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The anatomy and histology of the human eye online

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aly in anatomy. Then Briicke has an, inner layer of arteries,
which form the close capillary choroideal network. Also, an
anterior layer of posterior short ciliary arteries, which extends
from the capillary network to the roots of the ciliary processes.
Leber denies the existence of this arrangement of distribution
of the posterior short ciliary arteries, and his patient and suc-
cessful investigations in that direction seem to deserve cre-
dence. After repeated injections of the arteries with glycerine
and sulphate of baryta, and the veins with glycerine and Berlin


blue, he uniformly found that the injected material of the
arteries could only reach the veins through capillaries.

Membrana Chorio-capillaris. The capillary network forms the
inner layer of the choroidea, and extends from the entrance of the
optic nerve to the beginning of the non-fimbriated part of the
ciliary processes. Its meshes, or interspaces, are always radiary,
being always located at the terminal extremity of an artery or
a vein, i. <?., where an artery terminates by division into capil-
larie^3r a vein begins by the union of capillaries, giving rise
in this manner to the beautiful star-shaped figures seen on the
choroid. The meshes are finest in the posterior division of the
choroid, and become larger further forward. In the region of
the ora serrata retince the capillary network ceases, and only a
few capillaries are found between the straight veins of the
smooth portion of the ciliary processes.

Veins of the Choroid. The blood from the vascular tunic is
carried off mainly through the vence vorticosce. A small portion
escapes through the anterior ciliary veins. The posterior short
ciliary veins consist of a few fine venous trunks, which carry off
blood only from the sclerotica, and none from the choroid, as has
been taught by Briicke and others. The long posterior ciliary
veins, corresponding with the long posterior ciliary arteries, as
described in text-books, do not exist. The anterior ciliary veins
will be described hereafter.

The vence vorticosce consist of four trunks, which proceed to
the equator of the eye, and, before entering the sclerotica,
sometimes divide into two branches, so that sometimes six are
counted. They pass out of the sclerotica very obliquely, mak-
ing channels of IJmm. to 5Jmm. in length. In the chqroid,
the veins form the well-known whorls, as the veins approach
it from all directions in a radiary manner. The larger, 4 to 6,
branches, form complete vortices ; the smaller form only imper-
fect ones. The trunk of a vortex receives its larger branches from
all sides ; those coming posteriorly and from the sides are from
the choroidea ; those from the front are from the ciliary body
and the iris. It is not difficult to conceive in what manner



these vortices are formed. The curved manner in which the veins
have, necessarily to reach the vortex, accounts for the radiary
arrangement of those veins. Where the distance between the

FIG. 5. A, Border of the optic nerve.~ B, B. Equatorial region of the choroid. 1. Per-
fect vortices. 2. Imperfect vortices. 3, 3. Anterior venous connecting bows or curves.
4, 4. Parallel veins which enter the first connecting curving veins. 5, 5. Posterior con-
necting venous bows or curves. 6. Capillaries of the choroid. 7, 7. Large branches of
the posterior ciliary arteries. 8. Long posterior ciliary artery. (From Leber.)

vortices is great, the veins midway between them do not bend
over, but proceed in a straight manner, as seen at 4, 4, Fig. 5,
until they reach a curving vein, with which they unite.

The arteries^ and veins in the choroid have, by their close
contiguity, a mutual pressure on each other. By turgescence
of the arteries, the veins are compressed, and by engorgement
of the veins, pressure is made on the arteries.

The choroidea is soft, yielding, and easily torn. In its pos-



terior part it has a thickness of J 5 to 2 y ; in the region of the
equator, ^'". Anteriorly it is a little thicker again.

The woof (texture, stroma) of the choroid is a tissue interme-
diate between cellular and elastic tissues. In some localities the
one, and in other localities the other, predominates. The ex-
ternal part of the stroma of the choroid, in which the vessels
run, consists of nucleated, spindle-shaped and stellate cells,
which are quite irregular, colorless, or brown, with delicate
processes of variable length, which interlace freely, so as to
form a loose membrane, somewhat resembling the fibro-elastic
membranes. In the inner layer of the choroid the connective
tissue is less pigmented, and passes over into a homogeneous
nucleated tissue, quite similar to the elastic lamella of the inner

FIG. 6.

Fio. 7.

FIG. 7. Cell? of the black pigment of man
seen from the surface. (Kolhkftr.)

FIG. 6. Cells of the choroideal network.
a. Pigmented cells, b. Pigmentless fusi-
form cells, c. Anastomoses of the pig-
mented cells. (Kollikftr.)

coat of arteries. The investigations of Yirchow and Bonders
seem to have demonstrated that the stroma under considera-
tion belongs to the undeveloped form of elastic tissue. This
tissue is composed of cells with numerous prolongations or
processes, which form a network with straight and elegant
meshes, as seen in the cellular tissue of other parts, but differ
from ordinary cellular tissue in being pigmented. The black
pigment completely lines the inner surface of the choroid


membrane as a connected cellular lamina, and as far forward
as the ora serrata, as a single layer of beautiful cells, almost
regularly hexagonal, 0.006"' to 0.008"' in diameter (Fig. 7),
and 0.004'" in thickness, disposed in the manner of a mosaic.
The large quantity of pigment in them allows the cell nucleus
to appear only as a clear spot in the interior ; but this nucleus
is seen on a lateral view to be situated in the outer half of the
cells',, where they are poorer in pigment granules (Kolliker).
The pigment is not deposited on the vessels so much as in be-
tween them. It is more abundant immediately around the
optic nerve entrance, than in other parts of the choroid, and
is seen sometimes in a crescentic shape along the edge of the
nerve entrance. This stratum pigmenti uvce lines the whole
vascular membrane from the edge of the pupil to the optic
nerve entrance. The pigment-cells are so filled with fine pig-
ment granules, that their nuclei are seen as light, pale, pig-
mentless vesicular spheroids, with 1 or 2 floating nuclei. This
layer gives to the uvea the color of black silk velvet, to
which it has always been compared. In blonde persons this
pigment is but little developed. This pigment stratum is cov-
ered on its whole extent over the uvea by a structureless mem-
brane, covered by very bright granules. After covering the
posterior surface of the iris, it covers the whole free surface of
the ciliary processes and the choroid. It is attached at the ora
serrata retince, and there it is also connected with the membrana
limitans retinae. Stellwag names it the membrana limitans uvece.
On its outer surface, next to the sclerotica, we find that the
stroma of the choroid retains its characteristics, only the cells
become less numerous, the fibres longer, more fine, and less
resisting to the action of reagents, forming larger meshes.
This part of the choroid is named lamina fusea, but inaccu-
rately so (Manz), as histology teaches us that it is not inde-
pendent. How r ever, in a certain number of instances, it appears
as a continuous membrane, and not only as a layer of loose
cellular tissue. It forms the bed in which lie the vessels and
nerves that proceed to the iris and ciliary body, and it varies



much in thickness in different subjects. It ceases at the ciliary
body, and is gradually lost toward the optic nerve entrance.

Smooth muscular fibres were discovered in the choroid by
Heinrich Muller and by Schweigger. They are found near
the larger choroideal vessels, and are seen as bands of an opaque
tissue, <^ibout half the size of the arteries. On treating these
bands with acetic acid, a crowd of longitudinal nuclei are seen,
altogether similar to the fibre-cells of the ciliary muscle. Their
successful investigation seems to be extremely difficult.

In recent years nerves have also been discovered in the
choroid (H. Muller, Schweigger, Stellwag, Pope, etc.) They
are numerous, and are found only in the posterior part of the
globe, consisting not only of nerves of double contour, emanat-


Ganglion-cells and pale nerve-fibres found in the choroideal stroma.
(From Sckweigger.)

ing from the ciliary nerves after their passage through the
sclerotic, but also of a plexus of pale fibres, the extremities of
ganglion-cells. The emanations of this plexus seem to lose
themselves in the walls of the vessels, and in the bands of mus-
cular fibres described above. Schweigger says that the gan-
glion-cells, pale nerve-fibres, and the smooth muscular fibres,
are found in the inner vascular layer of the choroid, near the


The functions of the choroideal nerves are not known, but
probably they possess a regulating influence on the choroideal
circulation. It is not unlikely that they, in connection with the
smooth muscular fibres, have an influence on the accommoda-
tive process, acting as antagonists to the ciliary muscle (Manz).
The inner surface of the choroid is uniform, until within
2J' r/ from the anterior border of the sclerotica, where we ob-
serve a dentated line, the or a serrata. At this point the chorio-
capillaris ceases, and the second division of the vascular mem-
brane begins.

The corpus ciliare, which is bounded posteriorly by a dentated
line, the or a serrata retince, extends forward to the canal of
Schlemm, and the periphery of the iris. "When viewed from
behind, forward, it forms a beautiful deep-brown ring, from
2 \ nt to 3 /x/ broad, and a little more narrow on the nasal side than
on the temporal. It consists of the musculus ciliaris, of a part
of the choroideal stroma which passes over into the iris stroma,
and of the processus ciliaris.

The musculus ciliaris (Tensor choroidea, lig amentum ciliare)
consists of a layer of radiating smooth muscular bundles, and
near its anterior and inner part, also circular fibres, which,
doubtless, are antagonists to the former. Briicke and Bowman
almost at the same time positively determined the undoubted
muscujar character of this body. The
nuclei in these smooth muscular fibres are
oval, as seen in Fig. 9. The fibres form
small fasciculi, between which are found
cellular tissue, bloodvessels, nerves, and
ganglions. The direction of these bundles
is, from before, backward, with a slight
convergence at their posterior extremities
toward the antero-posterior axis of the
eye. The whole muscle is a rather thick
muscular ring, triangular or arrow-shaped,
its apex extending back as far as the ora

Muscular fibres from ciliary

muscle. (From Brucke.) serrata, whilst the thicker end proceeds



forward to the peripheral insertion of the iris, at the canal of
Schlemm, into the inner wall of which it is inserted by a short,
circular, white ring, w T hich forms a portion of the canal, or
venous simis, named. It gradually increases in thickness from
the apex to its insertion, and gains a thickness of f '" to \' n ', the
whole corpus ciliare being at the same place V" thick.

At its posterior thin portion the outer layer of the stroma of
the choroid divides, according to Briicke, the inner layer with

FIG. 10.

FIG. 10. Ciliary region of the human eye; section made along the antero-posterior
axis of the globe. Magnified 15 diameters.

C, cornea ; c e, epithelial layer of the cornea ; m d, membrane of Descemet ; f,
union of the cornea with the sclerotica.

S, sclerotica ; s e, epithelial layer of the bulb ; episc, episcleral cellular tissue (originat-
ing from the basis of the ocular conjunctiva) ; c s, canal of Schlemtn ; m ri, radiating
fibres of the ciliary muscle ; m c 2 , section of the circular fibres of the ciliary muscle.

/, iris ; lip, pectinate ligament of the iris ; p c, section of the ciliary processes.

-L, lens; c I, cnpsule of the lens; z, zone of Zinnius ; h, hyaloid membrane; c p,
Petit's canal. (From Manz.)

the larger vessels extending on the inner surface of the muscle,
to go to the iris, with the stroma of which it becomes continu-



ous. The outer layer passes over the external surface of the
muscle in the form of fascia. At the anterior border of the
muscle this fascia takes on again some of the characteristics of
the inner layer, and with it passes into a firm network of non-
nucleated fibres of a peculiar structure, which is, as above
stated, inserted into the inner wall of the canalis Sehleimni, as
a short ring-formed tendon of the ciliary muscle. It projects
furthest forward at the posterior part of the inner wall of the
canal of Schlemm, which is the so-called tendon of the muscle.
In its structure we find the fine, smooth fibres, consisting of
fine, granulated, very tender and easily destructible fibre-cells,

FIG. 11.

Non-nucleated fibres forming the tendon of the ciliary muscle. (From

which are somewhat shorter and broader than the ordinary
cells. The pale-gray fibres are so tender, and so easily broken
down, that their investigation is rendered difficult.

Recent investigations seem to prove that a portion of the
muscular fibres also originate from the fibrous net of the aque-
ous membrane. The inner strata' of the muscular ring extends
itself (within the limits of the ciliary processes) toward the
eye-axis, bending into the folds of the processes in such a


manner that their whole outer surface hangs over the muscle-
ends, like a brush.

The circular fibres interlace freely with the longitudinal
fibres, some assuming the arched form, their ends being turned
backward (Stellwag). I am not aware that it has been dis-
covered that these circular fibres belong to the striated class
of muscular fibres, although it is quite likely that they are
under the control of the will.

The inner surface of the muscle is lined by the middle layer
of the choroid, composed of choroideal stroma and the larger
choroideal vessels, and passes over into the stroma of the iris,
with which it becomes continuous. It is somewhat firmly
connected with the muscle by its external surface by a dense
fascia ; on its inner surface it is connected with its posterior
with the processus ciliares, whilst its anterior ^ turns toward
the origin of the iris, and to the tendon of the ciliary muscle,
so as to form, with the ciliary processes, an angular border.
(See Fig. 10.)

The third and innermost division of the corpus ciliare origi-
nates from the inner pigment-layer of the choroid. At the ora
serrata retince, %\" t to 3'" from the anterior border of the scler-
otica, the membrane is seen to swell into ridges, and as these pro-
ceed forward, they gradually increase in size, and constitute the
peculiar structure called the ciliary processes. In the posterior
half of this beautiful, dark-brown ring, the striae are small, so
as not to puff up the membrane much, and is called pars non
fimbriata corporis ciliaris ; the anterior half, being more puffed
up, is called the pars Jimbriata. In the latter division, the
choroideal stroma is intimately connected with the plaited
neck of the hyaloid membrane, the zonula Zinnii, and is quite
rich in pigment, having the appearance of being covered by
black velvet. These projections are from 70 to 80 in number,
gradually increasing in size as they proceed forward, and are
of a whitish-gray color. Each elevation begins an obtuse angle
from the middle layer of the corpus ciliare, and proceeds for-
ward toward the iris, but never coming quite in contact with


it, as some assert. It is difficult to determine the precise rela-
tions the ciliary processes sustain, in the living eye, to the lens
and the iris. Their relative position varies in accordance with
certain intraocular changes. They are placed to the outer and
anterior edge of the lens, but are never in contact with any part
thereof. According to some very interesting observations, re-
cently published by Otto Becker, on the position of the pro-
cessus ciliares, made on albinotic persons, and also on a case of
iridcemia, the processes in no case touch the margin of the
lens. He also ascertained that, during accommodation for near
objects, the pupil contracting, the ciliary processes are dimin-
ished in size, and are drawn backward. The same phenomenon
takes place under the action of the Calabar bean; the pupil
contracts, the ciliary processes diminish in size and recede, and
the eye becomes myopic. On the other hand, when the pupil
dilates, either for accommodation for the far point of vision,
or from the effect of atropine, the ciliary processes enlarge, and
protrude forward toward the optic axis, but without coming
in contact with the lens ; but they lie between it and the iris,
separated by a distinct space. It appears that the processes
do project forward and outward, between the iris and the
lens, into the posterior chamber, without touching the periph-
ery of the lens nor the iris, as seen in the figures of Arlt,
Jseger, Hasner, and Bowman. The cause assigned by Becker
for this swelling of the processes, is that, when the iris is di-
lated, the free circulation of blood through it is somewhat in-
terrupted, and, consequently, it is dammed up in the ciliary
processes, which it* swells up.

Each process has a breadth of T y to J'", a height of f '" to J'",
and a length of f" to If".

In structure they are the same as the stroma of the choroid,
only that here the stellate cells are more rare and more tender,
and that here, with the exception of the base of the processes,
it is but little pigmented.

The entire corpus ciliare is attached firmly with the sclero-
tica only at the tendon of the ciliary muscle, its outer surface


only having a slight vascular connection from the vessels en-
tering and leaving it through the sclerotica. With its inner
surface it is connected with the retina at the ora serrata, and
from that point forward it is inseparable from the hyaloid

The corpus dliare has a breadth at the nasal side of 2J' r/ , at
the temporal 3'", at the posterior edge it has a thickness of
3>V" to iV"> wMst its anterior part is V" in thickness.

The Iris. The iris is the third division of the vascular mem-
brane, the stroma of the choroid being continuous with the
stroma on its posterior surface, and lines it to the margin of
the pupil. It contains, however, elements quite different from
the choroid. It possesses true cellular tissue, which is loose,
and arranged in waving strice. This connective tissue is in
part radiary and in part circular, the latter arrangement pre-
dominating near its ciliary attachment. These bundles inter-
lace with each other very freely. This connective tissue
contains numerous cells, mostly spindle-shaped and stellate ;
more rarely round or connective tissue corpuscles, resembling
the cells of the choroideal stroma, are found, being heavily
pigmented, and with its fine processes they are connected into
a network (Stellwag, Pilz). In. dark eyes these cells possess
brown or black pigment.

In the anterior layer of the iris some of the fibres of the
ligamentum pectinatum iridis are mixed up with the cellular
tissue, but they do not extend beyond the half of the breadth
of the iris. Inclosed within the cellular stroma are bundles of
smooth muscular fibres, bloodvessels, and nerves.

Part of the muscular fibres are circular, and form a sphincter
muscle (sphincter pupillce) in the form of a smooth ring, J'" in
breadth, and located close to the pupil, nearer the posterior
than the anterior surface. Kolliker discovered another mus-
cular ring, very small, not more than the fa'" in breadth, sit-
uated nearer the anterior surface of the iris, close to the annu-
lus iridis minor. The appearance of the muscles of the iris,
under the microscope, is seen in Fig. 12.



The radiary fibres (dilatator pupillce) do not form a continu-
ous muscle, but extend from the ciliary border in a radiary
manner, and are collected into slender fasciculi, which, as they
approach the sphincter, divide, and are inserted into the cir-
cular fibres divergently, forming two bows, sometimes forming

Muscle of the iris as seen under the microscope. (Fioin Ptlz.)

regular arches, as shown in Fig. 12. The fasciculi of the dila-
tator pupillce, after reaching the circular muscle, are quite near
the posterior surface of the iris, immediately in front of the

In recent years the existence of the dilatator muscle of the
iris has been denied (Grunhagen, J.., Ueber Irisbewegung, Arch,
f. Path. Anat. und PhysioL, Bd. xxx, Heft 5 and 6, p. 481).
^ow, there is truth in this, if it is claimed that it is a continu-
ous connected muscle, which it is not. But, as described by
Henle, as a layer of very thin muscular fibres on the posterior
surface of the iris, covered and permeated by pigment-cells,
which renders its discovery difficult, the claim of its existence
must be insisted on.

The origin of the radiary muscular fibres has not yet been
positively determined. It is asserted by many that they origin-


ate from the ligamentum iridis pectinatum, and from the border
of the aqueous membrane of the cornea. Kolliker believes that
they arise from the circular fibrous layer on the inner wall of
the canalis Schlemmi, from which the ciliary muscle also origin-
ates. Stellwag believes that a large portion of these fibres
pass backward, with the iris stroma, vessels, and nerves, to
the space between the ciliary muscle and the outer border of
the ciliary processes, where they expand in a fan-shaped man-
ner. Here the _ outer fibres run in a parallel manner with the
ciliary muscle as far as the ora serrata retince, whilst the inner
fibres tiirn in toward the visual axis, almost perpendicularly
to the stroma connecting the ciliary processes with the zonula,
in which they disappear. He claims to have preparations
which demonstrate this origin of the radiary fibres of the iris.
The consonance of action of the ciliary muscle and the iris
during the accommodative act, would seem to point to some
such intimate anatomical connection between these parts.

Anteriorly the iris is covered by a single layer of round and
flattened epithelial cells, which is an immediate continuation
of the epithelial covering of the posterior surface of the mem-
brana Descemeti.

The posterior covering of the iris is called the iivea, and is
continuous with the choroid and ciliary processes, covers the
whole posterior surface of the iris to the pupillary edge, which
it hems in, so as to be visible on the front of the pupillary
margin. It consists of a thick stratum of cells, which are

O '

round, densely packed, and filled with dark pigment-mole-
cules. Between these pigment-cells and the iris itself is a
membrane, which Kolliker considers the same as the mem-
brana limitans choroidea ; others claim it only to be formed by
the anterior union of the walls of the pigment-cells. This
layer in light eyes is poor in pigment, and reflects blue or
gray ; in dark eyes it is richer in pigment, and gives to the
eye a dark brown or black appearance. In such eyes the pig-
ment is not confined to the posterior tapetum or layer, but is
found in the stroma of the iris, in its anterior epithelium, in



the form of golden yellow or brownish granules. They are
said also to be found free between the muscular fibres and

The manner in which the anterior surface of the iris is
formed by the anterior elastic lamina, or membrana Descemeti,
was first described by Bowman. The membrana Descemeti,
near the border of the cornea, begins to send off from its ante-
rior surface fine elastic fibrils, which constantly increase in
thickness, until at the margin of the cornea the whole thick-
ness of the membrane of Descemet is broken up into thicker
fibres and columns, which turn over on the anterior surface of
the iris to form the anterior fibrous membrane and pillars, or

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Online LibraryAbraham MetzThe anatomy and histology of the human eye → online text (page 3 of 14)