Abraham Metz.

The anatomy and histology of the human eye online

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urably small space filled with a fluid. These fibrillse are united
into larger lamellae about 0.004mm. in thickness, which are
placed into 15 to 20 layers concentric with the corneal surface.
The fibres of each layer run parallel with the corneal surface,
and with each other. The fibres cross each other, at an angle
of about 90, in two contiguous layers resting on each other.
In some places the fibres run from one lamella into another.

Between two contiguous layers are found, distributed at equal
distances apart, a large number of cells. These corneal cells
consist of masses of protoplasma, polygonal in form and without
nuclei. They are vertical to the corneal surface and flattened.
In the centre of each mass is found a vesicular nucleus with a
nucleolus. These masses that surround the nuclei measure
0.02 mm., and have projecting from their corners from six to
twenty processes, which run in various directions throughout
the corneal substance. The majority do not project beyond
two contiguous lamellae ; some, however, pass through the lay-



OF THE HUMAN EYE. 2B

ers at sharp angles. Some of these processes terminate free in
minute points ; others are connected with neighboring cells.
Each cell then is connected with other cells in the same layer,
and also with the cells of the layers above and below, so that
the whole corneal substance is connected by a penetrating net-
work of this protein material. Neither the cells nor processes
have membranes, but lie unenveloped within the inter-fibrillar
spsl5s, which they completely fill.

Besides the stellate bodies existing in the true .corneal sub-
stance, there are found, normally, a number of smaller cells,
without a membranous envelope, which constantly change their
form and locality, called the " wandering cells" They are found
in all the layers of the cornea proper, and do not move about
in bounded channels, but in the interspaces filled with fluid
between the corneal fibres. They seem to push aside ihejibrillce
that come in their way. These " wandering cells " are found
floating through all parts of the substantia propria, which is cer-
tain evidence that the interspaces between the fibrillse are filled
with a liquid substance. This understanding of the histology
of the cornea excludes a system of closed nutritive channels,
as taught by Yon Recklinghausen, B-owman, Samisch, Leber,
etc., which Engelmann claims to be artificial dilatations from
the injections used. This also excludes lymphatic vessels from
the cornea. He bases his claims of superior success in this in-
vestigation on the fact that he made his observations on en-
tirely fresh cornese without any hardening preparation or injec-
tions. The cornese were moistened only in the aqueous humor,
and examined a few minutes after separation from the living
frog.

The " wandering cells " cannot pass through the anterior
elastic lamella, which is more firm in texture than the sub-
stantia propria. There are found " wandering cells " in the an-
terior corneal epithelium, which exist in great abundance.
They are, however, of quite another kind, unlike those exist-
ing in the cornea proper, being much smaller than the latter,
and possess generally from two to three nuclei. Their length



24 THE ANATOMY AND HISTOLOGY

is about 0.01 mm., and their protein substance is granular. In
moving about, they pass through between the epithelial cells.

These masses of plastic material, containing cells with
nuclei, and having anastomosing processes, have been the
subject of most ardent discussions ; and whilst Engelmann's
method of investigation on fresh, living cornese seems to pos-
sess important advantages over the methods of hardening,
coagulating, and injecting, as practised by other histologists,
the time has not arrived for a decision on the character of
these "cornea-corpuscles" of Virchow. It is claimed (His,
Kolliker) that the nutrient canals of Bowman are artificial
productions brought about by his injections.

Hasner, Virchow, Strube, Luschka, and others, believe these
nutrient channels to be lymphatic vessels. Recklinghausen
and Leber, in quite recent publications, believe them to be
lymph channels, having a direct communication with the lymph
vessels of the conjunctiva. Leber made use of turpentine and
dragon's blood for injecting material, and made an incision near
the centre of the cornea. After some minutes the coloring
matter injected was seen following certain channels in the
conjunctiva, which he thinks were the lymphatic vessels.
Leber also claims that these channels possess a distinct en-
veloping membrane of their own. This much seems to be
established concerning these cornea-corpuscles, they are con-
nected with the nutritive process of the cornea.

The anterior elastic lamina (stratum Bowmanni) is found be-
tween the proper cornea and the conjunctival epithelium. At
the present day some doubt its independent existence. At any
rate it seems more rudimentary than the posterior elastic
lamina. Arnold considers it similar to the basement-mem-
brane of mucous tissues. It is coextensive with the cornea,
and Manz says that it is lost at the border thereof (certainly
at the superior and inferior borders), in the manner of the pos-
terior elastic lamina, to become blended with the cellular tissue
of the conjunctiva. According to Bowman it is a vitreous
membrane, perfectly clear, with a tendency to curl inward,



OF THE HUMAN EYE.



25



and it materially aids the cornea in retaining its exact curva-
ture. From its posterior surface a number of cords proceed
into the proper cornea, and into the sclerotica, which bind it
down firmly to those tissues. According to Arnold it has a
thickness of 0.0045 mm. On its anterior surface it is covered
by the conjunctiva! epithelial layer of the< cornea. This consists
of three layers : the first, a hyaline lamella of fused cells ; be-
neaiptrthis, a layer of cylindrical nucleated cells placed upright
and closely arranged ; and lastly, the layer in immediate con-
tact with the anterior elastic lamina, which consists of a layer
of round cells, with large nuclei, imbedded lightly in a viscid
fluid.

FIG. 3.




Vertical section of the Human Cornea near the surface, a. Anterior elastic lamina
f>. Conjunctival epithelium, c. Lamellated tissue, d. Intervals between the lamellae
showing the position of the corneal tubes collapsed, e. One of the nuclei of the larael-
lated tissue, g. Fibrous cordage sent down from the anterior elastic lamina. Magnified
300 diameters. (From Bou-man.)

The thickness of this layer is 0.01-0.05'". It can easily
be scraped off from the layer beneath it, and is readily regen-
erated.

Th,e posterior elastic lamina (membrana Descem.eti, seu De-
moursi) is a perfectly transparent structureless membrane,
which extends as far as the cornea, then divides into numerous
cords or fibres,-which partly terminate at the canal of Schlemm,



26 THE ANATOMY AND HISTOLOGY

and partly on the anterior surface of the iris, in a manner to
be hereafter described in connection with the vascular mem-
brane. It is a vitreous membrane, very hard, but easily torn.
It has a great tendency to curl towards its concavity, and as-
sists in retaining the proper curvature of the cornea, essential
to perfect vision. It is ordinarily called membrana Descemetii.
It has a posterior epithelial lamella, consisting of a single layer
of round cells with large nuclei.

The cornea in its centre has a thickness of about 0.436 '" .
Helmholtz says it is of uniform thickness until near the peri-
phery, where it is increased.

Under intraocular pressure the cornea is flattened and its
radial curvature is increased. The cornea may be compared to
a section of a smaller globe attached to a larger, and the greater
the pressure against the walls of the eye from within, the nearer
does the bulb .approximate a perfect globe. The posterior or
inner wall of the cornea is round, and has a diameter, in all di-
rections, of 5 //r , whilst its external or outer wall measures less
in consequence of the manner of its union with the sclerotica,
its vertical diameter being 4J /r/ , and its transverse diameter
nearly 5'". In penetrating the anterior chamber close to the
periphery of the iris, as in the operation of iridectomy, it is
necessary to make the incision V" behind the corneal border
above, f '" below, and \'" at the sides. The cornea is hard, re-
sisting, difficult to penetrate, but easily split into layers, a fact
realized by all young operators.

The fibrous tunic is poor in vessels, being nourished by a
peculiar system of nutrient canals. The arteries from which
this nutrient plasma is drained for the sclerotica, are the vasa
dliaria anticce, which proceed to the iris and ciliary body ; pos-
teriorly the arteriis ciliare posticis brevibus, which also mainly
supply the choroid.

Bloodvessels of the Cornea. It has been generally taught by
anatomists that the cornea is nourished from the palpebral and
lachrymal arteries, which form the superficial layer of vessels,
and project forwards about l r// , where they form loops. Ac-



OF THE HUMAN EYE. 27

cording to a recent publication by Leber (Anatomische untersuch-
ungen ilber die Blutgefdsse des Menschlichen Auges, Wien, 1865),
this seems to be a mistake. By careful and successful injec-
tions he has demonstrated that it is the anterior ciliary ar-
teries which form the loops at the margin of the cornea, and
that the palpebral and lachrymal arteries do not reach as far
forward as the annulus conjunctivas. The small branches spring-
ing from the fine loops of the anterior ciliary arteries run back
som^listance to meet and anastomose with the peripheral ar-
teries of the conjunctive, so that a belt of vascular network is
formed around the corneal border from which the branches of
the latter (peripheral) are excluded. A more full description
of the vessels supplying the cornea with nutritive plasma is
given in connection with the vascular system of the eye.

As regards the serous vessels of the cornea, described by
Arnold, Hasner, and others, to say the least, their existence has
not been satisfactorily demonstrated. Leber says that, in the
perfectly healthy eye, he has never been able to discover these
supposed serous vessels, and he does not believe in their exist-
ence. He further adds, that the matter does not deserve the
importance ordinarily attached to it, inasmuch as it is an
established fact, that the nutrition of the tissues does not take
place directly through the bloodvessels, but through lymph-
channels permeating through them ; and hence it cannot be of
much physiological importance, whether at the corneal border
there are or are not such fine vessels capable of circulating
serum only.

Nerves. It has not been positively demonstrated that the
sclerotica possesses nerves. Bochdalek believed that he had
traced nerves into its tissue; Luschka, Kolliker, arid others,
could not succeed in finding them. The cornea, however, is

O 7

plentifully supplied with nerves. Kolliker says 20 to 30 twigs
from the ciliary nerves run along the proper cornea ; the fine
branches, however, are directed forward, and in the corneal
centre a network is formed by free anastomoses, immediately
beneath the anterior epithelium.



28 THE ANATOMY AND HISTOLOGY

Schlemm, in 1830, first published the presence of nerves in
the cornea. Since then, the observations and opinions on those
nerves have been extensive and varied. Pappenheim, Luschka,
Valentin, Bochdalek, Engel, Beck, Purkinje, etc., have pub-
lished the results of their investigations, and, quite recently,
His, Samisch, Kiihne, Cohnheim, and Engelmann, have added
much to our knowledge on this subject. According to His and
Samisch (with whom Kolliker, Coccius and Arnold agree), the
nerves of the cornea are derived from the ciliary nerves and
from the nerves of the conjunctiva oculi, and form numerous
threads, which, at first, have a double contour, which, however,
change to a single contour, after the first division. These nerve-
fibres are very pale, and present, from the origin of their course,
numerous nuclei, located within an extremely delicate neuri-
lemma. These nuclei become more rare, the fibres become more
pale, and divide dichotomously in such a manner that the two
branches of the bifurcation run in nearly opposite directions.
The secondary nervous fibres anastomose, and form a network
located quite superficially in the cornea, where His and others
saw the termination of the nerves composing it. Samisch ob-
served the anastomoses of the secondary fibres, as well as com-
munications between the fibres of the first order. He thinks
that the terminal plexus is not constituted by the nucleated
fibres of His, but by a network of still finer fibres. Manz,
Krause and Kiihne have asserted that the plexus is not the
termination of the nerve-fibres, but that extremely fine, pale
fibres end free, after a certain course. Kiihne thinks the free
extremities belong to the order of motor nerves, and communi-
cate with the emanations of the cornea corpuscles. He says:
" Les cylindres axes nus, qui sortent enfin de ses divisions multi-
ples, deviennent legerement granuleux et se combinent continu-
ellement aux filaments du protoplasme de corpuscules de la
cornee. Ainsi il est probable, qu'il n'y a pas un seul corpuscule
(cellule) de la cornee, qui ne soit en combinaison direct e ou indi-
recte avec les elements nerveux. Quand au role des ces nerfs,
nous avons constate, qu'ils sont une espece de nerfs moteurs."



OF THE HUMAN EYE.



29



(Gazette hebdomadaire, tome ix, N~o. 15.) The triangular ex-
pansions at the nodal points of the nervous plexus, regarded by



FIG. 4.




The coarser ramification of the corneal nerves of a new-born child, outer half of the
cornea of the right eye. As far as 1 the stems are dark-colored, they contain nerves of
double contour. The net-like connection of the fibres is not seen in this degree of
enlargement. Magnified 14 diameters. Prepared with acetic acid. (From Samisck.)



30 THE ANATOMY AND HISTOLOGY

Coccius as true ganglia, Samisch considers merely expansions
of the primitive fibres.

Engelmann's observations differ from Kiihne. He says that
from the border of the sclerotica, the distance of two or three
laminae outward from the membrana Descemeti, are found a
large number of pale nerves with dark borders, which pass into
the cornea. A portion of these are to end within the true cor-
nea, and another portion penetrate the elastica anterior, to end
in the anterior epithelium. The dark-bordered fibres, visibly
medullated, are united into" larger branches, pass the border of
the sclerotica, to penetrate the cornea at six or eight places.
Each of these stems contains from five to fifteen dark-bordered
nerve-fibres. At the distance of about 0.3 to 0.5 mm. from the
border of the sclerotica, the medullary part of the nerve van-
ishes, and in their further progress they are completely trans-
parent and clear non-medullated fibres. In the second or third
lamina from the elastica posterior, these nerves ramify in all
directions, so that any one of the larger branches are con-
nected with all the other branches. As long as the fibres pos-
sess the dark-colored medullary layer, they divide only excep-
tionally ; as soon as they become non-medullated clear fibres,
they very rapidly divide, and become finer. The fibres in the
network, in the true cornea, form no true anastomoses, but
connections by contiguity only are found. Each dark-bor-
dered fibre is enveloped by a delicate sheath, containing nuclei.
This sheath, getting thinner all the time, is also continued on
the pale fibres. The nuclei are numerous near the corneal
border, but diminish toward the centre of the cornea ; they
are found only in the crossings of the plexus.

The corneal substance is not alone supplied by the dark-bor-
dered nerves ; there are also quite a number of very fine pale
fibres, which enter the corneal border from the sclerotica. At
many points at their entrance into the cornea they are united,
or, at least, in contact. In number they often amount to sixty.
They generally enter the cornea in the second lamella (count-
ing from within), and mostly remain in this lamina. They are



OF THE HUMAN EYE. 31

distinguished by their extreme fineness and want of nuclei or
visible sheaths. They seem to end mostly in the posterior
lamina of the cornea. All that Engelmann can positively say
concerning the termination of these fine fibres, as well as of the
dark-bordered fibres, is that they are seen as extremely fine
pale fibres, until they can no longer be seen in consequence of
their great fineness.

A large portion of the dark-bordered nerves that enter the
corne^ from the border of the sclerotica, proceed to the anterior
epithelium of the cornea. From the coarse network formed
by the ramification of the nerves in the proper cornea, as above
described, are given off numerous nervous branches which run
forward and penetrate the elastica anterior. They are given
off mostly from the crossings of the plexus, sometimes at a
true angle, and seldom less than at an angle of 60. In num-
ber they vary from forty to sixty for each cornea. The larger
of these branches are either bundles of the finest pale nerves,
or undivided pale thick axis-cylinders, which latter are found
mostly near the corneal border. When the nerve-fibres have
reached the elastica anterior, they perforate it in a vertical
manner. The perforations made by the nerve-fibres through
the anterior elastic lamina, Engelmann names nerve-pores. No
twigs are distributed in the elastic lamina. All the fibres that
have penetrated the elastica anterior consist wholly of the con-
tinuation of axis-cylinders. They ramify without nuclei or
sheaths, beneath and between the cylindrical cells, the layer
immediately in front of Bowman's membrane. They ramify
freely between and beneath the cylindrical cells, so that a some-
what close nervous network is formed. They do not appear to
reach the anterior layer of flattened cells, but terminate free
between the cells of the middle or posterior layers of the
epithelium, without end organs or a terminal network. As
already stated, the fibres terminate invisibly fine, and free
among the cells. It seems (according to Engelmann) that the
latest published results of Kolliker's ( Ueber die Nervenendigun-
gen der Hornhaut. Aus der Wiirzburger naturwissenseh., Bd. vi,



32 THE ANATOMY AND HISTOLOGY

1866) investigations of this subject coincide mainly with the
above views of Engelmann.

Kolliker also says that there are both an intra-corneal as
well as an epithelial expansion of nerve-fibres. He, as also
Engelmann, could not find any connection between the intra-
corneal nerves and the cornea-corpuscles.

The sclerotica may be compared to an ellipsoid, with its an-
terior segment cut off. When the anterior, open part is con-
sidered a plane, then its antero-posterior axis measures about
9J'", its vertical diameter at the equator is 10"', and the hor-
izontal 10J'".

The curvature of the outer corneal surface is considered as
the segment of a rotational ellipsoid, with a radius of 3.456 r ",
whilst the curvature of its inner wall has been considered as
the section of a revolutional paraboloid with the half para-
meter of 2'" (Pilz, Yolkmann, Kohlrausch, Stellwag).

The cornea is the most important part concerned in the re-
fraction of light. Its anterior surface may be considered the
section of a globe, with a radius of about 3J r// (Hasner). Its
refracting index is, according to Krause, 1.342, and is nearly
equal to that of the aqueous humor. It seems that its posterior
surface, which is frequently irregular, has but little influence
on the passage of the rays of light. Stellwag says the cornea
casts a focus 5'" behind the retina. Hasner gives its posterior
focal distance as 30.61 mm. (a little less than 15 r// ) ; and its an-
terior focal distance 22.81 millimetres (a little less than II 7 ").
The cornea is firm, and does not readily change its curvature.
Its power in directing the rays of light toward the axis of the
eye is great, acting as the object-glass of the eye, and the lens
as a collecting-glass.

Ttie Choroid (Choroidea, Tunica Uvea). The vascular tunic of
the eye extends from the entrance of the optic nerve to the.
pupil, thus lining all of the interior of the eye, with the* ex-
ception of the optic nerve entrance and the anterior chamber.
It consists of three divisions. The first and largest division
extends from the optic nerve entrance to a point a little in



OF THE HUMAN EYE. 33

front of the equator, and 2, 5'" to 3'" posterior to the anterior
border of the sclerotica, in the vicinity of the ora serrata retince,
and is called the choroidea.

The second division begins at the ora serrata retinas, where the
vascular membrane becomes thicker, and extends forward 2,
5'" to 3 r// to the junction of the sclerotica and cornea, called the
corpus ciliare.

The third division begins at the anterior border of the corpus
ciliare, and proceeds vertically toward the axis of the eye, as
far as the pupillary margin, and is called the iris.

Tfte bulk of the choroidea consists of a close, vascular net-
work, imbedded in a stroma of cellular tissue.

The arterial supply of the vascular membrane (choroidea,
ciliary body and iris) is derived from the ciliary arteries, the
posterior ciliary arteries being direct branches of the ophthalmic
artery, and the anterior ciliary arteries, which are branches from
the arteries supplying the straight muscles of the eye. We are
indebted to Leber for a clear description of the bloodvessels of
the vascular membrane, and we shall follow him in our descrip-
tions of the vessels within the eye.

The posterior ciliary arteries must be further divided into
the short posterior ciliary arteries, which are distributed only
within the choroid itself, and the long posterior ciliary arteries,
which proceed directly to the ciliary muscle, running in their
course between the choroid and sclera. In the ciliary muscle
the long posterior ciliary arteries form a connection with the
anterior ciliary arteries, to supply with blood the corpus ciliare,
the iris, and the anterior portion of the choroidea. The vascular
membrane, then, has an anterior and a posterior system of
bloodvessels, which are not independent of each other, but form
free connections. The anterior region is supplied by the long
posterior and the anterior ciliary arteries, and the posterior
region is supplied by the short posterior ciliary arteries.

The short posterior ciliary arteries proceed from the ophthalmic
artery in 3 or 4 small branches, which pass to the posterior part
of the sclerotica, around the optic nerve, to divide into numerous

3



34 THE ANATOMY AND HISTOLOGY

branches, to be distributed to the choroid, the posterior part of
the sclerotica, and the optic nerve at its entrance into the scle-
rotica. The branches destined for the choroid are about 20 in
number, which perforate the sclerotica in a direct manner
around the optic nerve. After having perforated the sclerotica,
the short ciliary arteries begin to divide, mostly at acute angles,
into smaller branches. They run for some distance in a tortu-
ous manner, in the outer layer of the choroid, but soon pass in-
side of the thick layer of the veins into the deeper layers of
the membrane. During their course they constantly give off
branches to the inner layer of the choroid, the capillary net-
work, where their terminal branches finally disappear. The
larger the branches are, the further forward they proceed for
distribution. The small branches that penetrate the eye close
to the optic nerve, supply only the posterior part of the choroid ;
whilst the larger branches pass forward with its twigs, as far
as the capillary network (membrana chorio-capillaris) which ex-
tends to the ora serrata. Only a few of the branches of the
short ciliary arteries pass beyond this limit, to anastomose with
the branches of the anterior and long ciliary arteries, to form a
connection with the anterior and posterior systems of vessels.

For years the text-books have been following Briicke in the
manner of distribution of the bloodvessels of the choroidea,
who describes an outer layer of arteries which do not termi-
nate in capillaries, but after having, through division, reached
a certain degree of fineness, they curve around, to end imme-
diately in the branches of the vence vorticosce, to form an anom-


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Online LibraryAbraham MetzThe anatomy and histology of the human eye → online text (page 2 of 14)