Maximilian Salzmann.

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

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bending is visible ophthalmoscopically as a hooked curve, the character-
istic index of the cilio-retinal vessel.

Cilio-retinal (or, as Elschnig calls them, retino-ciliary) veins have
been anatomically demonstrated in only a single instance (Kuhnt, 131);
in this case the vein entered the sclera.

In the older ophthalmoscopic observations, especially those of Nettleship (165),
there is much discussion about cilio-retinal veins. It has, however, been emphasized
by Elschnig (50) that cilio-retinal veins are very much more rare than cilio-retinal
arteries and, independent of Elschnig, I have come to the same conclusion. The well-
established retino-ciliary veins are often associated with other similar anomalies, e.g.,
with optico-ciliary vessels (Elschnig, 50) or with abnormal vortex veins (Czermak, 35).

The cilio-retinal vessels are relatively frequent; Lang and Barrett (133) found them
in 16.7 per cent of eyes, I, in 16.4 per cent. Elschnig (50) estimates their frequency
at only 7 per cent, but possibly has in mind only the larger vessels.

Most frequent are the small macular vessels (n per cent); these appear at the
temporal border of the papilla, and go directly to the fovea. Vessels of a caliber such
that the direction of their current can be determined with certainty ophthalmoscopically,
are found in some 6 per cent and are almost exclusively arteries. They often represent
those branches of the second or third order which circle about the fovea in a bow. Still
larger arteries are rare; they may have the dignity of an arteria papillaris and then their
place of origin is displaced just as much farther above or below as the vessel is large.
Upon one occasion, I observed the complete absence of the central artery and the sub-
stitution of two cilio-retinal arteries for this vessel, as has also Bloch (26).



1 84 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

The great majority of the cilio-retinal arteries occur in the temporal half of the
circumference of the optic nerve and supply part of the temporal half of the retina.
Nasal cilio-retinal arteries are very rare and often associated with anomalies of the
papilla.

The long posterior ciliary arteries (art. ell. poster, longae) are charac-
terized by larger caliber and course in the horizontal meridian; there are,
therefore, only two such arteries, one on the nasal, one on the temporal
side. The long posterior ciliary arteries pass through their emissaria
(p. 1 8) and the perichorioidal space (p. 51) without giving off branches,
press into the ciliary muscle at its posterior border, and divide therein.
These branches reach to the anterior surface of the ciliary body and there
bend about along the root of the iris, but go over into the circular direction
above and below, always coursing in the ciliary body. By union of these
branches, as well as through anastomoses which bridge across the divi-
sions, an arterial circle is constructed the circulus iridls major.

The anterior ciliary arteries (art. clliares anterlores) come from the
straight eye muscles, and accompany each tendon in pairs; the m. rectus
lateralls is usually accompanied by only one artery. The little trunks
pass over the insertion of the tendon in the episcleral tissue with great
tortuosity to within a distance of 3 to 4 mm of the cornea; then they
divide into superficial and one large perforating branch. The former
supply the episcleral vessel net, the border loop net of the cornea, and
the bordering zone of the scleral conjunctiva.

The perforating branches pass through the sclera steeply, often at
almost a right angle (emissaria; cf. p. 18), and then at once enter the
ciliary muscle. It anastomoses there, partly with the long posterior
ciliary arteries or its branches, partly with the circulus Irldls major.

The system formed by the long posterior and the perforating branches
of the anterior ciliary arteries supplies the anterior half of the uveal tract,
the ciliary muscle first, then the orblculus ciliaris, and the anterior portions
of the chorioidea by means of backward-coursing branches (art. recur-
rentes], while the ciliary processes (p. 115) and the iris (p. 133) are supplied
by the circulus Irldls major.

The vortex veins (venae vortlcosae] are the most important of the veins
of the ciliary system (cf. pp. 9, 18, 52). They carry away almost all the
blood from the uveal tract the blood of the chorioidea, of the ciliary
processes, and of the iris and, moreover, for the greater part, that of the
ciliary muscle. The blood takes another course only in the anterior parts
of the ciliary muscle, i.e., through small veins, the anterior ciliary veins
(venae clliares anteriores}, which go from the ciliary body over into the
sclera just behind the scleral roll, take up there the drainage of the



THE VESSELS AND NERVES OF THE EYEBALL 185

Schlemm's canal, and finally attain the episcleral tissue in the neighbor-
hood of the border of the cornea. Here they drain the marginal loop net
and the neighboring conjunctival zone, and with these and the episcleral
vein form a richly divided net, which, like the vortex veins, empties into
the orbital veins.

The posterior portions of the episcleral tissue have their own small
veins in the neighborhood of the optic nerve. The neighborhood of the
optic nerve is very poor in veins as a result of this, and larger veins do
not, in general, normally occur here. The posterior ciliary arteries, which
course in the dural sheath, are only occasionally accompanied by large
veins (Elschnig, 52).

Abnormal vortices have occasionally been observed in myopic eyes at
the border of the chorioidal foramen, more rarely in eyes of normal form.
Whether or not they are actually more frequent or can only be more easily
seen with the opthalmoscope in such eyes (on account of the atrophy
of the pigment epithelium), remains still to be decided. Most of the cases
have been observed ophthalmoscopically only. Axenfeld and Yamaschita
(12) alone have made a short report of such an anatomic finding.



There is not much to be said concerning the vessels of the eye, his-
tologically; their structure corresponds to the type. The muscularis
of the arteries is more weakly developed inside the eye than in the orbit;
this probably depends upon the fact that the walls of the intraocular
vessel have only to bear the difference between the blood pressure and
the intraocular pressure. One often comes upon the statement that the
muscularis is absent in the arteries (in the retina and iris). This is not
correct; smooth muscle-fibers of the wall can be followed even into the
finer branches.

The veins are everywhere provided with perivascular sheaths; in
general, their wall consists of connective tissue; muscle-fibers appear only
in the vortex veins in the neighborhood of the outer surface of the sclera.
According to my observations, these are directed crosswise or obliquely.

b) Lymph Passages

True lymph vessels occur only in the scleral conjunctiva, not, however,
in the eyeball itself, nor in the orbit. On the other hand, larger spaces
are present, which, among other purposes, serve to a greater or lesser
extent for the movement of lymph. In this category belong the inter-
vaginal space of the optic nerve, the perichorioidal space, and especially,
the posterior and anterior chambers.



1 86 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

In times past one laid very great weight upon the results of injections,
and all the pictures which arose with a certain regularity were attributed
to preformed lymph channels. But how very little such experiments
prove is shown by the example of the cornea, in which the idea of pre-
formed channels for the lymph stream has been entirely given up. Even
a splitting of tissue can occur systematically, especially where various
component parts border upon one another.

The investigation of the lymph circulation and its paths now demand
wholly other methods, which do not fall any more within the province
of anatomy and histology, I must, therefore, refer the reader to the proper
treatises, especially that of Leber (138), for an extended presentation.

c) Nerves of the Eye (N. ciliares)

Heretofore, posterior and anterior ciliary nerves were distinguished.
Axenfeld (10) has, however, shown that these so-called anterior ciliary
nerves are in most cases really posterior ciliary nerves which only vary
from the typical in their course. 1 The posterior ciliary nerves are, there-
fore, in short, the ciliary nerves. They spring partly from the ganglion
ciliare (n. oil. breves), partly from the n. naso-ciliaris (n. til. longi}; the
latter carry sensory nerves to the eyeball, the former, a mixture of three
kinds of fibers sensory, motor and sympathetic fibers. The long ciliary
nerves also lie in the neighborhood of ganglion ciliare and unite with the
short nerves in the neighborhood of the optic nerve inside the sclera.

As a result of this, it is possible to render the eyeball completely insensible by
cutting the ciliary nerves at their entrance into the eyeball (neurotomia optico-ciliaris) ,
also by injection of cocaine in the region of the ganglion ciliare (method of regional
anaesthesia of Elschnig; see Loewenstein, 145).

The number and, corresponding to it, the size of the nerve trunks
before their entrance into the sclera seems to be subject to a wide variation.
One can usually make out about ten of the larger nerves.

In the orbit the ciliary nerves show, outermost, a connective-tissue
coat (neurilemma), after which there follows a second coat of flat proto-
plasmic cells several layers in larger nerves and mixed fine collagenous
fibers. According to Gutmann (82), the nerve-fibers are almost exclu-
sively medullated; according to Hahn (84), they are exclusively so;
their caliber varies from 20 mu down to the greatest fineness. Most fre-
quently I find fibers of 7 to 12 mu thickness, between which the fine fibers
lie in small groups. All have nucleated (Schwann) sheaths. The spaces
between the nerves are filled out by fine collagenous fibers.



1 At the most genuine anterior ciliary nerves can occasionally be derived from the n. naso-ciliaris.



THE VESSELS AND NERVES OF THE EYEBALL 187

Furthermore, the ciliary nerves frequently contain ganglion cells in
the neighborhood of the sclera, indeed, even in the latter itself, and since
these ganglion cells often form little groups one may speak of accessory
episcleral ciliary ganglia (Axenfeld, n).

In the neighborhood of the optic nerve one finds many small nerve
branches in the sclera (often consisting of only a few fibers) ; the course
of these branches is very irregular. The larger ciliary nerves, however,
course in very oblique emissaria (p. 18). One ciliary nerve regularly
accompanies each of the long posterior ciliary arteries. While still within
the emissarium this gives off a weaker branch, which crosses the ciliary
artery, so that the artery at its entrance into the perichorioidal space is
accompanied by two nerves (p. 51).

With the entrance into the emissarium the ciliary nerve loses its
connective- tissue coat; the cell-coat is also reduced in its further course
through the perichorioidal space to a simple layer of flat cells. In the
place of them suprachorioidal lamellae appear as coats of the nerves in
this space; the nerve itself has an elliptical cross-section.

The ciliary nerves traverse the perichorioidal space in a meridional
direction and give off branches to the sclera and to the chorioidea while
doing so (p. 51). The scleral branches, however, only partly supply
this tunic (p. 25); in addition they bore through the sclera and course
to the cornea through the episcleral tissue or the sclera itself. On cursory
examination, these branches seem to be anterior ciliary nerves. The
intrascleral nerve loops (pp. 18-19), which are occasionally found, can
also give rise to the same confusion.

The main mass of the fibers of the ciliary nerves enters the plexus
ciliaris in the ciliary muscle (p. 114). This supplies the ciliary muscle
itself and provides the nerves for the ciliary processes, the iris, and for
the deeper layers of the cornea. These latter enter the sclera behind the
scleral roll and. course by a short route to the cornea. The superficial
layers of the cornea are supplied by the perforating branches lying farther
backward. Concerning the finer subdivisions of the nerve, the reader is
referred to the portion of the tissue concerned.



PART II

THE PHYSIOLOGIC CHANGES OF THE EYEBALL DURING LIFE
(Development and Senescence)



That which the reader has come to know in Part I is the structure of
the eyeball at the height of its development after the completion of the
body growth. Before this period in life there lies the period of develop-
ment, after it comes the period of senescence. And although some
details have already been reported on account of their connection with
development and senescence, there yet remains a great deal to say,
especially concerning the development, if the conception of the eye in the
different periods of life is to be a complete one, if the anatomic structure
and the significance of its parts is to be rightly understood.



CHAPTER XVI. THE EMBRYONAL AND FETAL DEVELOPMENT

So rich a material has now been brought together in the normal charts of Keibel
(116), by the thoroughgoing researches of Seefelder, 1 and many other authors, that we
may now consider the developmental history of the human eye to have been worked
out, in the strict sense of the word. I, myself, have to thank the friendly consideration
of the director of the I. Academic Institute of Human Anatomy of Vienna, Professor
Jul. Tandler for the opportunity of using the rich collection of the institute to orient
myself at least concerning the most important phases of the development of the eye by
personal observation.

I have not gone into the details of -histogenesis, but refer to the cited literature
in this connection. Such a task demands a specialist in embryology, and that I am
not. For the same reason I have intentionally avoided still undecided matters. I
must also emphasize that the following should be looked upon only as a sketch of
the developmental history of the human eye, making pretense to neither originality
nor completeness.

Since the estimation of the age of very young embryos is extremely difficult in the
human, it is now preferable to give only the greatest length (gr. 1.) This is a measure-
ment of the body-axis in the very first stages only, and these scarcely come into con-
sideration in the development of the eye. After the appearance of the neck bend, the
greatest length is the distance from the neck to the buttocks (neck-buttocks length) ,
later that from the skull to the buttocks becomes the greatest (skull-buttocks
length, SS). In the later stages of fetal life one measures this length or the skull-heel
length, i.e., the length of the fetus with outstretched legs (cf. Michaelis, 154). Unfor-
tunately, an accurate notation of the measurement is absent in many statements in
which simply the length (1.) appears to be given.

Even in the earliest phases of development one finds considerable individual varia-
tions in respect to the time of its appearance. In such cases I have usually taken only
the lower limit.



1 The large treatise by Bach and Seefelder, Atlas of the Developmental History of the Human Eye,
Leipsic, 1911, first began to appear while this work was in press, and could not, therefo re, be made use cf

191



i 9 2 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

Even before the complete delimitation of the medullary canal from the
rest of the ectoderm, therefore before the time when there is still an open
furrow at the apical end, two grooves, the optic grooves, appear on each
side of the median line on the floor of this furrow. That which presents
itself in the view from behind as a groove appears in the view from the
side of the ventral cavity as an evagination, and in cross-section as a fold of
the wall of the medullary canal ; the cap of this fold abuts on the ectoderm
(PL IX, 6; copied from Keibel, 116, No. 6, p. 24; Text Fig. 6 a).

This evagination takes on a lateral direction after the closure of the
medullary canal and becomes transformed into a vesicular structure (the
primary optic vesicle) in a gr. 1. of 2 . 5 to 3 mm. This is separated from
the medullary canal on the dorsal side by a constriction, the pedicle of the
optic vesicle ; at the same time the medullary canal has widened into the
forebrain at its anterior (apical) end. On the ventral side, however, the
wall of the optic vesicle goes smoothly over into that of the forebrain.
The pedicle of the optic vesicle is, therefore, very short, and the lumen
of the optic vesicle stands in wide-open communication with the lumen of
the forebrain.

At the same time, several layers of mesoderm have interposed them-
selves between the ectoderm and the summit of the optic vesicle (Fuchs,
Textbook of Diseases of the Eye, i2th German ed., Fig. 143; 4th English
ed., Fig. 162). The optic vesicle is very poorly separated off from this,
for the mesodermal cells enter into many protoplasmic connections with
the cells of the wall of the optic vesicle, as well as with the ectoderm
(Seefelder, 203).

This mesodermal layer has, however, only a short existence. Cirin-
cione (33) first demonstrated that this layer disappears with the invagina-
tion of the lens. Even at a gr. 1. of 3 . 5 mm (collection of the I. Anatomic
Institute of Vienna), the mesodermal cells have been reduced to a few
scattered remnants, yet the unions of the ectoderm with the primary
optic vesicle remain constant in the form of fine protoplasmic threads
(embryonal supporting tissue of von Szily, 217).

The primordium of the lens now begins (at a gr. 1. of 4 mm: Keibel,
116; Seefelder, 203) over the summit of the optic vesicle in the form
of a thickening of the ectoderm (lens plaque), and soon sinks into a
depression.

At the same time, but independently, the neighboring already thick-
ened, distal part of the primary optic vesicle becomes concave (convex
brainward) ; the primary optic vesicle begins to invaginate.

The much-used expression, "invagination," is not correct. The distal portion of
the primary optic vesicle does not actually grow inward, but the margins grow out
over the summit, and, indeed, from above (dorsal) and from the sides.



THE EMBRYONAL AND FETAL DEVELOPMENT 193

Therewith begins the transition of the primary into the secondary
optic vesicle or the optic cup (PL IX, 7). The optic cup has a double
wall; that portion of the primary optic vesicle which is not invaginated
forms the outer leaf (a), the invaginated distal portion, the inner leaf (i}
of the optic cup. Its lumen (a) has the form of a cleft and is connected
with the lumen of the forebrain (V) through the wide lumen of the pedicle
of the optic vesicle (5 1 ). The cavity of the optic cup arising through the
invagination shelters the lens primordium (L) and is open laterally, i.e.,
toward the ectoderm, and below.

Meanwhile, small vessels of a capillary nature have developed in the
mesoderm surrounding the optic cup.

To the extent to which the lens primordium sinks into a tiny sac, the
optic cup also deepens and more and more surrounds the lens primordium.
The opening of the optic cup thereby continues to differentiate itself more
plainly into a laterally directed rounded portion (the primitive pupil)
and into a downward-directed cleft (fetal cleft, optic fissure), which at
first ends at the beginning of the pedicle of the optic vesicle.

The primordium of the lens is now surrounded on all sides by the
margins of the optic cup and separated from this by the engirdling meso-
derm; the mesoderm borders the lens primordium below, only in the
region of the fetal cleft.

There is only a narrow interspace between the lens primordium
and the inner leaf of the optic cup, and this is filled out by the primitive
vitreous. This is nothing more than that layer between the ectoderm
and the optic vesicle present immediately before the invagination of the
lens; it consists, therefore, of von Szily's embryonal supporting tissue,
which, to be sure, is not completely free of cells at any stage.

The borders of the fetal cleft approach each other and narrow the
cleft more and more. Thereby an extension of the mesoderm becomes
more and more plainly separated off and presses in through the fetal
cleft into the cavity of the optic cup. A small vessel, a branch of a ring
vessel, develops in this process at the border of the cup. The newly
formed vessel ends blind behind the little lens sac at a gr. 1. of 5 mm
(Seef elder, 199), (at one of 7 mm 1. according to Elze, 56) ; by the twenty-
eighth day it extends through the entire optic cup, according to Calderaro
(29), and is united with the vessels of the region behind. This is the
primordium of the inner vessel system the primitive arteria hyaloidea
and the annular vessel at the border of the cup is the primordium of the
circulus iridis major.

At a gr. 1. of 6 . 5 mm (Keibel, 1 16), the little lens sac has been closed off
into a lens vesicle, the rounded lumen of which contains some desquamated
cells. The lens vesicle at first remains connected with the ectoderm.



i 9 4 ANATOMY AND HISTOLOGY OF THE HUMAN EYEBALL

At a gr. 1. of 8 . 5 mm the closure of the optic cleft begins (Seefelder,
203) ; the mesoderm between the edges of the cleft disappears, and these
fuse together, and, indeed, each leaf of the optic cup by itself, so that there-
after not even the slightest trace of this union remains. The closure of
the optic cleft begins in the middle and proceeds from there forward and
backward. The primitive pupil is thereby closed off into a round open-
ing directed toward the ectoderm (therefore at first still lateral, later
toward the front). At first, however, there is no closure at the posterior
end of the optic fissure.

At a gr. 1. of 9.75 mm (Tandler, 219) the lens vesicle becomes com-
pletely constricted off from the ectoderm, and its lumen begins to narrow
from behind (through elongation of the epithelial cells concerned). The
mesoderm between the ectoderm and the lens vesicle now also begins
to grow in, i.e., the formation of the primitive cornea begins.

In this stage (PL IX, 8) the pedicle of the optic vesicle (S) is always
still very short and thick, but its lumen is already considerably narrower.
A well-developed extension of the mesoderm passes into the cavity of the
cup at the transition of the pedicle into the optic cup: the fissure in
the cup is here still wide open (Mf). This extension (the primordium
of the inner vessel system) is, moreover, united with the neighboring
mesoderm at the border of the cup, although only below (ventral), and
the border of the cup still has a notch at this place. In between, how-
ever, the optic fissure is closed, and, therefore, the cleft-like lumen of the
optic cup (A) is continuous. The cavity of the optic cup is for the greater
part taken up by the lens vesicle (L); ventral to this lies the process of
mesoderm, and the remnant of the cavity is filled out by primitive
vitreous (G). Between the border of the cup and the ectoderm, mesoderm
is everywhere found, and this presses in like a wedge between the lens
vesicle and the ectoderm (H).

It is difficult to give a comprehensive description of the further
development of the eye; I, therefore, prefer now to treat the individual
portions of the optic primordium separately.

The pedicle of the optic vesicle becomes the optic nerve. It now
rapidly grows in length (Seefelder, 203). The suggestion of invagination,
which up to this time was present at the anterior end, deepens to a plain
furrow and extends farther backward. The pedicle of the optic vesicle still
consists of epithelial cells, however, and maintains the primary lumen.

At a length of 14 to 15 mm the first nerve-fibers appear in the periph-
ery of the pedicle of the optic vesicle, and the lumen narrows. At 23 mm
length the pedicle of the optic vesicle is solid; as a result, the furrow is
closed and the proximal portion of the arteria hyaloidea is closed into the



THE EMBRYONAL AND FETAL DEVELOPMENT 195

axis of the pedicle. The connection of the arteria hyaloidea with the
neighboring mesoderm persists only at the posterior end of the furrow,
where the central vessels enter later.

The nerve-fibers sprout in from the optic cup; at the same time the
train of epithelial cells becomes spaced apart and forms a syncytium, i.e.,
an area of protoplasmic framework permeated with cell-nuclei (Krueck-


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Online LibraryMaximilian SalzmannThe anatomy and histology of the human eyeball in the normal state, its development and senescence ; → online text (page 22 of 27)