Maximilian Salzmann.

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

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the chorioidea and base for the portion bearing the processes. Much as they are used,
they stand in direct conflict with the fact that elsewhere that side of the epithelium
turned toward the mesoderm is called the base (cf. the expression "basal membrane,"
"basal cells").

In general, the pigment epithelium of the chorioidea shows a very
uniform development; variations from the type are found only in the
region of ihefovea centralis and or a serrata.

In the region of the fovea (PL III, 10) the cells are higher (n to
14 mu) and narrower (9 to n mu), the cement ridges are more delicate
also; this is the main reason for the darker color of this region. In
the neighborhood of the ora serrata one encounters exceptionally large
cells, often with several nuclei, along with moderately large and normal-
sized cells (Text Fig. Ill, n); the diameter of such cells may attain
60 mu or more. The regularity of the epithelial layer also suffers thereby,
and the pigmentation is often quite lacking in uniformity. At the very
fore end of the retina the cells are again smaller and parallel to the border
of the retina; in this way stripes arise; sometimes these are plain, some-
times not (often somewhat lighter than the neighborhood) ; these separate



THE RETINA 63

the pigment epithelium of the chorioidea from that of the ciliary body.
These stripes follow exactly the course of the ora serrata or reproduce its
zig-zag form in a weaker way. In any case the border zone of the epi-
thelium is clearly seen, for the pigment epithelium of the ciliary body
appears much darker than that of the chorioidea.

The region of the ora serrata is probably very often the seat of light
pathologic changes, especially in older individuals, which have not
occasioned any functional disturbance in life. These changes are of
the same order as those resulting from light chorioiditis : partly a dis-
appearance, partly a hyperplasia of the pigment epithelium and a fusion
of the same to the retina.



CHAPTER VII. THE RETINA 1

The retina is a soft inelastic membrane, completely transparent
during life' except for the blood stream in the vessels; at the foramen
opticum chorioideae it is united with the intraocular end of the optic
nerve (papilla nervi optici}; in front it ends with a more or less dentate
border (ora serrata}. At both these places the union with the neighbor-
ing structures is a firm one; otherwise the outer surface of the retina is
only united to the pigment epithelium in the above-described way, and
this union is a relatively loose one; 2 a separation of this connection
(ablatio retina, detachment of the retina) is, therefore, frequent not
only under pathologic conditions but also as an artefact and post-
mortal appearance. Artificial and post-mortal detachment of the retina
stops, however, at the optic nerve and ora serrata (pathologic detach-
ment often extends beyond the ora serrata}. The inner surface of the
retina is smooth and aside from the slight bulgings occasioned by
the larger blood-vessels and from the fovea centralis, is even, as well,
and, therefore, reflects light in young individuals (the retinal reflex of
ophthalmoscopy) .

The retina is thickest at the border of the optic nerve, especially above
and below it (some o . 4 mm) , a little less on the nasal side and very much
less on the temporal (PL IV, 4). The retina thins out toward the
periphery to about o. 14 mm, rapidly at first, then more gradually. Only
the temporal side forms an exception.



1 In the stricter or clinical sense of the word; cf. the remarks on p. 13.

2 According to a newer conception (Halben, Die Kopulation der Netzhaut mil der Aderhaut durch
Kontaktverbindung, Berlin, 1910), this union is much more firm than one has heretofore held it to be.
A simple mechanical pull is not sufficient to separate the connection between the two membranes. A
primary insult to those organs concerned in the copulation, i.e., to the rods or the pigment processes,
is necessary.



64 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

* ,

Here lies the f ovea centralis ; this is an obliquely oval, flattened funnel-
form depression (PL V, 4) with its center some 3 . 5 mm away from the
border of the foramen opticum chorioideae and a little below the middle
of it; its size is about the same or a trifle greater than that of the
foramen opticum chorioideae (Dimmer, 40); the horizontal diameter may
measure up to 2 mm.

The edge of the depression forms a low wall (w) sloping off very
gradually into the normal level of the retina but is more steep toward
the center, and is uniformly bevelled (clivus, cl}. The larger the fovea
the more gradual, the smaller the more steep this declivity is, yet its angle
never attains more than 25. According to Kuhnt (130), a flat area
(fundus f oveae) lies in the center of the fovea, and exactly in the middle of
this is a small, very concave little depression, the foveola. According to
Dimmer, however, & fundus f oveae is found only in a very large flat fovea,
otherwise the clivus goes directly over into the foveola, in the region of
which the radius of curvature of the retina is only something like o . 04 mm.

In the same way, the relative thickness of the walls of the retina in the
region of the fovea is as follows : the wall is somewhat higher on the nasal
side (0.275 to 0.41 mm) than on the temporal side (0.22 to 0.35 mm);
in the foveola the thickness of the retina is 0.075 to 0.12 mm; all these
measurements are taken from Dimmer.

The so-called retinal reflexes, which are often so marked, especially in young
hypermetropic individuals, have an extremely changeable appearance in the extra-
foveal portions of the retina, since the slightest alteration of the direction of the
incoming light causes the reflex to disappear, possibly to reappear elsewhere. In the
foveal region, however, the retinal reflexes have a greater constancy in form and
presence because here there are only two places on the inner surface of the retina
which can so reflect light that it will pass through the aperture of the ophthalmoscope
and, therefore, become visible to the observer as reflected light (PL VII, i). These
places are the wall and the foveola: the wall has the form of a ring and gives a ring-
form reflex, the so-called macular reflex; the foveola acts like a concave mirror and
produces a reduced, inverted image of the part of the ophthalmoscope in front of
the pupil. The foveolar reflex, therefore, usually has a crescentic form, because one
almost always uses a perforated mirror (Dimmer, 39).

Since the clivus is directed so much away from the course of the rays which enter
the eye, the light reflected from it cannot again go out through the pupil; in its sphere
the reflex of the inner surface of the retina fails completely, therefore, and this is one
reason why the region of the fovea appears darker than the neighborhood in the ophthal-
moscopic picture; a second one is the slight thickness of the retina, a third the denser
pigmentation of the epithelium (cf. p. 61).

On anatomic study the district of the fovea centralis shows a citron
to orange-yellow color (yellow spot, macula lutea). The greatest inten-
sity of this color is found in the immediate neighborhood of the center



THE RETINA 65

(the foveola). The very center itself has a much more weak but still
yellow color (because of the lesser thickness of the cerebral layers in
this situation) ; the yellow color gradually fades away toward the periph-
ery; it is impossible, therefore, to give accurate dimensions for the yellow
spot although it is usually larger than the fovea.

The lighter color (citron-yellow) occurs in young persons (Cheval-
lereau and Polack, 31), the darker color (orange) in older persons.

The macula lutea is seen best in not-too-old cadaver-eyes, in which
the retina is already clouded. In entirely fresh eyes, enucleated during
life, one sees the yellow color as little as with the ophthalmoscope, pro-
vided the retina is transparent and in situ. The apparent reason for
this is that the transparent yellow color (lacquer-color) lies upon a
brown background, or, to the ophthalmoscope, a red background. In
the latter case there is added the factor that the source of light which
one ordinarily uses for this purpose is itself very yellow. If one uses
daylight, on the other hand, one can see the yellow spot in darkly
pigmented individuals with the ophthalmoscope, although under these
circumstances it seems considerably smaller than the macula in the
cadaver (Dimmer, 42).

When one detaches the fresh retina immediately after the enucleation
of the living eye, broadens it out on a slide, and studies it upon a white
background, one at once sees the yellow fleck, as recently re-established
by Chevallereau and Polack in a number of cases.

Gullstrand (81), on the other hand, accounts for the macula lutea
as a post-mortal appearance, because he has succeeded in so detaching
the fresh retina by patiently shaking it in a physiologic salt solution,
that it appears completely colorless.

a) Microscopic Anatomy and Histology of the Retina

The investigation of the structure of the retina is one of the most difficult tasks
in histology, not alone because its extremely complicated make-up can only be analyzed
by special methods, but because the retina is an extremely delicate and perishable
structure. The retina is subject to cadaverous changes much earlier than other tissues,
irrespective of whether the bearer of the eye dies or the eye is enucleated during life,
and with no other tissue of the eye must one so often and insistently ask himself the
question whether that which one sees under the microscope actually corresponds to the
relations in life or whether one has before him a post-mortem change, or an artefact,
wholly aside from the matter of pathologic alterations. The history of the investiga-
tion of the retina is rich in errors of this sort and has, indeed, even recently furnished
opportunity for criticism (cf. "macula lutea").

From this comes the demand that only absolutely fresh, healthy material be used in
the study of the retina, which unfortunately is not so easy to obtain in man. Naturally
the best are the normal eyes sacrificed because of large tumors of the neighborhood.



66 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

Severe injuries come second into consideration, eyes which must be enucleated immedi-
ately after the injury. Yet even the freshest material must be quickly fixed, otherwise
cadaverous changes at once supervene. Such rapidly working fixation fluids as osmic
acid, 3! per cent nitric acid, concentrated sublimat solution, Zenker's fluid, are best
adapted for this purpose and must work from the side of the vitreous; the eye must
come into the fixing fluid cut up and, indeed, it is then a piece of good fortune to
obtain a wholly unobjectionable preparation.

Every section perpendicular to the surface of the retina shows very
plainly, even on weak magnification, the stratification of this membrane.
This picture, surprising in its regularity, is brought about by the fact
that two non-nucleated layers, or layers which stain very poorly, alternate
with nuclear-rich ones; after nuclear stains, such as haemalaum, two
layers, especially, come out as sharply-stained stripes: the two nuclear
layers. A third less intensely stained stripe, inward to these, represents
the ganglion-cell layer. This much is sufficient for a cursory orientation
of the layers of the retina.

On more accurate study one recognizes nine layers in the retina; they
are given herewith from without inward: (i) layer of rods and cones;
(2) membrana limitans externa; (3) outer nuclear layer; (4) outer
plexiform layer; (5) inner nuclear layer; (6) inner plexiform layer; (7)
ganglion-cell layer; (8) nerve-fiber layer; (9) membrana limitans interna.

The connection between the individual layers is effected in part by
an extension of the elements from one to another, in part by a system of
special supporting fibers. In the description of the individual layers one
begins with the extrafoveal districts of the retina, not because one expects
to find the type of the retina in the structure of this district, but because
this structure extends over the greatest part of the retina and any section
through the eyeball can be made use of for the study of the extrafoveal
district; the extreme peripheral portions are of course excluded.

I. THE LAYER OF RODS AND CONES

(PL IV, 3, SZ)

On weak magnification this layer appears finely striated in a direction
at right angles to the inner surface of the chorioidea; this is due to the
palisade arrangement of its elements. The thickness of the whole layer
is greatest in the middle of the fovea centralisin my preparations 58 to
67 mu, according to Greeff (76) 85 mu; Dimmer (41), on the other
hand, considers the greater thickness of this layer in the fovea to be
an artificial effect. Toward the periphery the thickness decreases quite
rapidly, so that only i mm from the center of the fovea the whole layer is,
indeed, only some 40 mu thick. Farther on the thickness decreases much
more slowly (37 to 40 mu in the equatorial portions). Heinrich Mueller



THE RETINA 67

(158) gave somewhat larger figures (minimum 40 mu) for the peripheral
portions. Even by low power the rod-and-cone layers can be seen to have
two divisions, an outer less densely stained and an inner more densely
stained one; the border between the two portions lies about half-way
between the ends of the pigment processes and the membrana limitans
externa. It is the difference between the outer and inner members of the
elements which brings out this division into two portions.

The elements out of which this layer is constructed are of two kinds:
The rods are slender cylindrical structures with a length corresponding
to the thickness of the entire layer, a breadth, however, of only 2 mu
or less. Each rod is provided with a somewhat longer, slenderer, highly
refractile outer member (a), and a shorter, thicker, finely granular
inner member (i).

The outer member is double refracting and shows very fine longitudinal furrows
(stripes) ; it consists of a hull of neurokeratin and a contents made up of small diagonal
plates some o . 5 mu thick held together in roulette form by a cement substance. A
disintegration of the outer member into its platelets or amorphous droplets comes about
very easily after death or in various fluids, and a shepherd's staff or looped bowing
of the ends apparently represents a fore-stage of this destruction.

The inner member, of which the protoplasm is not so highly refractile, also shows
a fine longitudinal striation in the portions adjacent to the membrana limitans externa,
This is due to the last extensions of Mueller's supporting fibers, the so-called fiber-
baskets. The fiber apparatus, a system of fibers which course longitudinally on the
surface and within cross at narrow angles, lies in the outer third of the inner member.
Furthermore, the inner member contains a diplosome near the outer end, according
to Held (93). From this a thread (outer thread] is given off outward, going through the
hull of the entire outer member. A second thread extends inward (inner thread] ,
and can in any case be followed to the limitans externa.

With the exception of a 3 to 4 mm wide zone at the ora serrata, the
outer members of the rods contain the visual purple, a transparent red
coloring matter which bleaches out rapidly in the cadaver, but regener-
ates in the dark, as long as the union with the pigment epithelium is pre-
served. The visual purple makes the whole retina appear red, with the
exception of the above-mentioned peripheral zone and the rod-free area
of the fovea. Yet one only sees this in the fresh retinae of eyes which
have been previously kept in the dark.

One cannot see the visual purple in man with the ophthalmoscope because the
fundus (pigment epithelium and chorioidea) is already colored red; but in animals
which have a white fundus, like the crocodile, a so-called tapetum retinale, the visual
purple is visible and one can follow its blanching with the ophthalmoscope, likewise
in certain fishes (Abelsdorff, i).

The cones are flask-form structures likewise possessing a thinner outer
and a thicker inner member. The outer member is narrowed conically



68

toward the apex, the inner member bays out, yet the form and dimensions
of the cones vary a great deal with their location.

The longest and slenderest cones are found in the center of fovea (for
the measurements see p. 66), for their inner member is only 2.5 mu
thick (Greeff, 75). These foveal cones (PL V, 3, /Z) look more like
rods than they do like the other cones, but their cone nature is made
clear by the absence of visual purple. The extrafoveal cones decrease
in length toward the periphery, particularly in the outer member, which
is reduced to a miniature cone of 6 mu length at the ora serrata. The
inner member is some 3 mu shorter than the neighboring inner members of
the rods and takes on a more and more bellied form toward the periphery
(up to a diameter of 7 . 5 mu, according to Greeff).

With respect to the finer structure there is great similarity between rods and cones:
both show the same constituents. The thread apparatus is especially well developed
in the cones; it consists of a thick fiber mesh and occupies two- thirds of the inner
member.

The cones contain no visual purple.

The distribution of the rods and the cones and their relations to each
other are best studied in surface preparations of the retina in which
the thicker cones appear as larger discs, the thinner rods as smaller discs.

In the center of the fovea is a district containing only cones; it has a
diameter of about 0.5 mm according to Fritsch (63), only 0.15 mm.
But the very slender cones are found only in the middle of this district
and in irregular arrangement ; toward the border of this field the cones are
notably thicker and are arranged in oblique rows (Fritsch, 63) ; a beauti-
ful drawing of the cone-mosaic of the fovea has been published by Heine
(92). Outside this field the rods appear; at first they are strewn about
among the cones; soon, however, they become united into a simple circle
about each cone. Farther on the rods become more and more numerous
and the cones wider apart until some three to four circles of rods intervene
between two cones (PL V, 2). This distribution is attained some 4 to
5 mm away from the center of the fovea and is then maintained quite
constantly to the periphery. In the most extreme periphery the cones
are again relatively increased.

2. MEMBRANA LIMITANS EXTERNA

(PL IV, 3, Le)

This is an extremely delicate, sieve-like, perforated membrane,
visible as a fine, continuous, or streaked line only in absolutely perpen-
dicular sections. The holes correspond exactly to the elements of the
rod-and-cone layer in number and position, for they serve for the exit of
the fibers going out from these elements.



THE RETINA 69

The m. limitans externa belongs to the supporting tissue of the retina.
It is connected with Mueller's fibers on one side and fine fibers, so-called
fiber-baskets (see above), go off from the other side of it (outside).

According to Leboucq (139), it, as well as the fiber-baskets, is, as a
whole, a remnant of the original intercellular cement of the fetal retinal
cells.

The greater length of the foveal cones makes the m. limitans externa
show a slight bulging inward in the center of the fovea, the so-called
fovea externa. What was said concerning the greater thickness of the
rod-and-cone layer in the fovea holds true, of course, for these as well.

3. THE OUTER NUCLEAR LAYER
(PL IV, 3, a*)

This layer consists mainly of thickly placed, rounded, or weakly
oval structures (outer nuclei) in which a thin protoplasmic mantle and a
densely-staining nucleus can be made out. The nucleus forms so large
a portion of the outer nuclear element that the whole layer seems to be
made up solely of nuclei.

The thickness of this layer is about 4.6 mu nasal to the optic-nerve
entrance and is there some 8 or 9 nuclei wide. From here toward the
periphery its thickness decreases gradually but only very slowly without
essential alteration of its appearance. Temporal to the optic nerve the
layer is, in general, somewhat thinner, and in the direction of the center
of the fovea its thickness decreases still -more to 22 mu at the border of
the rod-free field, and here it is only 4 nuclei thick. From here on it
again increases in thickness, partly from an increase in the number of
nuclei, but especially because these are more widely spaced apart, and
attains a maximum of about 50 mu in the center of the fovea (PI. V, 4).

Even in ordinary preparations one can distinguish two kinds of nuclei,
yet in my experience this difference is but little noticeable in preparations
from Mueller's fluid and much more striking in formalin and sublimat
fixation. This difference affects only the nuclei. The one kind of nuclei
is smaller (5.7 mu), more rounded, and more densely stained (PL IV, 3,5);
the other is larger, plainly oval (5X7 mu) and more weakly stained (PL

IV, 3, *)

The main bulk of the outer nuclear layer is made up of the first form
while in the extrafoveal portions of the retina those of the second form
are found only outside, right next to the m. limitans externa. In the
territory of the rod-free field of the fovea only nuclei of the second form
are found and the entire thickness of the nuclear layer is, therefore, made
up by them. About this field the nuclei of the first form appear, at first
in the innermost layers of the nuclear layer, then they rapidly increase in



7 o ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

number, while the more sparse appearing nuclei of the second form make
an unbroken layer immediately along the m. limitans externa only. Farther
away from the fovea the number of these nuclei continues to decrease, they
are more and more separated from one another, and the intervals are filled
with the nuclei of the first form, which henceforth make up the main mass.

From this distribution alone, it may be suspected that the smaller,
denser stained nuclei of the first form belong to the rods, the larger, more
weakly-stained nuclei of the second form to the cones. As a matter of
fact each element of the rod-and-cone layer is bound to an outer nucleus
by a fiber, which one can see without the employment of unusual meth-
ods of preparation, for example, in sections through the middle of the
fovea, where the outer nuclear layer is especially loose (PL V, 3).

Each rod, for example, is extended inward as a fine, tortuous, and
varicosed fiber (rod-fiber); this goes through the corresponding hole in
the m. limitans externa into the outer nuclear layer and at a varying
distance from this membrane distends into the rod granule containing
one of the smaller, denser stained nuclei; the protoplasmic mantle of
the granule is extremely thin, so that it seems to consist almost solely
of the nucleus. The fiber continues beyond the cell and ends in a small
bud in the outer plexiform layer.

In the same way each cone goes over into a fiber (cone-fiber), but
this is much thicker in its distal part, i.e., between the cone and its granule,
than is the rod-fiber, and very short, because the cone granule is placed
just within the m. limitans externa. The cone granule is somewhat more
spindle-form, because its nucleus is oval and somewhat more protoplasm
is present at the two ends of the granule. The proximal part of the
fiber is throughout longer than the distal and more slender than it, but
still is always heavier than a rod-fiber. The cone-fiber, likewise, ends in
the outer plexiform layer, but at a deeper level than the rod-fiber, and,
indeed, with a conical swelling (cone swelling, or cone-foot) from which
short lateral branches go off.

Only in the center of the fovea, where cones only are present and the
cone granules must be placed over one another in layers, is the distal
portion of the cone-fiber thinner and longer.

The direction of the rod-and-cone-fiber in the extrafoveal parts of the
retina is perpendicular to the surface. In the region of the fovea the direc-
tion becomes an oblique one and in keeping with this the outer nuclei
are arranged in oblique rows.

Concerning the presence of cross-striations in the outer nuclei, i.e., an arrangement
of the chomatin substance in cross-bands, opinions are much divided. I can discover
nothing of this sort in my preparations.



THE RETINA 71

Here and there the cone nuclei lie beyond the m. limitans externa (the nucleus then



Online LibraryMaximilian SalzmannThe anatomy and histology of the human eyeball in the normal state, its development and senescence ; → online text (page 8 of 27)