
retina (R}.

It is apparent, therefore, that a shifting occurs in this region of the
eyeball in further development. The ciliary processes shift out of the ter-
ritory of the iris into that of the ciliary muscle and the border of the
retina removes itself more from the ciliary processes. The backward
displacement of the ciliary processes cannot well be an actual one, for its
foundation is mesodermal tissue ; the form only of the ciliary body really
changes. But the border of the retina actually shifts backward in its
relation to the posterior end of the ciliary muscle. Thereby the origi-
nally short projections of the border of the retina become drawn out to
the long, sharp teeth of the or a serrata (cf. p. 88).



As already reported, the optic vesicle is laid out laterally, i.e., the
primitive pupil looks to the side. After the closure of the optic cleft the
direction of the eyes changes w T ith the greater development of the skull;
the pupils turn more toward the front, and that which was the posterior
(caudal) half of the eye in the primordium becomes the lateral (temporal)
half in the developed eye.

However, no notable rotation of the eye about its axis occurs, for the
entrance of the central artery into the optic nerve (the place correspond-
ing to the posterior end of the optic cleft) also lies below in the adult. The



THE EYEBALL OF THE NEWBORN



203



fovea centralis has, therefore, no relation to the optic cleft, wholly aside
from the fact that it develops at a much later period of fetal life. The
physiologic excavation has just as little to do with the optic cleft.



CHAPTER XVII. THE EYEBALL OF THE NEWBORN

(Text Fig. 5)

The size of the eyeball varies considerably at the time of birth;
according to E. von Jaeger (108), the sagittal diameter varies between 16 . i
and 19.1 mm. The mean of these, and, furthermore, the measurements of
Koenigstein (121), Merkeland Orr (152), Weiss (235) and von Pflugk (172),
is 17.3 mm.

The form of the eyeball also is
subject to much change; while Weiss
finds the equatorial diameter smaller
than the sagittal, Merkel and Orr, as
well as von Pflugk, estimate a some-
what higher average for it. All are
agreed, however, that the variation
from the form of a sphere (cf. p. 4) is
greater in the newborn, and especially
that the postero-temporal part is more
markedly curved out. The distance
of the cornea from the optic nerve is,
therefore, found to be o . 3 to o . 5 mm
less than the sagittal axis (Koenig-
stein); this asymmetry comes out
plainly in some drawings of Weiss, also in the photographs of von Pflugk.

According to von Reuss (181), the horizontal diameter is usually
9 mm, according to Koenigstein, 10 mm. The cornea is, therefore,
relatively large ; its relations to the optic axis in the newborn is something
like 1:1.8 (in the adult something like 1:2), or the cornea must grow
one-fifth to one-fourth, the optic axis, however, at least one-third, in
order to attain its definitive size.

The radius of curvature is given as 6.59 by von Reuss, as 7.3 by
Merkel and Orr. According to the latter, the cornea is more curved at
the border than in the central portions, therefore, a relation which is
exactly the opposite of that in the adult eyeball.

Extensive investigations have been made concerning the insertions
of the eye muscles by Weiss. They give so great a variation in the posi-
tion and direction of the lines of insertion, that I must limit myself to




TEXT FIG. 5. Left eyeball of the newborn.
Schematic cross-section closely following a
photograph of von Pflugk's. Magnification 3.



204 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL


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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 23 of 27)