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The anatomy and histology of the human eye online

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the eye for the near point of vision, as has been the favorite
theory of many ophthalmologists. In its physical character-
istics, the vitreous body is colorless, perfectly transparent,
highly elastic, yielding, but not very compressible. By filter-
ing, it separates into a fine hyaline substance, and into a fluid
clear as water, thin and slimy. In form, the corpus vitreum
presents a not quite regular rotational ellipsoid, of which the
larger axis in the larger diagonal diameter of the ball is 9}'" to
10|'", the vertical axis in the diameter of the bulbus oculi is
9J"' to 9| /;/ . The depression in its anterior wall, the fossa
hyaloidea, is 4"' in diameter, and is quite round. The optic
axis through the vitreous body, is shortened 0'".2 to 0'".3 by
the hyaloid fossa. The connections of the vitreous body with
the surrounding parts are at the entrance of the optic nerve,
at the ora serrata retinae, where it is connected with the retina
and choroid, and, through the zone of Zinnius, it is connected
with the lens. It never contains as much fluid as its volume
seems to permit, and hence it allows the alterations in size of
the lens diameter during accommodation, and the changes in
form that must follow the action of the muscles ; and its in-
herent elasticity enables it to resume its form.

Hanover says, that the division of the zonula, going to the
posterior lens-capsule, is not corrugated. This seems to be a
mistake.

Heiberg says, that many of the fibres of the zone of Zinnius,
when they arrive at the point of separation to send a plate to
each lens-capsule, divide dichotomously, one branch going to
the anterior surface, and the other to the posterior, of the lens,
to be there expanded and attached. On the posterior surface
they are, however, distinctly seen only on the periphery of the
capsule. That there is not an intimate union between the
posterior laminae of the zone and the hyaloid membrane,
forming the hyaloid fossa, is obvious, from the fact that the



102 THE ANATOMY AND HISTOLOGY

cataractous lens is sometimes removed, with, its capsule, leav-
ing its bed in the hyaloid fossa with an unruptured membrana
hyaloidea.

The vitreous body in the adult is wholly without bloodves-
sels or nerves. In the embryonic state it has a system of ves-
sels of its own, which at or before birth rapidly shrink or dis-
appear. In the foetus it is also richly supplied w T ith cells,
which after birth rapidly disappear, until few are left. They
are partly composed of round or oval, finely granular nucle-
ated cells, of cystoid character, and partly of larger cells, with
several nuclei, with well-defined boundaries. They are found
particularly near the ora serrata, behind the lens, and in front
of the optic nerve entrance. According to Bowman and
IwanofF there is a fine dense net-work of fibres with dark
nuclear corpuscles at the points of interlacement. This view
is being corroborated by Stellwag and Kolliker.

In the adult the nutritive plasma is, according to Pilz, de-
rived exclusively from the bloodvessels of the ciliary processes.

The Aqueous Humor.

The Aqueous Humor (Humor Aqueus) fills up the space be-
tween the zonula Zinnii, the processus ciliares, the anterior
capsule of the lens, and the cornea, and weighs from 3J to 5
grains, with a specific gravity of 1.0053. It is thin, clear,
slightly viscid, colorless, without smell, with a slightly salty
taste. It does not distend the chambers to a high degree, so
as to permit the necessary excursions of the surface of the lens
during accommodation, and the movements of the iris.

It consists chemically of 98.687 parts water, and of solid
matter 1.313 (0.467 organic and 0.486 inorganic), of which there
are albuminates 0.1223, extractive matter 0.421, table-salt
0.689, chlorate of potash 0.0113, sulphate of potash 0.0221,
earthy phosphates 0.0214, and lime 0.0259.

The iris divides the cavity occupied by the aqueous humor
into two chambers, the larger called the anterior chamber, and



OF THE HUMAN EYE. 103

the smaller, the posterior chamber of the eye. The posterior
chamber is the space between the ciliary muscle, ciliary pro-
cesses, and the anterior surface of the lens, and the uvea, or
posterior surface of the iris.

The anterior chamber is the space between the anterior sur-
face of the iris and the cornea. The chambers communicate
through the circular opening in the iris, the pupil. According
to Budge, the posterior chamber has J the diameter of the an-
terior chamber. Petit gives as the distance from the axis of
the pupil to the lens J"' to J'", at the periphery of the pupil $'"
to f "', and in the anterior chamber from the axis of the pupil to
the middle of the cornea \'" to V". Whilst it has been denied
by Stellwag, Cramer, and others, that there is a posterior cham-
ber, it cannot be denied that a small portion of aqueous exists
between the lens-capsule and iris ; in the frozen eye a small
pellicle of ice will be found between the two. However, the
volume of the chambers constantly varies, according to the
position of the lens and the curvature of its surface, as is evi-
dent on observing the variations of size in the anterior cham-
ber during fixation for the far and near points of vision, which
also proves a laxity of tension in the chambers permitting these
variations.

The quantity of aqueous humor between the iris and the
capsule of the lens is smallest in the narrow pupil ; and in the
dilated pupil this distance is considerably increased.

The opinion that formerly prevailed, that the aqueous humor
is secreted from a serous membrane lining the chambers, is
now known to be an error. It is derived from the vessels of
the ciliary processes, and not from those of the iris. When
lost, it is re-accumulated in a very brief time.

In the operation of paracentesis oculi, the surgeon frequently
empties the chambers at intervals of from five to ten minutes.
It is not known whether it is continually secreted and ab-
sorbed, or whether such speedy regeneration ensues only after
a loss thereof.



104 THE ANATOMY AND HISTOLOGY



The Orbit.

The eyeball lies in the bony cavity, the orbita, surrounded
by a soft cushion of adipose tissue, and is sustained by the
muscles of the eye, and by the processes of the tunica vaginalis
bulbi. Through the optic nerve, which enters the orbit through
the foramen opticum, and through sensitive, motor, and gan-
glionic nerves, it is connected with the brain and spinal mar-
row, and through the conjunctiva, with the lachrymal appa-
ratus and the lids.

The orbits, the bony sockets in which the eyes are lodged,
are two pyramidal cavities of irregular quadrilateral form,
with their bases directed forward and outward, and their
apices backward and inward, so that their prolongation back-
ward would form an angle of 45 on the sella Turcica. The
temporal side of the base does not project so far forward as
does the nasal side, which permits a wider lateral field of vision.
The superior boundary or roof is arched, and is formed by the
orbital plate of the frontal bone, and by a part of the lesser
wing of the sphenoid bone, the base of which is traversed by
the optic foramen, a little to the inner side of the apex of the
orbit, and gives entrance to the optic nerve and the ophthalmic
artery. There is to be noticed on this upper boundary, the
roughness or spicula for the attachment of the pulley of the
superior oblique muscle, situated at its inner and anterior part ;
also the depression for the lodgment of the lachrymal gland,
at the outer margin of the angular process of the frontal bone,
just within the margin of the orbit ; also the supra-orbital
notch, or foramen, for the exit of the supra-orbital artery and
the frontal branch of the trigeminus nerve. The inferior boun-
dary of the orbit is formed by a part of the malar bone, and by
the orbital processes of the superior maxillary and the palate
bone, and is inclined downward and outward, and forms the
roof of the antrum Highmori. There is to be observed a groove,
which runs forward and ends in the infra-orbital foramen, which



OF THE HUMAN EYE. 105

gives passage to the infra-orbital branch of the internal maxil-
lary artery, and to the middle and anterior dental nerves, being
branches of the maxillary division of the trigeminus.

The internal boundary of the orbit is formed by the lachry-
mal bone, the os planum of the ethmoid bone, and part of the
body of the sphenoid bone. There is to be observed in this
boundary, immediately behind the border of the processus
frontalis of the superior maxillary bone, the fossa lachrymalis,
which terminates below in the canalis nasa lachrymalis, which
is formed posteriorly by the ungual bone, and where it termi-
nates in the inferior meatus of the nose, it is completed by the
inferior spongy bone. There are also to be noticed here the
anterior and posterior ethmoidal foramina, for the passage
through the first of the nasal nerve and the anterior ethmoidal
artery, and through the latter, the posterior ethmoidal artery
and vein. The external boundary of the orbit is formed by
the orbital process of the malar bone, and the great ala of the
sphenoid bone. There are here some small foramina, through
w^hich junctions of the branches of the fifth and seventh nerves
are made. We have further to observe at the superior ex-
ternal angle of the fissura orbitalis superior, and at the outer
inferior angle, the longer but more narrow fissura orbitalis in-
ferior, through the former of which enter the third, fourth,
first divisions of the fifth, and sixth nerves, and through it the
ophthalmic vein passes out of the orbit. The latter fissure is
filled with fat, and gives passage to the infra-orbital nerve and
bloodvessels. The orbit is lined by a thin and rather loosely
connected bone-membrane, the periorbita, and it is in connec-
tion with the dura mater through the foramen opticum and the
fissura orbitalis superior, and with the periosteum of the bones
of the face, through ihejissura orbitalis inferior, the canalis zy-
gomaticusfacialis, foramen supra-orbital and canalis infra-orbitalis.
This periorbita sends out fibrous processes, partly to the tunica
vaginalis bulbi, and also to the ligamentum palpebrale internum,
and externum, and is here connected with a mass of firm con-
nective tissue containing but little fat ; it also sends out



106 THE ANATOMY AND HISTOLOGY

cesses to the lachrymal sac, to the lachrymal gland, to the
tendon of the levator palpebrce superior is, and the tendon of the
obliquus super iori ; at the border of the orbit it passes over to
the outer convex edge of the cartilage of the lid, the liga-
mentum tarsi latum, coming here in connection with a process
of the tunica vaginalis bulbi, which will soon be described.

At the optic foramen, the periorbita is condensed into a
thick fibrous ring, the annulus fibrosus, which forms an ellipti-
cal ring around the foramen opticum, and the middle part of
ihejissura orbitalis superior, the latter of which it divides into
three divisions ; into the middle, which leads into the pyra-
mid formed by the muscles of the eye, and which gives pas-
sage to the nervus oculo-motorius abducens, and the ramus naso-
ciliaris nervi trigemini, and the vena ophthalmica superiori ; in the
upper division, which leads into the space between the upper
part of the bulbi, with its muscles, and the wall of the orbit,
and gives passage to the nervus frontalis, the nervus trochlea-
m, and the nervus lachrymalis ; in the lower division, which
enters the space between the globe and its muscles, and the
wall of the orbit, and which gives passage to the vena ophthal-
mica inferior.

The tendinous ring gives origin to six muscles : the four recti,
proceeding forward to be inserted into the anterior part of
the eyeball, form a pyramidal space ; the two other muscles,
originating from the common tendinous ring, are the superior
oblique, and the elevator of the upper lid. The space between
the eyeball and the walls of the orbit is filled up with a
loose connective tissue, quite rich in adipose matter. This
connective tissue is condensed on some points, and constitutes
sheaths for the muscles within the orbit, for the nerves and
bloodvessels ; it also forms fascia-like processes, which con-
nect some of the parts within the orbit with the periorbita.

The tunica vaginalis bulbi (Bonnet's capsule, capsule of Tenon)
is continuous with the optic nerve sheath, and envelops the
eyeball loosely until it arrives in front of the equator, where
It is perforated by the straight muscles of the eye ; forward



OF THE HUMAN EYE. 107

of that point it is firmly connected with the sclerotica and
the conjunctiva, and proceeds forward as far as' the border
of the cornea, where it ceases. It also sends out some pro-
cesses to the edges of the cartilages of the upper and lower
lids, forms a connection between the conjunctiva and the facia
tarsi orbitalis ; it also sends membranous bands to connect with
the ligamentum canthi internum and externum ; and the carun-
cula lachrymalis (along with the plica semilunares), rests on
such a band, which has its practical importance in connection
with the operation for strabismus (Liebrich). The posterior,
loosely connected partition is smooth on its inner surface, so
as to allow the free rotatory movements of the globe. At the
point of perforation, it is intimately connected with the mus-
cles, so as not easily to permit a separation of the parts, and it
sends off processes backward toward the* optic foramen for
some distance as firmly connected sheaths to the muscles.
Anterior to the perforation, the tendons are not enveloped by
the capsule, but for some distance proceed without any en-
velope, until, just before their insertion, they expand between
the capsule and the sclerotica, to be inserted into the latter,
but also in close connection with the former. Anterior to the
line of insertion of the recti muscles, the capsule becomes much
thinner, and it is extremely difficult to detach it from the con-
junctiva and the sclerotica, so intimately are the three mem-
branes connected. Liebrich compares this anterior belt as a
lid of a half sphere to a half globe shelL The lid has a circu-
lar perforation on top, just the size of the cornea, at the border
of which it ends. The portion in front of the perforation of
the muscles is often named the capsule of Tenon, whilst the
loose posterior partition has also been known as Bonnet's cap-
sule. In the anterior part of the capsule, it is intimately con-
nected with the conjunctiva, and quite firmly up to an irregu-
lar circular line, which is known by the fact, that by eccentric
movements of the eyeball, a fold is produced at the line re-
ferred to. The formation of this fold prevents the corruga-
tion and projection forward of the conjunctiva, which would



108 THE ANATOMY AND HISTOLOGY

otherwise take place, at the caruncula, on turning the eye in-
ward. A complete continuation from one of those divisions
of the capsule into the other does not take place, as, at the line
formed by the fold referred to ahove, the border of the poste-
rior division in part turns out to the border of the orbit on the
membranous expansions in the manner explained above. Back
of the equator, near the optic nerve, it is perforated by the
obliquus superior and a little further forward, on its outer and
posterior surface, by the obliquus inferior.

The Muscles of the Eye.

There are seven muscles within the orbit, the levator pal-
pebrce, rectus superior, rectus inferior, rectus internus, rectus ex-
ternus, obliquus superior ', and obliquus inferior.

The four recti muscles, according to Alt's observations on
frozen eyes, procqed in a straight line, up to the greatest
diameter of the eyeball, and at the equator they first come in
close connection with the bulbus oculi, but still are outside of
the ocular sheath, and it is only near the insertion of their ten-
dons into the sclerotica that the capsule is perforated. From
this point back to the equator of the globe, there is only a
rather loose cellular connection between the capsule and mus-
cles. The muscles for some distance back have a sheath formed
from processes of the tunica vaginalis. After perforating the
ocular sheath, the tendons are naked for a short distance, but
they soon expand, between Tenon's capsule and the sclerotica,
to be inserted. The expansions of the tendons do not meet each
other, so as to form a continuous membrane, but are only con-
nected in a manner by the tunica vaginalis bulbi. The tendons
of the recti muscles are quite short, are from 3J r// to 4 /r/ broad,
with the exception of the rectus externus, which is \'" nar-
rower. The common muscle plane for the rectus internus and
the rectus externus rests in the horizontal meridian ; the com-
mon muscle plane of the rectus superior and the rectus inferior
is on the vertical meridian. This is, perhaps, not strictly cor-



OF THE HUMAN EYE. 109

rect, as the inferior inclines J'" inward toward the nose. A
line drawn through the points of insertion of the recti muscles,
will strike a point between the equator of the eyeball and the
border of the cornea, in the region of the processes ciliares, and
its* vertical diameter is 8'", and its horizontal diameter 9'",
whilst the vertical diameter at the equatorial plane is 10'"
to 10|'", and the horizontal 10J"' to 11'" (Pilz).

FIG. 55.




A view of the muscles of the eyeball, taken from the outer side of the right orbit. 1.
A small fragment of the sphenoid bone around the entrance of the optic nerve into the
orbit. 2. The optic nerve. 3. The globe of the eye. 4. The levator palpebree muscle.
5. The superior oblique muscle. 6. Its cartilaginous pulley. 7. Its reflected tendon.
8. The inferior oblique muscle ; a piece of its bony origin is broken off. 9. The superior
rectus muscle. 10. The internal rectus, almost concealed by the optic nerve. 11. Part
of the external rectus, showing its two heads. 12. The extremity of the external rectus
at its insertion, the intermediate portion of the muscle having been removed. 13. The
inferior rectus muscle. 14. The sclerotic coat. (From Smith's Anatomical Atlas.)

The superior rectus (musculus rectus superior, attollens oculi),
takes it's origin in connection with the musculus palpebrce, from
the upper part of the tendinous ring, and proceeds forward in
the same direction as the nervus opticus ; yet in consequence of
its direction to reach the upper end of the vertical diameter of
the globe, it forms an angle with the optic axis of 20. It is
the most thin among the recti muscles, about an inch and a
half in length, and is inserted 3-f '" from the corneal border,
with the inner side of its tendinous expansion V" nearer the
cornea than its outer.

The inferior straight muscle (musculus rectus inferior, seu de-
primans oculi), takes its origin from the lower part of the com-
mon tendinous ring, is thicker than the superior straight
muscle, and V" to 2'" longer, and proceeds forward, bearing
about the same relation to the optic nerve axis, as the rectus



110 THE ANATOMY AND HISTOLOGY

superior, and is inserted 3"' behind the lower border of the
cornea, \'" inward from the end of the vertical meridian, and
with the inner border of its tendinous expansion V" nearer
the cornea than the outer.

The inner straight muscle of the eyeball (musculus rectus in-
ternus, sen adducens oculi), arises from the common tendinous
ring, proceeds forward parallel with the inner wall of the
orbit, and is inserted 2J r// back of the inner border of the cor-
nea, in the horizontal meridian. It is the thickest of the
straight muscles, is about an inch and a half in length, and
its tendinous expansion is quite broad, sometimes being divided
into two tendons.

The outer straight muscle (musculus rectus externus, sen ab-
ducens oculi), arises by two heads from the common tendinous
ring, and passes along the outer wall of the orbit, and in
consequence of its direction outward is the longest of the
recti, being 3'" longer than the rectus superior, and is next
in thickness to the rectus internus. It is inserted further back
from the border of the cornea than any of the other straight
muscles, being 3J'", and at this point the lachrymal gland
rests on it. Its muscle-plane is in the horizontal meridian.

There are two more muscles that run along the roof of the
orbit, the levator palpebrce superior, and the obliquus superior.

The musculus levator palpebrce superior originates from the
upper part of the common tendinous ring, in common with the
musculus rectus superior, as a triangular flat muscle, is separated
from the rectus in the region of the bulbus, and becomes broader,
and pa'sses out of the orbit below the marga supra-orbitalis, and
behind the ligamentum tarsi superioris, and terminates in a
flattened tendon, one half inch in breadth, to be inserted into
the superior border of the tarsal cartilage, and into the fascia
extending from that point.

The musculus obliquus superior, seu trochlearis, sen patheticus,
originates also from the common tendinous ring, between the
tendons of the rectus superior and the rectus" internus, proceeds
along the upper and inner angle of the orbit forward to the



OF THE HUMAN EYE. Ill

pulley or trochlearis beneath, the internal angular process of
the frontal bone, which is located about 5J /r/ above the hori-
zontal middle of the cornea, 6f //r inward from the vertical
diameter of the cornea, and about on the same plane with the
base of the cornea (Pilz), over which its slender tendon passes,
where it changes its direction, becomes broader, passing under
the rectus superior. It is the longest and thinnest of all the ocu-
lar muscles, and by a tendon 3'" in length, with its convexity
turned backward and outward, is inserted into the temporal
side of the eyeball, with its inner end 3}'" to 4 /r/ from the
nervus options, whilst the outer end is inserted 6'" to l f " fur-
ther forward. The musculus obliquus inferior arises from a
depression in the orbital edge of the superior maxillary bone,
a little to the outer side of the lachrymal sac, runs along the
floor of the orbit, beneath the rectus inferior, first for a distance
of 3'" running backward and outward, and then turns up-
ward and backward, forms a fibro-cellular connection with the
rectus inferior, and passes between the globe and the rectus ex-
ternus, and is inserted by a broad thin tendon, with short fibres,
on the temporal side of the posterior part of the eyeball, with
a line of insertion of 5"' in length, with its convexity directed
forward and upward, the anterior end of which is 7 /r/ , and
the posterior 2'" to 3'" from the optic nerve. The seven mus-
cles of the orbit that have been described, one of which raises
the upper lid, and the six others rotate the ball around the
turning focus, consist of transversely striated muscular fibres.
They are furnished with nerves from the nervus oculo-motorius,
or third pair, with the exception of the rectus externus, which
is supplied by the sixth pair, and the obliquus superior by the
fourth pair of cerebral nerves. Consequently all are volun-
tary muscles.

Action of the Muscles of the Eye.

Although it is not the plan of the present treatise to enter
extensively into the physiology of the parts described, a few
words on the actions of the muscles above considered seem to



112



THE ANATOMY AND HISTOLOGY



be necessary here ; and first a few definitions must be given,
in doing which we shall follow Pilz. The optic axis (a b, Fig.
56) is the extension of the lens axis forward to the cornea, as



FIG. 56.




well as to the back part of the eye, and is lOf '" to 11'" in length.
The points that are touched by the horizontal line along the
surface of the eyes, are the poles, of which there are two, the
anterior (a d) and posterior (bf) poles of the eyeball. The
equatorial plane is a line vertical to the optic axis (o p, r s),
through the greatest diameter of the eyeball, and is located
'" to V" nearer the posterior pole than to the anterior, and its
longest diameter is neither horizontal nor vertical, but extends
from the nasal side above to the temporal side below. It
measures the same as the optic axis, whilst the vertical diame-
ter is \ f " to \'" shorter. The circumference around the equa-
torial plane is the equator, and the sections of the globe thus
made, are the anterior and posterior hemispheres. The meridi-
onal planes are drawn through the optic axis, and cut the globe
into lateral segments ; and those curved lines on the surface of


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Online LibraryAbraham MetzThe anatomy and histology of the human eye → online text (page 8 of 14)