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the globe, centering in the meridional plane, are the meridians.
The optic nerve axis is a line drawn through the middle of
the optic nerve entrance to the middle of the optic axis, and



OF THE HUMAN EYE. 113

from thence is extended to the anterior surface of the cornea,
1 \'" outward from the anterior pole (g, A), and the posterior end
of the same axis is just the same distance to the inner side of
the optic axis. It cuts the optic axis at an angle of 20 32'.

The eye must be considered as a globe, with a fixed centre
(centre of motion Drehpunct) around which latter it rotates with-
out change of locality. A muscle plane is the direction in
which a muscle exerts its force, and is obtained by drawing a
line from the middle of its origin to the middle of its insertion,
and a line which stands perpendicularly upon this plane in the
turning-point, is the axis of turning (Drehungsachse). Con-
sequently the axis of turning of the musculus rectus externus
and internus, is vertical, corresponding with the vertical diam-
eter of the eye, leaving the vertical meridian unchanged. The
common muscle plane of the rectus superior and the rectus in-
ferior does not coincide with the vertical meridian, but for the
simplification of the matter, they are so considered by authors.
It inclines itself a little from behind and forward, and within
outward. This plane, which cuts the antero-posterior axis
of the eye, under an angle of 20 degrees, does not represent a
meridian of the globe, and does not contain the centre of the
eye, but leaves it outward. The perpendicular axis of turn-
ing to this plane, forms, with the horizontal diameter, an angle
of 20 to 25 degrees, and with the antero-posterior an angle of
70 degrees (Wecker). These two muscles contracting separately
will draw the centre of the cornea upward and downward,
but from the anatomical conditions described, the superior and
inferior straight muscles, besides each one drawing the centre
of the cornea to its own side, when acting in concert, they will
cause a slight deviation of the cornea inward. The rectus su-
perior contracting, will draw the superior extremity of the ver-
tical meridian inward, the superior extremity of this meridian
being the one always noted in determinating the ocular move-
ments, whilst the rectus inferior, in contracting, draws the in-
ferior extremity of the same meridian inward, and conse-
quently the superior extremity is inclined outward, and we

8



114 THE ANATOMY AND HISTOLOGY

say the vertical meridian is inclined outward. The contrac-
tion of this group of muscles is not always the same as regards
the position of the centre of the cornea, as well as the inclina-
tion of the meridian, and varies according to the position of
the antero-posterior axis of the eye (optic axis) at the moment
of contraction. If the centre of the cornea is directed out-
ward in such a manner, as that the axis of turning of the
group forms with the optic axis an angle' approaching closely
to 90 degrees, and that the muscle plane tends progressively to
confound itself with the vertical meridian, the muscular con-
traction will necessarily produce its effects upon the deviation
above or below the centre of the cornea, whilst the inclination
of the meridian will be very little affected. Inversely, if the
optic axis is directed inward, and forms with the axis of turn-
ing a less and less open angle, and if the muscle plane forms
with the vertical meridian an angle more and more open, the
influence of the contraction of the muscles of this group will
have a decided influence upon the inclination of the vertical
meridian, whilst the deviation in height will be quite feeble
(Wecker). For the sake of convenience in studying the ac-
tions of the oblique muscles, a voluntary mistake is necessi-
tated in adopting a common muscle plane. This plane does
not accord with the equatorial plane of the eye, but turns
aside by its internal circumference forward and by its exter-
nal circumference backward. The centre of the eye is located
in this muscle plane, and which deviates more from the equa-
torial plane, forward and inward, then backward and out-
ward. Its axis of turning does not, then, accord with the optic
axis, but according to Graefe forms with it an angle of 35 to
40 degrees. The anterior extremity of this axis ends outward
from the anterior pole of the eye. The action of each of the
oblique muscles is trifold as in the group preceding this one.
The superior oblique draws the centre of the cornea down-
ward and outward ; but its effect is to draw the superior end
of the vertical meridian inward, and consequently to incline
the meridian inward. The inferior oblique draws the centre






OF THE HUMAN EYE. 115

of the cornea upward and outward, and draws the inferior
extremity of the vertical meridian inward, which inclines this
meridian outward. The effect that will be produced by the
action of these muscles, whether on the deviation of the height
of the corneal centre, or whether it be on the inclination of the
meridian, will vary much according to the angle that the optic
axis will form with the axis of turning ; or in other words,
according as the anterior pole of the eye will be directed in-
ward or outward. The effect that the oblique muscles pro-
duce upon the height of the cornea, is the more decided, when
the eye is strongly carried outward ; for then the axis of turn-
ing of the obliqui forms with the optic axis an angle more and
more approaching a right angle. On the contrary this angle
diminishes by the approach of the two axes, when the cornea
is directed outward, and for the reason that the muscular
contraction then produces its effect nearly exclusively upon
the meridian which it deviates, and not upon the height of the
centre of the cornea. In looking over what has been said on
the actions of the above muscles, we find that the obliqui are
the antagonists of the recti. The consentaneous action of the
recti, draws the eye backward into the orbit. The obliqui
pull the globe out. The former are retractors, whilst the latter
are protractors. The division of the recti, would enable the
obliqui to draw the eye forward and make it protrude from
the socket ; to divide the obliqui, would cause the recti to re-
tract the ball within the orbit. The action of the recti is
greatest on the height of the cornea, when the optic axis is
directed outward. The reverse is true of the obliqui, which
have their maximum effect upon the vertical meridian when
the cornea is turned out, whilst the contrary is true of the
recti superior and inferior. The combined action of the rectus
superior and the obliquus inferior deviate the cornea Upward,
and these muscles are antagonists as relates to the deviation
laterally and to the inclination of the meridian. The same re-
lations exist between the rectus inferior and the obliquus superior.
Now briefly as regards the actions of the individual muscles ;



116 THE ANATOMY AND HISTOLOGY

In looking straight forward, all the muscles are in a state of
equilibrium, and the vertical meridian is perpendicular. In
looking inward on the horizontal plane, the only muscle con-
cerned is the rectus internus, which leaves the vertical meridian
unchanged. In looking outward on the horizontal plane the
rectus externus is the only muscle in action. Hence in moving
both eyes on the horizontal plane, the only muscles in action
are the rectus internus and externus. In looking directly up-
ward the vertical meridian remains vertical. To bring about
this movement the muscle chiefly concerned is the rectus supe-
rior. Now this muscle acting singly would also incline the
vertical meridian inward, and consequently it must associate
itself with an antagonist in order to keep the meridian vertical,
which we find to be the obliquus inferior. In looking directly
upward with both eyes., four muscles then are in a state of
activity. So in looking directly downward, the vertical me-
ridian remaining vertical, the rectus inferior associates with
itself the obliquus superior. In looking upward and outward,
the vertical meridian is inclined outward, and the muscles
chiefly concerned in bringing about this result are the rectus
externus and the rectus superior. But in order to counteract
the tendency which the rectus superior has to incline the ver-
tical meridian inward, the obliquus inferior is brought into
action, to draw on the inferior extremity of the meridian, and
assist in inclining outward. Then in looking upward and
outward with both eyes, six muscles are in action. The rec-
tus externus simply turns the eye outward, the rectus superior
rotates it upward, and the obliquus inferior inclines the meri-
dian. In looking downward and outward, it has been de-
termined that the vertical meridian is inclined inward. The
rectus externus rotates the eye outward, and the rectus inferior
rotates it downward, with the vertical meridian slightly in T
clined outward. To correct this the obliquus superior is
brought into action to correct the inclination of the meridian.
Then, we find, in looking downward and outward, six mus-
cles act. The rectus externus to rotate outward, the- rectus in-



OF THE HUMAN EYE. 117

ferior to rotate downward, and the obliquus superior to incline
the meridian inward. In looking upward and inward, the
vertical meridian is inclined inward. The rectus internus
simply rotates the cornea inward ; the rectus superior ro-
tates the eye upward, and also inclines the vertical meridian
inward, and with the greater force, the optic axis being di-
rected inward. But a third muscle is necessary to modify
the power of the two muscles named, in their tendency to in-
cline the meridian inward ; this we find to he the obliquus in-
ferior, which partly counteracts the action of the rectus supe-
rior, at the same time that it assists the latter in elevating the
centre of the cornea. To look upward and inward, then,
employs six muscles, the rectus internus, to rotate the eye in-
ward ; the rectus superior, to rotate the eye upward, and in-
cline the meridian ; and the obliquus inferior, to assist in ro-
tating the centre of the cornea upward, and also to modify
the inclination of the meridian. In looking downward and
inward, the vertical meridian is inclined outward. The ro-
tation inward is effected by the rectus internus, and the centre
of the cornea is rotated downward by the rectus inferior, which
at the same time finds the eye in a favorable position for in-
clining the meridian outward. We shall find, again, that a
third muscle is required to modify the excessive influence of
the rectus inferior, which we find to be the obliquus superior.
The action of both eyes for vision downward and upward,
requires the activity of six muscles : the rectus internus, to ro-
tate inward; the rectus inferior, depressor and de viator of the
meridian ; and the obliquus superior, a depressor of the cornea!
centre, and modifies the deviation of the meridian. It will be
perceived that when the eye is moved either strictly vertically
or horizontally, the meridian remains vertical ; in the positions
outward, it is the obliqui that determine the inclination of the
meridian ; in the positions inward, it is the rectus superior and
inferior that incline the meridian. In these different positions,
the muscles act with the greater facility, the more the optic
axis confounds itself with their axis of turning.



118 THE ANATOMY AND HISTOLOGY

The Optic Nerve (Nervus Opticus.)

The optic nerve is somewhat tortuous in its passage from
the optic foramen to the globe, forming a curvature outward and
downward. Its length from the eyeball to the optic foramen is
13'" to 14'", which is more than the distance of the posterior pole
from the same foramen, which is only 12'". This is owing to
the fact that the optic nerve is inserted to the inner side of the
posterior pole. It is surrounded by the posterior ciliary arte-
ries and the ciliary nerves, which run along close to its sheath
and accompany it through the loose fatty tissue to the bulbus
oculi. During its course it receives, and, in its axis, carries
within it, the arteria centralis retinae. It is 2'" thick, and has
a considerable constriction as it passes through the sclerotica.
(See Fig. 1.)

The optic nerve is a nerve of special sense, and its exclusive
office is to conduct to the nervous centre the luminous im-
pressions made upon the retina. It runs a long course in the
encephalon before its termination in the eye. It is in immedi-
ate connection with the optic tracts of the brain. What is
said below in reference to the cerebral origin of the optic nerve
is mostly translated from Galezowski.

The optic tracts are white, and rather broad, and proceed di-
rectly from the corpora genwulata, external and internal ; they
are in contact with the cerebral peduncles, around which they
pass horizontally, and proceed forward to form the chiasma.
In half their course they are in contact with the cerebral ped-
uncles. They are in relation with the cerebral peduncles above
and within ; below and outward they are free. Further for-
ward they are in contact with the membrana perforatus and the
sphenoid bone.

The optic tracts proceed, as stated above, from the corpora
geniculata, located on the inferior and posterior surface of the
optic thalami. The internal is smaller than the external, but
is more prominent. On its free surface it is enveloped by a thin
white layer, which is prolonged backward to the posterior or



OF THE HUMAN EYE. 119

quadrigeminal tubercle (testes). A nucleus of gray substance
is found in it.

The external corpora geniculata form an oblong eminence,
which passes around the posterior and inferior extremity of
the optic thalami, and by means of a small white medullary
band, communicate with the anterior quadrigeminal tubercle
(nates). Its free surface is also white, but of a more murky
color. The existence of gray nuclei in these bodies leads to the
opinion that they are not simply conductors, but that they
perform some special act connected with vision.

The tubercula quadrigemina are formed by four separate emi-
nences divided by grooves. The two anterior projections are
called nates, and the posterior testes. They are in front of the
cerebellum and above the cerebral peduncles, with which they
form adhesions.

In structure, they are very complex, and are covered exter-
nally by a thin, white substance. In the posterior tubercle are
found round nuclei, of a reddish-gray color, more dense than
the cortical part.

Another portion of gray matter, separated by the two nuclei
described, serves to connect the tubercles of the opposite sides.
Beneath these pass the white medullary fibres pertaining to
the band of Reil, and to the cerebellar peduncles.

It then seems that the tubercula quadrigemina are in direct, or
indirect, connection with the several parts of the brain, as with
the cerebellum, the medulla oblongata, and the spinal cord.
Thus, there is given off from each posterior tubercule a small
white band of medullary matter, which passes in front of the
triangular lateral fascicule of the isthmus, to end in front in
the corpora geniculata interna. There is another prolongation
forward and outward, which proceeds directly to the corpora
geniculatum externa. The pineal gland is attached to the
tubercula quadrigemina by four white fasciculi, as also by the
choroid plexus. The valve of Yieussens communicates with
the posterior extremity of the tubercula, by its filament ; the
cerebellum communicates with the tubercula quadrigemina



120 THE ANATOMY AND HISTOLOGY

through the processus cerebelli ad testes, which take their origin
in the white nuclear substance of the cerebellum, cross above
the inferior peduncles of the same organ, and pass beneath the
band of Reil and in the tubercula quadrigemina. Between the
medulla oblongata and the tubercula quadrigemina, there is a
communication through the antero-lateral fasciculus of the bulb
or band of Reil.

M. Schroder van der Kolk has demonstrated that the ante-
rior roots terminate wholly in the anterior cells, forming a
group ; from these proceed the ascending or encephalic fibres,
which form the anterior and lateral cords. The posterior
roots contain two orders of fibres, the cerebral and the reflex ;
the first ascend directly into the brain ; the others end first
in the cells of the posterior cornu, and thence proceed to the
brain. Hence, it is inferred that the superior crura of the
cerebellum form a continuation with the posterior medullary
fascicule, and that the fibres of the band of Reil are, according
to Longet, the continuation of the antero-lateral fascicula of
the spinal cord.

According to this, -then, the tubercula quadrigemina are in
relation both with the posterior or sensitive, and with the an-
tero-lateral or motor columns of the spinal cord, which ex-
plains why affections of the latter so frequently cause amauro-
sis (Galezowski).

The chiasma or commissure is nearly quadrilateral, and is
situated on the olivary processes of the sphenoid bone. It is
bounded in front by the lamina cinerea, by the tuber cinereum
behind, and on either side by the anterior perforated space.

In the chiasma, the optic nerves of the two sides partially
cross. It contains two kinds of fibres (see Fig. 57): the ex-
ternal, which proceed to the external part of the retina, and
which do not decussate ; the central, which cross in the chiasma ;
and the inner fibres of the optic tract of one side cross over
to supply the inner side of the retina of the other side, and
vice versa.

Another class of fibres, the internal do not decussate,



OF THE HUMAN EYE.



121



but those on the anterior surface of the chiasma pass from
the retina of one side to the retina of the other side, without
crossing, and are called the inter-retinal fibres ; those on the
posterior surface of the chiasma do not cross, but pass from
one optic tract -to the other, and are called the inter-cerebral
fibres. (See Fig. 57.)



FIG. 57.




a. a. External fibres of the optic nerve and of the retinae coining from the corresponding
hemispheres, b, b. Internal fibres of the same nerve which originate in the opposite
hemispheres, f., c. Chiasma, with inter-crossing of the optic fibres, d, e. Optic tracts.
f. Testes. g. Nates, h. Internal carotid, i. Middle cerebral artery, k. Posterior
communicating artery. /. Anterior optic artery.

1. Central nucleus of the testes. 2. Gray substance of the nates. 3. White band
which separates the nates in two parts. Below the gray part 4, are seen parallel white
striae, which are the continuation of the processus cerebelli ad testes. (From Galezowski.)



The former constitute a communication between the retinae
of both eyes ; the latter becomes continuous with the optic
tracts. Also see Fig. 63, of which o, o, is the commissura arcu-



122 THE ANATOMY AND HISTOLOGY

ata anterior ; r, r, the commissura arcuata posterior ; p, p, q, q,
the commissura cruciata. Up to the chiasma the fibres are
medullary ; at the commissure the optic tracts present modifi-
cations that are important in a physiological point of view.
Each one, before crossing over, receives on its superior -face a
large fascicule of gray fibres, emanating from the gray mass
which covers the internal surface of the optic thalami. (Wecker.)
At the chiasma the pia mater forms a sheath for the optic
nerves, which accompanies them to their insertion into the
globe. It sends numerous processes inward in a manner pres-
ently to be described. At the optic foramen each optic nerve
receives a strong outer sheath, generally considered as a con-
tinuation of the dura mater. It is believed by some (Stellwag)
to be derived from the periorbita ; but the weight of testimony
seems to preponderate in favor of its origin from the dura
mater. According to Donders (Archive fur Ophthalmologia,
Band i, Ab. ii, Bl. 82), the optic nerve, as regards the structure
of its sheath and its processes between the individual nerve
fasciculi, differs from other nerves.

In ordinary nerves Donders found a firm fibrous texture as
a common sheath (see Fig. 58), which sends extensions inward
(a'), and is crossed by a loose cellular tissue (6), which (the

- FIG. 58.




Transverse section of the ischiatic nerve of a man. Magnified 50 diameters.
(From. Donders.)



OF THE HUMAN EYE. 123

latter) separates the tertiary fasciculi, and the larger branches
of vessels (2, 2), as well as inclosing fat-cells (i) ; besides, there
is around each secondary nerve-bundle a thin, firm, fibrous,
lamellated enveloping membrane (e), the neurilemma proprium,
which has no connection with the outer, common sheath;



FIG. 59.




Transverse section of the optic nerve 1 mm. from the sclerotica. (From Dondcrs.)



whilst some loose cellular tissue divides the secondary, bundles
into the primitive, and takes up the smaller vessels (3).

In the optic nerve, on the contrary, we find in the longitu-
dinal section (Fig. 60), as also in the transverse section (Fig.
59), two firm fibrous sheaths, an outer and thicker (a), and an
inner and thinner (&), both rich in connecting elastic elements.
Between these two sheaths there is a loose, connective tissue
(<?), in consequence of which the inner sheath, firmly connected
with the nerve, can slide on the outer sheath. The outer
sheath and the loose connective layer between the sheaths do
not come in contact with the nerve-bundles at any point. The
inner sheath sends in firm fibrous extensions, which separate
the nerve-bundles (<?), and there is no loose connective tissue



124



THE ANATOMY AND HISTOLOGY



either between the secondary or tertiary fasciculi. The elastic
elements here are less developed, and seem only extensions with
connecting small elongated granules. The outer sheath (#/,
Fig. 60), as seen in the longitudinal section, is lost in the outer
two-thirds of the sclerotica (a' b'). The inner sheath (6), on



FIG. 60.







Longitudinal section of the optic nerve and the tunics of the eye. Magnified
10 diameters. (From Danders.)

the contrary, encircles the nerve-stem close to the choroidea,
with which some of its fibres undoubtedly connect, whilst the
others turn out immediately beneath the choroid, and are
blended with the inner sclerotica. From this inner portion of
the sclerotica a number of elastic elements are given off, which
proceed between the individual nerve-bundles of the optic nerve
to form the so-called lamina cribrosa (g). Donders asserts, dif-



OF THE HUMAN EYE. 125

fering from Kolliker and Heinrich Miiller, that the fibres are
in contact with only a very small part of the choroidea, and
that within the eye, in fact until within the retina itself, the
nerve-bundles are separated by the inter-fascicular tissue.

There are in the optic nerve a great number of small arterial
and venous branches. Those on the outer sheath are partly
continuous with those of the sclerotica ; those in the loose cel-
lular tissue, between the outer and .inner layers of the sheath
(which, immediately behind the lamina cribrosa, becomes con-
tinuous with the inner sheath, and at this point, where it is
already within the sclerotica, it is thickest), pass on the inner
sheath, and on the processes separating the nerve-bundles, and
are distributed on the lamina cribrosa, and some pass into the
papillce nervi optici, and surrounded by nervous substance, are
distributed in the retina itself. This is true especially of a
few small branches in the immediate neighborhood of the vasa
centralia.

As regards the distribution of the central vessels themselves,
every branch is completely surrounded by the optic fibres, not
one reaching the membrana limitans. Capillary vessels are
seen on every part of the retina, except in the bacillar or rod-
layer, and in the granule-layer, contiguous to it. As the
bloodvessels are placed on the inner part of the retina, the
inquiry presents itself whether in the papilla nervi optici the
vessels come to the surface. Bonders satisfied himself that in
nearly all cases they are also surrounded by nervous fibres, and
that they seldom come in contact with the membrana limitans.
The vessels of the retina are an independent system of vessels,
having no communication with any other system of vessels.
At the anterior border of the retina they end in loops.

The Arteries.

The eyeball is (and its appendages mostly) supplied with
blood from the ophthalmic artery, which arises from the internal
carotid artery, just as the latter vessel is emerging from the


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Online LibraryAbraham MetzThe anatomy and histology of the human eye → online text (page 9 of 14)