Maximilian Salzmann.

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

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(and one should study the inner lamellae; the nerve-fibers and ganglion cells are found
here) and then one comes upon the larger vessels. One grasps the wall of one and pulls
in the direction in which the finer branches lie. In this way one can gradually remove
the whole layer; finally there remains a thin unpigmented layer: this contains the
capillaries and glass membrane. A separation of these two in an anatomic way is
impossible, yet by tearing this membrane one very easily produces step-like borders,
i.e., the capillary layer clings to one fragment, the glass membrane, a slight distance in
front, to another in a way entirely adequate for the study of the finer structure of these
layers.

The cohesion between the vessel layer and the capillary layer is decidedly less than
that between the other layers, otherwise one could not make a pure preparation of
a great expanse of the capillary layer, but a real space does not exist between these
two. In well-stained specimens the connective-tissue stroma is seen to be continuous.

If the suprachorioidea, which was described in the preceding chapter
as a tissue filling out the perichorioidal space, be not considered, this
anatomic preparation of the chorioidea shows a division into three main
layers: (i) The vessel layer (stratum vasculare); (2) The capillary layer
(choriocapillaris} ; (3) The glass membrane (lamina vitrea, s. elastica
chorioideae) .

I. THE VESSEL LAYER

(PL IV, 3, Gf)

This layer forms the main mass of the chorioidea and is the bearer of
the macroscopic markings those visible with the ophthalmoscope.

A further division of this layer into one with vessels of larger caliber
and one, inward, with vessels of lesser caliber (Sattler, 187) can be made
out in the thicker parts of the chorioidea. The richer development of the
whole vessel system, and the principle of the arrangement whereby the
caliber of the vessels decreases from without inward, condition this strati-
fication.

The matter is not to be taken thus literally: often enough one is in
doubt where to place the limits of the two layers, and transitions in
caliber are numerous. I cannot, therefore, accept the view of Nuel (166)
that the separation of these two layers is equally as sharp as that between
the vessel layers and the choriocapillaris, and I do not, therefore, recog-
nize any "intervascular space."

According to Nuel, the arteries predominate in the layer of larger
vessels; this, also, is probably true only for the posterior (polar) portions
of the chorioidea and is apparently only the result of a sharp separation
in space which the arteries and the venous trunks of the chorioidal vessel



THE CHORIOIDEA 55

system show (cf. chap. xv). On the other hand the veins very markedly
predominate over the arteries in the layer of the middle-sized vessels.
This latter statement is without doubt correct, at least in so far as one
encounters more venous lumina than arterial lumina in this layer. Yet
the increase in venous lumina is certainly in part only an apparent one, for
the veins are more tortuous and, therefore, more often cut across.

In the region of the fovea centralis, where the chorioidea attains its
maximum thickness, the layer of larger vessels and the perichorioidal
space disappears entirely, according to Nuel, while the smallest veins
increase appreciably in size and many times lie over one another in layers.
In the equatorial parts of the chorioidea the separation of the larger
from the smaller vessels is likewise lost, for the smallest arteries and veins
go over into the capillary layer, while the rest of the vessels are broadened
out into a single layer.

Histologically, the vessels show the ordinary structure (PI. Ill, 6).
The arteries (A) have a plainly developed muscularis, which can be
followed into the arterioles (precapillary branches); there it becomes
reduced and, according to Wolf rum (240), consists of polymorphous
structures whose branches surround the vessel lumen like polyps. An
adventitia of finely fibrillated, almost homogeneous, collagenous tissue,
traversed by thick elastic fibers, follows upon the muscularis.

The veins (F) have perivascular sheaths (p), i.e., a second proto-
plasmic tube, provided with flat nuclei, about the endothelial lumen (e),
whereupon follows the connective-tissue adventitia. This is relatively
better developed about the small vessels than about the larger ones, yet
varies greatly in its development with the age of the individual.

The chorioidal stroma fills out the vessel interspace. This consists
mainly of the same elements as the suprachorioidea, except that here
collagenous fibrillae are added. From without inward the stroma very
gradually changes in its make-up ; the outermost layers still contain many
endothelial membranes and flat chromatophores, so that they scarcely
differentiate themselves from the adjacent suprachorioidal lamellae.
Indeed, the farther inward one goes the more the dimensions of the
chromatophores increase; their bodies become smaller, their processes
more slender. Their position changes, also, in that they become related
to the vessel walls, i.e., their processes broaden out parallel to the vessel
wall. The endothelium becomes less prominent or goes over into con-
nective-tissue cells, the elastic fibers become finer, collagenous tissue
increases in amount.

The small vessels lying immediately outside the capillary layer still
contain a few chromatophores in their interspaces, but inside this layer,



5 6 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

i.e., inside the border line between the vessel and capillary layer, no more
chromatophores are found. Here the stroma consists only of collagenous
and elastic fibers mixed with a few flat cell-nuclei.

Numerous nerve-fibers, the last branches of the ganglionated plexus
beginning in the suprachorioidea, traverse the chorioidal stroma, especially
in company with the arteries (). Moreover, one also encounters ganglion
cells (g) in the inner strata of the vessel layer, although only scatteringly.

Its further branchings and endings have been studied by Bietti (24)
by Golgi's method. According to this author, a meshwork of the very
finest sort is interposed between the above described nerve-fibers. Other
nets surround the arteries ; their fibrillae show many varicosities and end
with club-form or spherical swellings in the vessel musculature. Another
finer nerve-plexus lies beneath the lamina vitrea.

The statements concerning the presence of smooth muscle-fibers in
the chorioidal stroma (outside the vessel walls) are founded principally
upon a contribution of H. Mueller (159).

According to this author, the smooth muscle-fibers together with a
plexus of small nerve bundles lie along the sides of the arteries. They
are easiest to find by the side of the long posterior arteries (cf. p. 51),
are present, as well, however, along the short posterior arteries. Accord-
ing to Schweigger (195), these smooth muscle-fibers lie by the side of a
plexus of clear ganglionated nerve-fibers in the innermost vessel layer.
Wolfrum (240), on the other hand, absolutely denies the presence of
muscular elements outside the walls of the arteries.

The demonstration of smooth muscular fibers in the chorioidal stroma
is one of the most difficult things in histology, and I can say that"! have
not been able to see them there with certainty up to this time, i.e., in the
stroma of the chorioidea proper. It is certain, on the other hand, that
smooth muscle-fibers are present in the suprachorioidea and, furthermore,
far behind the or a serrata (cf. chap, ix, i).

These muscle-fibers play a very subordinate r:le, in any case. The generalization
of Fukala (70), therefore, that the whole uveal tract, with the exception of the posterior
pole, is invested with a muscle-net, certainly is improbable. It can only be explained
by a misunderstanding and is apparently a resuscitation of the statement of Iwanoff
(115) concerning the endings of the ciliary muscle, and the data of F. Eilhard
Schultze (196) concerning the reticular arrangement of the muscle tissue along the
inner surface of the ciliary muscle, and the above statements of H. Mueller.

2. THE CAPILLARY LAYER (Choriocapillaris)
(PL IV, 3, C)

The layer can be spoken of as characteristic of the chorioidea, for the
most important difference between the equatorial portions of the chori-
oidea and the most posterior zone of the orbiculus ciliaris consists in the



THE CHORIOIDEA 57

presence of this layer in the one and its absence in the other. Further-
more, the chorioidea is different from all the other vessel-containing
portions of the eye in that its capillaries are broadened out into one
plane, i.e., they do not build a meshwork, as elsewhere, but do form
a net, which is, moreover, characterized by the width of the individual
capillary vessels; while elsewhere the capillaries are so narrow that the
blood corpuscles can only pass one after another in single file, and
even then must often stretch out, there is room for several blood
corpuscles side by side in the capillaries of the chorioidea. In addition,
local widenings of the vessel by sac-like distentions of the capillary wall
are frequent.

The network of the capillary layer is especially thick in the posterior
parts of the chorioidea, i.e., in the region of thefovea centralis retinae and
its immediate neighborhood; its meshes are rounded and the interspaces
of the capillaries smaller than the lumina themselves (PI. IV, 7). The
smallest arteries and veins pass from without fairly perpendicular into
the capillary layer (Passera, 168), and break up, star-like, into capillaries,
i.e., each of these small vessels divides up at once into capillaries upon its
entrance into the capillary layer, and these radiate out in all directions.
In teased preparations one sees these small afferent and efferent vessels in
optical cross-section only (#); one recognizes these relations better on
cross-sections (PL IV, 3, V).

Farther toward the periphery the meshes of the capillary net con-
tinually become wider and longer (PL IV, i). The main difference is
due, however, to the fact that not only do the capillaries show distentions
and great variation of caliber, but also that the smallest arteries and
veins course in the capillary layer itself, and the break-up into capillaries
takes place by feathery or dendritic branchings.

In the region of the ora serrata the network becomes remarkably
loose and finally ceases with irregularly projecting loops or exten-
sions. The capillary net is more or less interrupted over the recurrent
arteries (A).

A net of narrow capillaries lying inside the choriocapillaris (cf. chap, ix, 4), is
found on the border between the chorioidea and ciliary body, according to Sattler
(187).

The capillaries (PL III, 7), as elsewhere, consist of simple endothelial
tubes strewn with oval, quite densely-staining nuclei (e) , which, according
to Wolfrum, lie either in the interspace or on the outer side of the
capillary wall, i.e., on the vessel-layer side. The interspaces (interstices)
of the capillary net (f) are filled out by an almost homogeneous non-
nucleated stroma; special stains are necessary to show its very fine
collagenous and elastic fibrillae.



S 8 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

The collagenous tissue forms the main mass of this stroma; it is an
extremely delicately fibrillated tissue, staining only a faint rose-red by
Van Gieson. Outward it is continued into the stroma of the vessel layer,
which in its innermost layers, as already reported, contains the same con-
stituents as the tissue filling out the capillary interstices. The elastic
fibers are especially fine, and, according to Wolfrum, are found mainly
in the immediate neighborhood of the capillary wall; they are united
inwardly with the elastic lamella of the glass membrane and outwardly
with the subcapillary fibrillar net.

This net (/"), likewise, consists of collagenous and somewhat larger
elastic fibrillae and separates the capillary layer from the vessel layer.
Since the places at which the smallest arteries and veins go over into the
capillaries are separated from one another by relatively wide intervals,
these expansions of the stroma lying between the capillaries and the
layer of the smallest vessels take on a flattened-out or reticular character,
and occasionally give the impression of an independent layer, especially in
teased specimens. Cell-nuclei also lie in the subcapillary fibrillar net;
Sattler, in his time, considered them to be the nuclei of an endothelial
membrane, but Wolfrum now looks upon them as nuclei of connective-
tissue cells, because they belong to the branched cells and have granular
protoplasm (sc).

One can see the nuclei in question best in surface preparations of the
capillary layer treated with a nuclear stain, such as haemalaum. Two
systems (or kinds) of cell-nuclei then come forth in the otherwise unstained
tissue. One system belongs to the capillaries; the nuclei are rounded or
oval and stain quite densely; they lie partly in the contour of the capil-
laries, partly (apparently) in the lumen. The other system (the sub-
capillary nuclei) belongs to a plane lying farther outside; a slight turn
of the micrometer screw is, therefore, necessary to focus upon the sub-
capillary nuclei if one has previously had the capillary wall in focus. The
nuclei are larger, of more irregular form and less densely stained, and do
not correspond in their position to the capillaries, i.e., sometimes they lie
over the capillaries, sometimes over the interspaces.

With respect to these nuclei, I can only corroborate the view of
Wolfrum; an endothelial membrane such as Sattler postulates would
presuppose at least a cleft space. But no such thing is to be seen in well-
stained sections; the stroma of the capillary layer goes uninterrupted over
into the stroma of the vessel layer and, indeed, its collagenous as well as
its elastic elements.

A fine nerve-plexus lies beneath the glass membrane, therefore between
it and the choriocapillaris, according to Bietti (24) and Smirnow (208).



THE CHORIOIDEA 59

3. THE GLASS MEMBRANE (Lamina vitrea s. elastica)

This possesses the physical and on weak magnification also the
morphologic peculiarities of glass membranes in general. One must use
a very high magnification to recognize, on its outer surface, not a homo-
geneous, but an extremely fine and faint, network. The torn edges of the
surface preparation (PI. Ill, 7) very often show a terraced form, i.e.,
there are present two contours which do not coincide, an indication that
the glass membrane itself consists of two further lamellae.

In cross-section (PI. IV, 3, Lv) the glass membrane appears as a
very highly refractile membrane, some 2 mu thick, firmly grown to the
stroma of the capillary layer especially, whose elements, as already
stated, arise at least in part from those of the glass membrane. Yet a
stroma is present only in the interspaces of the capillary net; the indi-
vidual endothelial tubes lie immediately on the glass membrane without
any intervening layer.

The outer contour of the glass membrane that turned toward the
capillary layer is sharper, darker, often not exactly straight, and finely
granular; the inner contour, turned toward the pigment epithelium,
is more delicate "softer" in the sense of the artist and entirely straight
and uniform, except for the depositions which appear in old age. These
variations are also simply expressions of the fact that the glass membrane
is made up of two lamellae.

Toward the entrance of the optic nerve the membrane, which in general
has a quite uniform thickness, becomes much thicker (3 to 4 mu), and the
two lamellae can be made out more easily. Concerning the ending of
the glass membrane at the optic-nerve entrance, compare the anatomy
of the optic-nerve entrance (chap, viii, a).

From their position the two lamellae making up the glass membrane
can be called the outer and inner; from their nature they may also be
distinguished as elastic and cuticular lamellae.

The outer or elastic lamella (PL III, 7, el] bears the above reported
clear network, first described by Sattler. Smirnow (208) later demon-
strated a dense plexus of finest elastic fibers in this network by the orcein
stain. The Sattler network corresponds to the larger bundles of this
elastic mesh only, and Smirnow has called it the stratum elasticum supra-
capillare. As stated, this is united with the elastic fibers of the capillary
interstices and to a certain extent closes off the whole elastic system of
the capillary layer inward. In general, this lamella has no measurable
thickness; by itself it appears only as a contour on cross-section, and,
moreover, the elective elastic fiber stain only makes this contour sharper,
not broader. In the region of the optic-nerve entrance, alone, the elastic



60 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

lamella is thicker and to the same extent that the glass membrane, in
general, is thickened. Its fibrillae come out more plainly and take on a
more and more circular course.

Some o . i mm from the edge of the optic-nerve canal, where the
choriocapillaris disappears as a closed layer, the subcapillary elastic fiber
net appears over the glass membrane and this latter gives off larger and
more numerous elastic fibers. A circular mesh of elastic and collagenous
fibers of considerable thickness arises in this way; on cross-section it
appears as a layer of some 2 mu thickness and, on account of the rich
mass of elastic fibers which 'it contains, it stains quite otherwise than the
neighboring structures. As a result of the circular course of the fibrillae,
this layer appears finely punctate or granular upon meridional section of
the papilla, but, on the other hand, longitudinally striated in sections
tangential to the border of the papilla.

The inner lamella (PL III, 7, cu) is entirely homogeneous, like
other glass membranes, and is apparently a cuticular formation of the
pigment epithelium, as shown by the pathologic cases in which a certain
kind of regeneration or hyperplasia of this lamella takes place; it should,
therefore, be called the cuticular lamella.

By ordinary staining this lamella seems to form the main mass of the
glass membrane, i.e., the cuticular lamella makes up almost the entire
thickness of the glass membrane.

Wolf rum (240) recognizes a lamina elastica chorioideae and a basal membrane of the
pigment epithelium. The latter is only about one-half as thick as the former and gives
a protoplasmic as well as a collagenous tissue-staining reaction. A space, traversed
by finest collagenous fibrillae, lies between the two membranes. It appears that these
details can only be demonstrated by special staining methods, especially such as Held's
protoplasmic stain, and are, therefore, not visible after ordinary stains.

I have ribt seen collagenous tissue between the two membranes in my preparations
up to this time, yet the presence of this tissue has nothing improbable about it, since
such can be easily demonstrated in an analogous situation in the ciliary body without
special staining.

CHAPTER VI. THE PIGMENT EPITHELIUM OF THE CHORIOIDEA 1

To the naked eye this forms a thin, uniformly brown covering over
the inner surface of the chorioidea. A darker, indistinct area about
twice the size of the papilla (Usher, 230) comes out only in the region of
the fovea centralis retinae. Even moderate magnification shows a fine
flecking, due to the fact that not all the cells are so pigmented as to be
equally dark; this is also visible by the ophthalmoscope: the character-
istic granulation of the fundus comes from the pigment epithelium.



1 For the justification of the term see p. 13.



THE PIGMENT EPITHELIUM OF THE CHORIOIDEA 61

The pigment epithelium is made up of a single layer of protoplasmic
cells with a diameter of about 16 mu, most of which appear six-sided
upon surface view (PL III, 8). This hexagonal form is due to the
regular arrangement and uniform size of the cells; when individual cells
are smaller than the rest, they have less than six corners, when larger,
more. Viewed in this way, the cell-body appears uniformly filled with
pigment; the round or weakly oval nucleus, measuring about 7 mu in
diameter, is more or less surrounded by pigment ; the cell borders, on the
other hand, come out as sharp, unstained stripes of almost i mu width.
The so-called cement substance consists of neurokeratin, a material which,
too, covers over the surface of each cell on the side toward the chorioidea
in a thin layer (Kuhnt, 128).

Seen in cross-section (PI. IV, 3, P), the cells appear more quadri-
lateral, their height is only about half their breadth (8 mu), the pigment
fills out only the inner part of the cell and leaves a thin layer of the pro-
toplasm bordering the glass membrane outward entirely free, so that a
part of the nucleus is very plainly visible. The outer half of the proto-
plasm shows a radial structure, according to Kuhnt. The cement sub-
stance is only partly seen on cross-section where it runs exactly at right
angles to the direction of section. The form of a pigment epithelial cell
is a low six-sided prism, according to this.

Each pigment epithelial cell carries a large number of fine processes
(PF) along its inner surface; these project inward between the outer mem-
bers of the rods and cones. Since it is extremely difficult to obtain the
retina in situ, these pigment processes are very rarely seen in the human,
especially, since on account of their fineness they are only to be made out as
such in extremely thin sections. In sections of ordinary thickness (15 to
20 mu) they are fused into a uniformly pigmented stripe. I c<jkh, there-
fore, give nothing more in detail concerning their form; I suspect they
represent moulds of the interspaces between the outer members of the
visual cells.

The relation of the outer members of the visual cells to the pigment epithelium is
much better seen in the retina of amphibians (frogs, tritons), because these animals
have colossal rods and pigment epithelium in comparison to warm-blooded animals.
The phototropic shifting of the pigment is also much more easily demonstrated in
these animals.

I have measured the length of the pigment processes in a faultlessly
fixed human retina and found it to be 5 mu; it is, however, possible that
the processes are still longer and that the pigment only mounted up to
this height. Whether a shifting of the pigment (phototropic pigment
displacement) by illumination also takes place in mammalia, and in man



62 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

in particular, has not yet been definitely settled (Garten, 73); the dis-
placement of the pigment must in any case be very slight.

The pigment granules (PL III, 9) are rounded in the outer portion of
the cell, longish, like crystal needles in the inner portion, notably in the-
processes.

With the ultramicroscope, the rounded granules appear deep red-brown, the long
ones, on the other hand, a light brownish yellow (Raehlmann, 176). Kuhnt (126)
calls the pigment of the epithelium fuscin on account of its color and form; it is very
resistant to chemical influences, but bleaches in acids when in the light.

The crystal-needle form is not marked in man and the granules look more like
rods or spindles. According to Raehlmann, a large number of very small particles in
the form of light-yellowish bacterial rods are to be seen only by ultramicroscopic
means, in addition to the above-described pigment granules. Moreover, some of the
fuscin needles consist of and are made up of several such short rods bound to a longer
one by a protoplasmic substance. This latter substance is reddish in the darkened
eye of an animal and bleaches out in the light, just as visual purple does, and for this
reason Raehlmann holds it to be identical with the visual purple.

The pigment epithelium of the chorioidea is especially distinguished from all other
pigment cells of the eye by the elongated form of the granules. When, therefore, the
epithelium of the chorioidea breaks up and its granules are carried away, their origin
can be recognized from their longish form. It is, nevertheless, incorrect to reverse the
conclusion that round pigment granules exclude origin from the pigment epithelium of
the chorioidea, because first, it also contains rounded granules in the normal state,
and second, its pigment may be completely transformed into roundish granules in
pathologic conditions.

As one may note, I have avoided the term cap for the part of the cell turned toward



Online LibraryMaximilian SalzmannThe anatomy and histology of the human eyeball in the normal state, its development and senescence ; → online text (page 7 of 27)