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The anatomy and histology of the human eye online

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cells are found. It is seen that in the middle region of the
retina (see Fig. 26) the fibres of the membrana limitans, when
traced to the nerve-fibre layer, each presents, at its external
extremity, the same conical expansion that it has on its inter-
nal extremity. Bitter says they are not simple elements, as is
indicated by a manifest striation, which indicates complexity
of structure.

The size of these fibres depends on the size of the inter-
spaces in the optic nerve layer, and the spaces between the
ganglion-cells. The simple fibres are distinct and bright ;
those in the central parts of the retina generally have small
nuclei, difficult to perceive, whilst the larger fibres near the
ora serrata contain a great number of large nuclei. Hence, it
will be perceived that the fibres of the limitary membrane pre-
sent numerous varieties of form, partly determined by the
proportion of nervous tissue contained in the different parts of
the retina. In parts where the latter is less abundant, the cel-
lular tissue seems to take its place. In points where the globu-
lar nervous elements exist, as in the gangli6*n-cell layer, the
cellular tissue fills up the cavities, whilst in places where the
nervous substance is found in the form of fibres, the cellular
tissue forms a network. At the outer limit of the ganglion-
cell layer these fibres suddenly expand in all directions, and
divide into extremely fine fibres, as seen in Fig. 28, which are
so delicate that it is extremely difficult

FIG. 28.

to estimate their dimensions. These fibres
form a network in the fibrous layer of
extreme tenuity, so that with a moderate
illumination it gives to this lamina a
granular appearance.

m , TT -, .. n. ,, ~, Transformation of fibres

The cellular tissue of the fibrous layer of the mem brana





presents considerable variations in the into fibrils of the fibrous

. PI layer of the human retina.

dilterent regions ot the retina. The Magnified 500 diameters.
fibrous layer is most uniform in its (From Ritter -)
thickness of all the retinal layers. Even at the ora serrata it
is this layer that furnishes the larger part of the cellular tissue



74 THE ANATOMY AND HISTOLOGY

found there. In the centre of the retina the fibres are perpen-
dicular to such an extent as to almost deprive the lamina of
its granular aspect, and in the middle of the macula lutea this
is most manifest. On the contrary, toward the periphery, the
granular aspect of the retina is more decided, the anastomoses
of the fibres are more marked, and a larger number are seen
running obliquely ; nuclei are also found, which, in appearance,
nearly approach the fibres of the limitary membrane.

The network in the fibrous layer receives the prolongations
of the ganglion-cells, the distribution of which determines the
size of the meshes of this network and their reciprocal disposi-
tion. In the centre of the retina the meshes are most narrow,
and the fibres composing it are least oblique, as the prolong-
ations of the ganglion-cells have in this locality a direction
nearly perpendicular to the retinal plane. The meshes of the
cellular tissue increase toward the periphery, in proportion as
the ganglion-cells take, in this region, more volume, and, as
they form numerous anastomoses, the arrangement of the
meshes becomes more complicated. At the external border of
the fibrous lamina the fibres compare with the inter-granular
layer as coarse fibres with distinct outlines. These fibres, not-
withstanding their analogy with those of the limitary mem-
brane, are yet distinguished from them, inasmuch as they con-
stantly run in a perpendicular direction to the retinal plane,
and do not form any anastomoses. They also form at certain
points small cavities similar to those found between the cells
of the ganglion-cell layer. These cavities inclose a small
number of cells of the granular layer, and their walls are per-
forated by the filamentous processes of those cells.

The external prolongation of these cellular fibres form a new
network of fibres, which constitutes the cellular tissue of the
inter-granular lamina. In man the intermediate granular
layer exactly resembles the fibrous lamina, and is distin-
guished only by perpendicular strise, which represent the fili-
form anastomoses situated between the cells and the granules
of the granular lamina. These filiform prolongations belong




OF THE HUMAN EYE. 75

to the nervous tissue, and are interlaced by a fibrous network
of more narrow meshes than those of the nerve-fibre lamina,
as seen in Fig. 29. These fibres assume a regular disposition
in forming quadrangular, pentagonal, and
hexagonal figures, so as to have the ter-
mination and origin of these small ele-
ments correspond. The angles of these
figures send off anastomoses to neighbor-
ing fibres of similar character, so that in
the constitution of these figures several
fibres are always added. Toward the Cellular tissue fibres in

the inter-granular layer of

two surfaces of the inter-granular layer, man . Enlarged 500 diam-
this network terminates by dark-colored eters - (From Rifter.)
fibres, which, in the region of the ora serrata, take the place of
the other fibres through the whole thickness of the lamina.

o

In the granular layer a simple cellular tissue fibre is found
proceeding from the fibre-cells, and which are not much finer
than the fibres of the limitary membrane. They are distin-
guished from the fibres of Miiller in not inclosing granules,
and they contain only a simple nucleus. The enveloping
membrane of each cell is largely dilated, and by its external
prolongation generally undergoes a vitreous metamorphosis.
The reunion of these cells is made by anastomoses in arches.
In the central part of the granular layer two series of anasto-
moses are observed, whilst toward the periphery the whole
thickness of the lamina is occupied by a single system of
arches, contiguous to the rod layer. As these external ramifi-
cations of the cells which constitute the arches are, in great
number, the seat of vitreous metamorphosis, in certain prepara-
tions the external limit of the granular layer seems to be ar-
rested by a continuous layer. It is this that Schultze named
the external- limitary membrane. It does not seem proper to
name it a membrane, as in the central parts of the retina this
expansion of the cells is entirely wanting, and in the middle
region it is often interrupted. It can only be termed a mem-
brane in the region of the ora serrata, where it is continuous.




76 THE ANATOMY AND HISTOLOGY

The last fibres of the retinal cellular tissue are situated at
the inner termination of the rod and cone layer. They consti-
tute at this point the most external arches, and, as far as is
now known with certainty, the cellular fibres of the retina
cease there. Bitter thinks it possible that an intimate con-
nection between this cellular tissue and the enveloping mem-
branes of the cones and rods will hereafter be demonstrated.

These investigations of the cellular tissue of the retina were
made by Eitter, mainly on the peripheral region, as it is in
FlG 30 that region that its characteristics are

most conspicuous. Toward the centre
of the retina the elements of cellular
tissue are very fine, and their direction
is parallel to the retinal plane, which
readily leads to its confusion with the

External termination of the ,

cellular tissue network of the nerVOUS fibres.

retina of the whale. Magnified The retina, as a whole, has a surface

300 diameters. (From Rl.tt.er.)

of about three hundred square lines.

At its peripheral termination at the ora serrata it is firmly
attached to the hyaloid membrane. It gradually diminishes
in thickness from its central portion to its periphery. At the
equator of the eye it yet possesses one-half of the thickness at
the centre ; but from this point it rapidly diminishes, so that
a few lines from the ora^ serrata it measures only one-third,
and at the ora serrata only one-fourth of the central thickness.
Bitter says the ora serrata is only a conventional limit, from
which point the retina diminishes under an appreciable angle
to reflect itself at the distance of two millimetres on the hya-
loid membrane as a simple vestige. Up to the equator of the
>eye all the retinal layers participate equally in the diminution ;
after this the granular layer and the layer of ganglion-cells
disappear. At the distance of four millimetres from the ora
serrata it becomes thin, and at the ora serrata not a trace of
the nervous tissues remain. At this point the retina is rep-
resented only by its cellular tissue, which also is decidedly
vdiminished at this point.



OF THE HUMAN EYE. 77

The macula lutea is elliptic ; lias a length of 1'".44, and a
breadth of 0'".36. With its inner end it is 1.'" to 1"'.2 from
the middle of the optic nerve entrance. It loses its yellow
color soon after death, and in the fresh eye it can only be seen
with the microscope. The layer of ganglion-cells is increased
in the macula lutea, eight cells being superimposed on each
other. The rods are entirely wanting, and are replaced by
closely packed cones. As regards the granular layer in this
spot, the granules are diminished, whilst the layer of granule-
cells is increased. The fibrous layer has its normal thickness.
The fibres of Miiller converge toward the centre, which un-
usual disposition is explained by the fact that the augmenta-
tion in number of the granular cells and of the ganglion-cells
shows that the fibres of Miiller do not fuse so frequently, as the
number of the elements of the bacillar layer are not greater
than in other parts of the retina. Consequently the elements
of the rod and cone layer, which correspond to a ganglion-
cell, are less numerous. Hence the fibres of Miiller take in
the first place a parallel direction, and then a convergent, in
proportion as they are nearer the centre of the macula lutea.
The increase in number of the granule-cells is so great that
the contiguous parts of the retina do not furnish enough of
the fibres of Miiller, and they are in some measure derived
from neighboring parts. The layer of optic nerve fibres is
very much diminished, and sometimes imperceptible, as all
the bordering fibres end there. Hence, it appears that, ana-
tomically, the distinguishing point between the yellow spot
and other parts of the retina consists in the substitution ot
cones in place of rods. Bitter believes that this particular
arrangement of the cones has some relation to binocular
vision, which man and the monkey alone possess. The views

\J IT

of Max. Schultze on this point have been referred to, and
all things considered, it does seem that he is right in the
opinion that the function of the cones is to distinguish
color, whilst that of the rods is to furnish quantitative light.
The peculiar arrangement of the rods and cones in man, as



78 THE ANATOMY AND HISTOLOGY

well as the facts derived from comparative anatomy, favor this
theory. Vision is most acute in the yellow spot, where we find
cones alone, an increase of granule-cells, and an increase of
ganglion-cells, the latter being eight layers in thickness here.
Still other parts of the retina also possess vision, being most
acute nearest the yellow spot, and gradually diminishing in
acuteness to near the ora serrata.

In the macula lutea, a little toward the .inner extremity
from its centre, there is a colorless, depressed spot of O r// .08
to O'".l in diameter, called the fovea centralis. In this depres-
sion, according to the measurements of Heinrich Miiller and
Schultze, the cones are smaller and thinner than in other
parts of the yellow spot (where they are smaller than in other
parts of the retina), being in the macula lutea 0'".002 to
//r .0024, and at the fovea centralis only 0.0022 millimetres in
breadth. Kb'lliker says the yellow color is produced by a
diffused pigment, saturating the parts of the retina, with the
exception of the bacillar layer. The use of the coloring matter
in the macula lutea is not known positively. Within the past
year Max Schultze (Ueber den gelben Fleck der Retina, seinen
Erufluss auf normales Sehen und auf Farbenblendheit ; Bonn,
1866) published the results of his investigations on this pig-
ment, and he concludes that it is the ends of the rods and
cones imbedded in the chproideal pigment that are the per-
ceptive organs of light, and that all perceived light must pass
through the . yellow coloring matter before it can reach the
perceiving rod and cone ends. This yellow-colored point neces-
sarily absorbs some of the violet rays of light of the spectrum.
It has been determined that the refracting media have but
little to do with the fact that the violet and ultra-violet rays
of the spectrum of the human eye can only be seen by a weak
light, and hence this feeble illumination must have its cause
in the torpid retina ; and the yellow coloring matter in the
region of most acute vision is intended to bring about the
want of irritability of the retina, to modify the bright day-
light, so that we do not prefer twilight to daylight, as the owl



OF THE HUMAN EYE. 79

does, which, has scarcely a trace of yellow coloring matter in
the retina.

Papilla Nervi Optici. The optic nerve entrance is round or
slightly oval, and has an area of 0.44 square lines, with a
diameter of not quite three-fourths of a line.

In the optic nerve itself, the nerve-fibres, with distinct out-
lines, are united in several fasciculi, among which the thick
sheath of the optic nerve sends cellular partitions, which
separate them from each other. The largest part of the cel-
lular tissue of the nerve (and especially the outer, thicker
layer) is reflected on arid blended with the sclerotica. At the
inner sclerotical limit, the cribriform plate marks the termina-
tion of the cellular tissue of the optic nerve. Bonders, how-
ever, asserts that the connective tissue envelopes of the nerve-
fibres sometimes follow them until within the retina. The
lamina cribrosa, as its name indicates, forms a sieve. Its
meshes are a little serrated, and it is in contact partly with
the inner layer of the sclerotica, and with the outer layer of
the choroid.

The elements of the cribriform plate consist of cells, identi-
cal with those of the stroma of the choroid ; sometimes they
are intermixed with pigment-cells, which can be recognized by
the ophthalmoscope. It is tense and slightly concave anteri-
orly. In front of the cribriform plate, or, at least, soon after
having passed that point, the optic nerve loses its sheath, and
also its inner neurilemma, so that the tubules are expanded in
every direction, divested of their connective-tissue envelopes.

Seen through the ophthalmoscope, the papilla appears white,
like the full moon, surrounded by a red, or dark red, ground,
which is the vessels of the choroid. It is sometimes called the
blind spot of the retina, from the fact that it is not suscepti-
ble to the impressions of light, not possessing the retinal lay-
ers essential to the performance of the function of vision,
consisting solely of the optic nerve tubules, covered by the
membrana limitans.

The central vessels of the retina, the arteria centralis retinas,



80 THE ANATOMY AND HISTOLOGY

and the vena centralis retinw, spring from the centre of the
optic papilla, as seen in Fig. 31. About J'" external to the
ring of the optic nerve, outside the eye, the vena centralis sep- ,
arates itself from the arteria centralis, and takes an oblique,
outward direction, whilst the artery continues for some dis-
tance a central direction, until it arrives at the optic nerve
entrance, within the eye, where it bends in a direction oppo-
site to the vein, in an oblique or knee-shaped manner, to
ramify in the retina. In their crossings, sometimes the arteries
are in front, and sometimes the veins, more frequently the
latter. Sometimes, in the centre of the papilla, there is a



FIG. 31.





thin covering of nervous matter between the artery and the
vein, as seen in Fig. 32, and, as they approach the border of
the papilla, they plunge deeper into the nervous matter, and,
in their ramification forward, the vessels are completely im-
bedded by the expansion of the optic fibres. They continue



OF THE HUMAN EYE.



81



to divide into branches as they proceed forward to the ora
serrata, where they pass into a very fine capillary network.
These vessels diminish so rapidly in size, that, at the ora
serrata, no separate vessel can be distinguished. These vessels
send off- no branches into the vitreous body, or choroid, the
retina possessing its own system of vessels. At the beginning

FIG. 32.




of the zonula Zinnii they terminate in a somewhat imperfect
ring of vessels, the sinus circularis venosu-s retince, from which
the returning veins proceed. The vessels of the retina have
an intimate connection with the cellular tissue of this mem-
brane. As seen by the ophthalmoscope, the arteries are lighter
and smaller than the veins ; the veins are darker, larger, and
more tortuous than the arteries.

The retina still remains a fruitful field for histological and
physiological discoveries. Very much remains to be discovered.
Investigation is needed, and not theorizing. Carl Bitter and

6



82 THE ANATOMY AND HISTOLOGY

Schultze have recently made a real progress in our histological
knowledge of the retina. According to Bitter, the impressions
of light are first made on the central fibres of the rods and
cones, then on the granules, from thence to the granule-cells
and to the ganglion-cells, through the optic fibres, to the brain.
The rods and cones perceive the most minute luminous im-
pressions, which, in the granules, is converted into a nervous
irritation. The granule-cells unite a determined number of
the fibres of Miiller, and give the sense of color. The ganglion-
cells collect all the impressions made on them by a certain
number of rods, cones, granules, and granule-cells, and then
transmit them to the nerve-fibres, through which the whole
is conducted to the brain, where consciousness is enforced.
The ganglion-cells, he thinks, may, perhaps, 'preside over the
conceptions of space and form. Still, our present knowledge
does not enable us to decide whether the ganglion-cells are
endowed with a psychical central action, or whether they pos-
sess simply and purely a reflex action.



The Crystalline Lens.

The lens (lens crystallind) is like an optical bi-convex lens,
and has its only organic connection with the anterior termi-
nation of the hyaloid mejtnbrane, or zone of Zinnius, which
sustains it in its place. It is bounded anteriorly by the iris
and the aqueous humor, and posteriorly by the vitreous body,
resting in a fossa, called the hyaloid fossa.

The lens may be considered a rotational body, of which the
anterior elliptical segment measures, through its larger axis,
4'" to 4"'.l, and its smaller axis 1|'" to 2J'", whilst its pos-
terior surface presents a parabolical curvature of 3}'" to
Vo'" parameter. It is difficult to give the measurements of
the lens, as its diameters constantly vary, as well as the de-
grees of curvature of the anterior and posterior surfaces, the
former varying far more than the latter. The lens is the organ
acted on by the accommodative force, and, consequently, its



OF THE HUMAN EYE. 83

relations to the solid parts surrounding it constantly vary. In
the living eye, the distance of the poles of the lens is not con-
stant. The distance from the anterior pole of the lens to the
middle of the cornea is V" to 1J"', and from- its posterior pole
to the posterior pole of the retina 5f "' to 6f '". The weight of
the lens is 4 to 4J grains.

In consequence of the variations of its radial curvatures, as
regards other parts of the globe, it is somewhat difficult to
determine accurately its relative location. Pilz lays down the
following rules:

A line drawn through the anterior pole of the lens, perpen-
dicular to the eye-axis, touches the insertion of the tensor
choroidecB, and is V" distant from the centre of the cornea.

A line drawn through the posterior pole of the lens strikes
the origin from the choroid of the corpus ciliare, and measures
8'", and at its centre is 3J r// distant from the middle of the
cornea, and 6| r// to 7f '" from the posterior pole of the eye.

Two-thirds of the processus ciliares fall anterior to a line
drawn through the equator, perpendicular to the axis of
the eye.

The iris rests on the lens, and its degree of anterior curva-
ture is dependent on the extent to which the lens projects.

Helmholtz says that the axis of the lens does not correspond
with the anterior pole of the cornea, as the centre of curvature
of the latter is located to the nasal side of the lens-axis.

The lens is a perfectly transparent body, and is to be dis-
tinguished into the capsule and the lens proper.

The capsule of the lens (capsula lentis) is a perfectly trans-
parent, structureless membrane, elastic, yet easily torn, and
surrounds the lens like a mould, completely inclosing it on all
sides. The capsule of the lens measures, at its anterior wall,
0'".005 to 0'".008, which thickness does not extend as far back
as the equator of the lens, but immediately behind the attach-
ment of the zonula Zinnii it becomes thinner, and measures
only 0"',002 to 0'".003.

The thickness of the lens-capsule varies very much T and



84



THE ANATOMY AND HISTOLOGY



FIG. 33.



seems to increase with old age. It is always thinnest at the

posterior pole, in the adult measuring at that point, according

to Becker, 0.009 mm.

Although the lens-capsule is a continuous membrane, yet,

for the sake of convenience, it is described as consisting of an
anterior and a posterior capsule. Histol-
ogists do not agree on the structure of
the capsule. Kolliker, Pilz, and Becker
assert that it is striated, which indicates
that it is lamellated. Becker says that
this striated appearance is seen in the
larger ruminants. It was seen by Ley-
dig, in the heifer, and by Kolliker, in
man. Becker asserts that, when suffi-
ciently magnified, this superficial stria-
tion is always seen. On the contrary,
Bitter, Stellwag, and Hulke confidently
assert that the capsule is wholly struc-



Section of the capsule of

e lens of a calf, at the lo- tureless. The latter writer says that the

the embryonic



cality w

-cells are situated.



fied 230 times.

(From Becker.}



converging Uneg

from the attachment of the suspensory
ligament, for some distance toward the
anterior pole, and the similar, but fainter, lines upon the outer
surface of the posterior half of the capsule, are due to the
peculiar arrangement of the fibrous cordage of the suspensory
ligament. When torn, the capsule contracts by its own elas-
ticity, and curls in, so as sometimes to permit the lens to
escape spontaneously.

The posterior surface of the anterior capsule is lined by a
delicate layer of pavement epithelium, which is located be-
tween the capsule and the lens proper.

Becker says that, strictly speaking, it is not correct to say
that the posterior wall of the anterior capsule is lined by an
epithelial membrane, inasmuch as beneath the attachment of
.the zonula are found irregular granules of various sizes, and of
clearly defined outlines, closely packed, and which have around



OFTHEHTJMANEYE. 85

them but a small quantity of protoplasma. They often possess
distinct divisions. Everything points to the fact that the
bodies in question are quite young, and, to a certain extent,
imperfect cells, which are destined for a further metamorphosis,
and hence are named embryonic cells (bildungs zellen). To-
ward the anterior part of the capsule, these cells gradually are
changed into a pavement epithelial covering ; and backward,
toward the equator of the lens, are found the small round
cells, which, still further back, sprout out into true lenticular
fibres, as will be explained hereafter.

These nucleated epithelial cells also perform an important
office in connection with the nutrition of the lens.

Immediately opposite, on the outer side of the capsule, are
placed the processus ciliares, which are made up in structure,
almost wholly, of vascular loops ; and osmotic circulation will
readily take place through so permeable a membrane as is the
lens-capsule.

Hulke, who seems to have very thoroughly investigated the
human lens, agrees with Becker, and, for convenience of de-
scription, divides the anterior hemisphere of the capsule into a


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