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The anatomy and histology of the human eye online

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the so-called ligame.ntum iridis pectinatum (Bowman and Kol-
liker). It will thus be perceived that the iris has no peripheral

FIG. 13.

FIG. 13. Union of the Membrnna Descemeti with the Hgamentum iridis pectinatum.
a. Membrane of Descemet. b. Tendinous ring or wall of canal of Schlemm. c. Liga-
mentum pectinatum expanded on the anterior surface of the iris. (From Mam.)

attachment, but is immediately continuous all around w^ith the
membrana Descemeti. All the fibres of the membrane of Des-
cemet at the margin of the cornea, do not turn in to be ex-


panded on the anterior surface of the iris, a part of them only
assuming this direction ; another portion of them pass into the
anterior two-thirds of the wall of the canal of Schlemm. The
nature of these fibres has not been settled. They have been
considered as connective tissue, as serous fibres, in part elastic,
"and partly cellular, and as intermediate between the elastic and
connective tissues (Henle, Luschka, Bowman, Kolliker, Pilz).

The vascular system of the iris is intimately connected with
that of the ciliary body and the choroid. We are indebted to
Leber for recent clear elucidations on the circulation of the
iris, ciliary body, and the choroid.

The iris is supplied with blood from the arterm ciliares pos-
terior es long 02, the arterm ciliares anterior es, and the arterm
ciliares postwar breves. The posterior long ciliary arteries are
two in' number, one passing forward on the inner side of the
eye, and the other on the outer side. They perforate the
sclerotica a little anterior to the posterior short ciliary arte-
ries, passing it so obliquely that there generally intervenes a
space of several lines between the points of entrance and exit.
They run forward between the sclerotica and the choroid, en-
veloped by the lamina fusca, to the posterior border of the
ciliary muscle, where they divide into two branches, which
rapidly divide into smaller branches.

The arteri& ciliares anteriores are branches from the arteries of
the four straight muscles : (exceptionally one arises from a palpe-
bral artery.) They pass through the tendons of the muscles to
the sclerotica, and run toward the cornea, near the margin of
which they perforate the sclerotica somewhat directly from
without inward to the ciliary muscle, where they join the
branches of the long posterior ciliary arteries to form the cir-
culus arteriosus iridis major. This is a complete, closed, vascular
ring, in some places being double, and in others threefold. It
is situated in the ciliary muscle, in its anterior border near the
periphery of the iris. From this arterial circle, arteries pass
forward to the iris, inward to the ciliary processes, and back-
ward to the ciliary muscle and the choroid. The arteries



passing to the iris and to the ciliary processes generally arise
from one common trunk, which divides into two branches, one
proceeding to the iris and the other to the ciliary processes.

Besides the large arterial circle of the iris, the branches of
the posterior short ciliary arteries, and the anterior ciliary
arteries, form an imperfect circle or row of anastomoses far-
ther back in the ciliary muscle, from which proceed the arte-
ries of the ciliary muscle, and the arteries passing back to the
choroid, which are generally from ten to twelve in number.
It will thus be perceived that, according to Leber, the ciliary
body and the iris are much more independent of the short
posterior ciliary arteries than has ordinarily been supposed.
The ciliary muscle possesses a very large number of small ves-
sels, which spread within it in an arborescent manner, and form
a quite close capillary network, the mass of which follows the
direction of the radiary muscular fibres in a parallel manner
on its external surface. On the internal surface it is more
irregular. The arteries of the ciliary processes and of the iris
generally originate in one common trunk from the large arte-
rial circle of the iris. Those of the former rapidly pass into a
mass of fine twigs, which anastomose freely, and pass to the
free border of the processes in a curved manner, where they
terminate in the commencement of the veins, which seem
larger than the arteries.

The arteries of the iris form in it a loose capillary network,
especially on the sphincter pupillce. Some branches form at a
certain distance from the pupillary border the well-known cir-
culus arteriosus iridis minor, which is, however, not a perfect
circle. At the pupillary border the fine arterial branches pass
in a looped manner into the beginning of the veins.

Veins of the Corpus Ciliare and the Iris. The blood from the
vascular membrane is carried off through the vasa vorticosa, the
vence ciliares posterior es, and the vence ciliares anterior es. The
vencB ciliares posteriores breves are small, and, according to Leber,
receive blood only from the sclerotica ; and none from the
choroid. The vasa vorticosa have been described. The long


posterior ciliary veins, Leber says, cannot be found, as deline-
ated in works on the anatomy of the eye. Briicke says they are
somewhat less in size than the arteries, and accompany them.
The blood from the vascular membrane, then, is carried off
by the vasa vorticosa, with the exception of a small portion
which is conveyed through the venae ciliares anticce, being much
less in quantity than the arterice ciliares anticce carry into the
eye. The veins of the iris run back between the ciliary pro-
cesses, to empty into the vence vorticosce. Leber never saw any
of the veins of the iris empty directly into the canalis Schlemmii,
as has generally been taught. They are joined by the veins
from the ciliary processes, and by some also from the ciliary
muscle. The veins of the ciliary processes arise from the vascu-
lar network of that structure, a larger vein generally running
along the free border of the process. In the space between the
processes, the veins returning from the iris run back, and, with
the veins of the processes, form a venous network, which lies
immediately beneath the inner surface of the ciliary body, the
trunk passing over the smooth part of the corpus ciliare to the
choroid, and at the border of the latter membrane it passes to
the outer layer thereof. The veins of the iris and the ciliary
processes pass inside of the ciliary muscle, and are clear of it,
whilst the arteries running to these parts all have to pass
through the muscle. Consequently, during the contraction of
the ciliary muscle, for accommodation for near objects, the
compression of the arteries lessens the amount of blood thrown
into the iris and processus ciliares, whilst the veins are left free
to disgorge themselves. It has been the common opinion that,
during accommodation for near objects, the ciliary processes
become engorged, which, from the arrangement of the vessels
in the parts concerned, is not possible. In the observations of
Otto Becker, recently made on albinotic persons, he observed
that, during the contraction of the ciliary muscle for adjust-
ment for the near point of vision,- the ciliary processes receded
and diminished, and that, in adjustment for the far point of
vision, they increased in size.


As regards the vence ciliares anticce wlietlier some derive
their blood immediately from the so-called canalis Schlemmii or
from the ciliary muscle, has been a disputed matter among
authors. It has generally been believed that some of the veins
of the iris empty into the canal of Schlemm, and that the venw
ciliares anticce convey it from thence out of the eye. Briicke,
who is very high authority, takes this view. In Note No. 27,
annexed to his Treatise on the Eyeball, he says that this is the
only instance in which he did not follow the results of his own
investigations, but that he followed Arnold and Retzius, and
that he could never succeed in tracing any veins from the iris
into the canalis Schlemmii, nor inject any, from the canal out-
ward, with mercury. Leber could never trace any of the
veins from the iris into this venous sinus. As regards the
canalis Schlemmii itself, it has been considered as a venous
sinus, regularly round, and belonging to the system of the vence
ciliares anticce. The anterior two-thirds of its wall consists of
elastic tissue, the fibres of which originate from the membrana
Descemetii, and the posterior third is a lamella of tendinous
substance, which springs from the sclerotica, and is identical
with it in texture. Its posterior wall is much thicker than its
anterior. At the point where the elastic and tendinous tissues
unite, the ciliary muscle and the radiary fibres of the iris arise
from a common origin. From the anterior elastic portion of the
wall of this canal the elastic fibres start that form the lig amen-
tum iridis pectinatum. Its horizontal diameter, according to E.
Jaeger, is 0"'.3058, and its horizontal plane, from side to side,
measures 12'".5666. It is situated in the wall of the sclerotica,
and much nearer its anterior border, quite close to the corneal

Leber asserts that it is not a circular canal, as has been
taught, but that it is a circular venous network, in the innermost
layer of the sclerotica, and immediately external to the in-
sertion of the ciliary muscle. He thinks it ought to be de-
nominated plexus ciliares venosus. In this plexus of veins there
are, at certain points, 6 to 7 veins of nearly the same thickness,


which, by free intercommunication, form a close network of
veins. In other parts of the circumference one or two large
veins are accompanied by a few smaller, which freely communi-
cate with each other. This circular plexus seems, at first view,
like a canal, but close investigation, Leber says, will prove that
it is not. The loose connective tissue around the veins will
permit a small probe to force a passage. In some places small
islands are seen, formed by a vein running out and returning
again to the main trunk.

The anterior portion of the ciliary muscle sends forward
and outward numerous small venous trunks, which perforate
the sclerotica as vence ciliares anticce. Other small veins enter
the posterior wall of the canalis Schlemmii, or, rather, plexus
ciliares venosus, and at each point of the entrance of a vein
from the ciliary muscle, several small branches perforate the
sclerotica, to form anterior ciliary veins. But all the blood
passing off through the vence ciliares anticce is much less than the
amount thrown into the eye through the arterice ciliares an-
teriores, the larger portion escaping through the vence vorticosce.
Hence, it seems, according to Leber, that the so-called 'canal of
Schlemm receives blood only from the veins of the anterior
part of the ciliary muscle ; that it has no direct communication
with the anterior ciliary veins, but through its elongations to-
ward the ciliary muscle ; and that it must be considered a
venous reservoir for the ciliary muscle, into which blood can
escape during its contraction, and return again when the con-
traction ceases.

The iris and ciliary muscle are supplied with nerves mostly
from twigs from the trigeminus and the oculo-motorius. Some,
however, also are derived from the sympathetic and abducens.
They enter the eye mostly as the nervi ciliares breves from the
ciliary ganglion, and perforate the sclerotica around the optic
nerve entrance, and pass forward through the lamina fusca, to
be distributed in the ciliary muscle, iris, and cornea. There are-
two (sometimes only one) nervous twigs, called nervi ciliares
longae, derived from the ramus nasa ciliares, the first branch of


the trigeminiis. They perforate the sclerotica immediately be
hind the insertion of the musculus trochlearis, and pass for-
ward to the ciliary muscle, in which they ramify with the
short ciliary nerves. In recent years it has been discovered
that many of the nervous branches of the ciliary muscle are
surrounded by ganglion-cells, or by small agglomerations of
these cells (H. Miiller, Liebreich, Krause, Manz). They con-
tribute to the formation of the nervous network in the ciliary
muscle. From this network the iris is provided. They run
somewhat like the bloodvessels, and, like them, form anasto-
moses, arches, and circles, one of which corresponds with the
drculus arteriosus minor, and another circle is formed a little
more outward. These nerves being motor nerves, it is proba-
ble that they lose themselves in the muscular structure.

We know that the third pair of nerves sends twigs to the
ciliary muscle and iris, as paralysis of that nerve causes paraly-
sis of accommodation. We know also that there are both
sympathetic and cerebro-spinal nerves in the eye, from the
fact that the antagonistic action of opium and of belladonna
are witnessed, the former causing contraction of the pupil and
spasm of the accommodation, whilst atropin causes dilatation
of the pupil and paralysis of the accommodation, the cerebro-
spinal nerves acting on the circular fibres, and the sympathetic
on the radiary fibres (Graefe).

The iris, considered as a "whole, is a quite soft, loose tissue,
highly yielding ; it can be stretched out more than half with-
out tearing. It has but one border, the pupillary border, the
ligamentum iridis pectinatum being continuous with the mem-
brana Descemetii anteriorly, whilst posteriorly its stroma is
continuous with the choroidal stroma. The pupil is located
J"' nearer the nasal than the temporal side, and varies in
diameter from V" to 3 r// . According to Jaeger the iris is
thickest in the middle, 0'".45 ; at the ciliary and pupillary
borders it is 0'".30. Its color depends on the number and
arrangement of the pigment-cells.

The anterior surface of the iris is divided into two zones by


a zigzag line, of which the outer iris circle is the larger. In
light-colored eyes the small circle is darker than the large,
whilst in dark-colored eyes it is often the lighter in color.
The color varies from gray brown to dark brown. The direc-
tion of the fibres is radial in this zone, which, the nearer they
approach the small zone, the more they separate, and leave
black, oblong spaces between them, which often pass over the
boundary line between the zones. The inner circle proceeds
from below and behind the larger circle, and at the pupillary
border there is a raised ridge or rim derived from the uvea.
The color of this circle is generally gray or bright gray, and
with a dark larger circle it is rust-colored. The degree of
prominence of the anterior surface of the iris varies very
much. During accommodation for the near point of vision,
the small circle projects forward, and the ciliary border is
drawn back. In accommodation for the far point of vision
the reverse of this takes place. When the ciliary processes
are congested they doubtless push the larger circle of the iris
forward. The pupillary border, doubtless, is in contact with
the anterior capsule of the lens, having only the moisture of
the aqueous humor between them. The posterior chamber is
much smaller than the anterior, yet a small quantity of the
aqueous humor is always between the posterior wall of the iris
and the ciliary processes, and the capsule of the lens.

The Hetina (Tunica Retina).

The retina extends from the entrance of the optic nerve,
being in part in continuous connection with it, to within y ff
of the corpus ciliare, near the or a serrata retinas, where its proper
nervous character ceases, and where it is firmly connected
with the choroid. At the point named there is a slight
puffing of the membrane, and when the retina is torn loose
for investigation, a finely serrated edge is left, and hence
the name of or a serrata. The delicate gray membrane, which
has a thickness of 0'".018 to 0'".02, and lies on the inner


surface of the membrana limitans uvece, and is closely connected
with it, called the pars ciliares retince, is a continuation of the
connective tissue fibres of the retina, and not an epithelial mem-
brane, as taught by Hanover, Pilz, and others. The retina is
an entirely transparent substance, and becomes visible by ca-
daveric changes, or by detachment from the choroid, or when
subjected to a hardening solution (Ritter).

It has a thickness in the middle of O'".l, whilst toward the
ora serrata it is thinned out to 0'".04. It has a surface of
about 300 square lines. Externally it is in close contact with
the vascular membrane, and internally with the hyaloid cover-
ing of the vitreous body, over the convex surface of which it
is expanded.

In its structure the retina is a very delicate, complicated
tissue, on which histologists by no means agree. A history of
the various opinions of microscopists on this subject would be
too voluminous for these pages, and perhaps without much
advantage. Recent histologists, however, all seem to agree
that the late Heinrich Muller was the great pioneer in this
lield of labor, and that he laid the foundation deep and solid
for other investigators. It seems that Carl Ritter (Die Struc-
tur der Retina dargestelt nach Untersuchungen tiber das Walfisch-
auge ; UAnatomie de la Retine, written for Wecker's Etudes
Ophthalmologiques, etc.) has. been most successful in this part
of the vast histological domain, and what follows on this sub-
ject is mainly drawn from his labors. ,

Notwithstanding the variations in thickness of the retina,
the following layers can be distinctly traced from without in-
ward :

1. The layer of rods and cones (Stratum bacillorum, Mem-
brana fucoli).

2. The granular layer (outer granular layer).

3. The outer fibrous layer (the intermediate granular layer).

4. The layer of granule cells (inner granular layer).

5. The inner fibrous layer (layer of gray nervous substance,
fine granular layer).


6. The layer of ganglion-cells.

7. The expansion of the optic nerve fibres.

8. The limitary membrane (Membrana limitans retinae).
The external limitary membrane of Max Sdhultze, between the

granular layer and the rods and cones, is not in reality a mem-
brane, and will be considered hereafter. Mixed in with the
nervous structure of the retina there is that kind of connective
tissue that Virchow has discovered in the brain, called neuro-
glia. This connective tissue extends from the internal limi-
tary membrane to the inner surface of the rod and cone layer.
The Nervous Tissues of the Retina. In the retina are found the
terminations of the optic nerve fibres in connection with the
sensorial apparatus on w T hich the luminous impressions are
made. This connection of the apparatus on which the lumin-
ous impressions are made and the optic nerve fibres which
conduct those impressions to the brain, takes place through
the medium of the ganglion-cells. There are numerous sensi-
tive points to each conducting fibre, many rods and cones
being in immediate communication with it, through the
medium of fine nerve-fibres (fibres of Muller), which are in con-
nection with all the nervous layers, which layers run parallel
with the retinal plane. Bitter says that the true nervous
layers consist of

1. The layer of rods and cones.

2. The granular layer (external granular layer).

3. The layer of granule-cells (internal granular layer).

4. Layer of ganglion-cells.

5. Layer of nervous fibres.

The outer and inner fibrous layers are simply layers of con-
ducting fibres, without possessing any peculiar structure or

The Layer of Rods and Cones (Membrana Jacobi}. This is the
outermost layer of the retina, and consists of two kinds of
elements, the rods and the cones. The outer surface of this
layer rests against the pigment-layer of the choroid. The
inner surface is connected with the granular layer. It con-


sists of regularly arranged rods and cone!, placed vertically,
in palisade form, and packed closely together. In some parts
of the retina, the rods, and in other portions, the cones, pre-
ponderate. At the macula lutea there are no rods, but the layer
is entirely made up of cones. At the border of the yellow spot
the rods already predominate, at the middle of the retina
there are still less cones in proportion to the rods, and further
toward the periphery of the retina, near the or a serrata, the
disproportion at the expense of the cones is still more marked.
(See Fig. 14.)

The rods and cones form a single layer, with a thickness
at the centre of the retina of 0'".036, fur-
ther forward only O'^.OSO, and nearer the
periphery it diminishes to 0'".028 (Kolli-

The Rods. The true character of the rods
and cones remains a matter of dispute. The
figures of rods and cones given by different
Baciiiar layer seen from authors vary very much. As heretofore

stated ' u seems that Ritter has been emi -


the boundary of this spot; nently successful in his investigations of

3, from the middle of the , .. -, ,1 r , i

retina; , the cones, or the retina, being the first author to present
the spaces corresponding clearly the character and termination of

to them; b, rods of the ......... , , mi , -r,.,

cones, the terminal sur- Muller's fibres. The figures given by Pilz,
face of which is often sit- Kolliker, H. Muller, Tick, and nearly all

uated deeper than the

ends of the proper rods, who have written on the retina, represent
c. Magnified 350 times. t k e ro( j s or t ^ e cones, or both, as termi-

(From Ki'Utker.)

nating with a broad triangular base on
the membrana limitans inter naz. The subject is an extremely
difficult one to investigate. The idea of a cone or rod, on
which the luminous impressions are made, running through
all the layers of the retina, to terminate with a broad base
in the membrana limitans interna, instead of terminating in
a ganglion-cell, or an optic nerve fibre, is a physiological in-
comprehensibility. Ritter, by his investigations on the eyes
of the whale, has arrived at the conclusion that every rod and


every cone communicates, by means of the fibres of Miiller,
with a ganglion-cell ; and that the optic nerve fibres all termi-
nate in ganglion-cells is a known fact. This offers us a con-
nected, comprehensible view of the different elements of the
retina. The triangular expansions seen in vertical sections of
the retina, terminating by a broad base on the membrana limi-
tans interna, are the connective tissue of the retina, which
starts with a broad base from the limitary membrane, but
narrows so as to pass between the ganglion-cells, in a manner
to be hereafter described.

The rods, says Hitter, are complete cylinders, which measure
in the adult 0.05mm. in length, with a thickness of 0.003mm.
In the fresh state, each one presents a yellow reflection, and
has four well-defined surfaces, of Which the two largest, which
are exactly parallel, are cut by the two smallest at nearly right
angles. At the inner extremity of each rod there is sometimes
seen a fine filament, hardly perceptible.

This filament is always in communication, in
its course, with a granule of the granular layer. t 2

This filament is best seen after the retina has J|
been subjected to some hardening process ; but |
even in the perfectly fresh state, a faint line

J Rods in the fresh

may be seen running upward and downward, state, i. of man.
to terminate in the filament at its ends. The l' t (

Magnified oUU di-

outer extremity of the rod is dilated like a ameters. (From
club ; the inner more pointed extremity forms a
dehiscence. Under careful examination it is discovered that
the rod really has an opening or canal, and that the filament
runs through its middle to the outer extremity, where it ter-
minates in a rounded enlargement. "In man the rods then are
composed of an enveloping membrane and a central filament.

The Cones. The cones are composed of a middle, large portion,
which is granulated, and of two appendages, of which one pro-
jects outward and the other inward. The internal append-
age is continued by an elongated thread, which contains in its
course several granules of the granular layer ; the external ap-



FIG. 16.

Rods hardened in chromic acid.
1. Rods of man, showing the cen-
tral filaments. 2. Rods of the frog,
with central filament and a medul-
lary substance. Magnified 300 di-
ameters. (Ritter.)

pendage ends by a rounded extremity. The length of the cone

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