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in the adult is 0.03mm. to 0.04mm.
Its greatest thickness is 0.006 mm. ;
its appendages only 0.005 mm. Un-
der the influence of hardening sub-
stances a globule is discovered to-
ward the inner extremity of its
middle portion, which is generally
considered as a nucleus. Ritter be-
lieves it to be only a dilatation of
the central filament. The external
appendage or filament is not continu-
ous with the surface of the cone, but penetrates the internal
appendage, which it completely fills. From thence it is pro-
longed toward the internal extremity of the
middle part of the cone, where it sometimes
forms a globulous swelling, and sometimes ter-
minates simply, without change of diameter.

As regards the difference between the rods
and cones, Ritter says it is only necessary to
compare the composition of the rods with that
of the cones to prove that no important differ-
ence exists in their character. It is only in
man that the cones alone are found in the
yellow spot." The importance of the cone is di-
minished since the discovery of the central
fibre ; for all the physiological value of the rods and cones rests
upon the existence of this filament, of which the external ex-
tremity constitutes the point explained above. The rods and
cones are only two forms of the same element. Their enveloping
membrane approaches near to the cellular tissue, and it is pos-
sible that there exists between it and the arches of connective
tissue (that has been traced up to the inner surface of the layer
of rods and cones), a connection, which, however, remains to
be proved.

The distribution of the rods and cones is such in man that in




Cone from an adult
human eye seen with
the central filament.
(Ritter.)



OF THE HUMAN EYE. 61

the macula lutea cones only exist, and they also predominate in
that neighborhood, but diminish toward the periphery of the
retina. In animals, no general law of distribution of these ele-
ments exists. Toward the periphery of the retina the rods and
cones diminish in length. The internal, gaping extremity of
the rods and the arches, most external of the cellular tissue,
touch, which arrangement has given rise to the belief of the
existence of a membrana limitans externa.

The above opinion of Ritter, that no important difference
exists between the rods and cones, does not seem probable, for
several reasons. It is true that it is a settled matter, that both
the rods and cones are nervous elements, and that both receive
luminous impressions, as the cones in the macula lutea ex-
clusively prevail in man, where vision is most acute. In some
animals, on the other hand, rods only are found in the retina,
and yet, undeniably, such animals can see. This proves that
both elements are susceptible to the impressions of light. Yet
there are important anatomical and functional differences. (See
Max Schultze, Zur Anatomie und Physiologic der Retina, Bonn,
1866.) Some of the differences between the rods and cones, very
briefly enumerated, are the difference in size and form. The
filaments proceeding from the inner extremity of each rod and
cone also differ, those from the latter being thicker, and can be
traced inward further than the former, which is very fine, and
often ends by an enlargement. Each rod and each cone has a
granule in connection with it, those of the cones being con-
siderably larger than the rod granules. In many animals the
cones are wholly wanting, and we always find this to be the
case in the retinae of such animals as live in darkness, as the
bat, the mole, the mouse, and many others. Birds with acute
vision have the retina plentifully supplied with cones, as much
so as the retina of man. Those birds who prefer twilight, as
the owl, have but very few and small cones in the retina. There
is a peculiarity connected with the cones of birds. Each cone
has a powerfully-refracting globule of an intensely yellow or
red color connected with its extremity, through which the light



62



THE ANATOMY AND HISTOLOGY



has to pass. Again, in the owl species the few cones found in
the retina have pale yellow, or colorless globules, with the red
ones entirely wanting. In certain reptiles, as the lizard and
the snake, cones only are found in the retina. This arrange-
ment of the rods and cones is found throughout the animal
kingdom, as far as investigated. Where only imperfect vision
is needed, and no distinct perception of colors is required, and
a supply of quantitative light only demanded, the rods are found
at the expense or to the exclusion of the cones. On the other
hand, wherever we find acuteness of vision, with a nice dis-
tinction of colors, there we find the cones largely or exclusively
prevailing. To perfect vision three things are essential: the
perception of light, the perception of colors, and the conception
of space (Raumsinn). The first functions may he performed
hy the rods alone ; the cones, evidently, are connected with the
second function, and, perhaps, with the third, also.

The Granular Layer. This lamina has generally been di-
vided into the outer granular layer, the intermediate granular

FIG. 18.



Vertical section of the human re-
tina near the yellow spot. Magni-
fied 300 diameters.
- 1. Layer of rods and (rones.
2, 3, 4. Nuclear layers.

2. Layer of granules.

3. Intergranular layer.

4. Layer of granule-cells.

5. Fibrous layer.

6. Layer of ganglion-cells.

7. Membrana limitans.

(From Ritter.)



layer, and the inner granular layer. Eitter has discovered that
the outer and inner layers are different in structure, whilst the
intermediate granular layer is wholly composed of the (nerve)
fibres of Mutter, and of connective tissue, and could, with more




OF THE HUMAN EYE. 63

propriety, be named the " external fibrous layer," as suggested
by Max Schultze. This lamina separates the external and in-
ternal granular layers, the former consisting of granules, and
the latter forming a layer of cells. The whole granular layer
(including the three layers just named) has a thickness in the
central parts of the retina of 0.75mm. Of this the external
granular layer has a thickness of 0.35 mm., the internal lamina
of 0.18 mm., and the intermediate 0.22 mm. Near the ora serrata
the entire layer diminishes to about one-fifth of the above thick-
ness. The granular layer constitutes more than one-third of
the thickness of the retina. The granules are round or ellipsoid,
and have a diameter of 0.005 mm. to 0.01 mm. Some have a
depression on the surface (see Fig. 19), looking toward the
observer. In certain animals the round granules predominate,
as in the lamb ; in other animals the ellipsoid predominate, as
in the calf; whilst in man, both exist in about the same pro-
portion. The ellipsoid have their long diameter vertical, as




FIG. 19.
3







FIG. 19. Granules or nuclei. 1. Filament of the granules. 2. Transverse striae of the
granules of the lamb. 3. The same in man. 4. The same in the calf. The right granule
in 3 shows the central depression. (From Hitter.)

regards the retinal plane. In 1864, Henle (Nachrichten von der
Koniglichen Gesellsehaft der Wissenschaften und der G. A. Uni-
versitdt zu Gottingen) discovered the transverse strise in the
granules. He describes the granules as ellipsoids, and that
each one has three dark transverse strise surrounding it, and
running parallel with the retinal plane. Ritter subsequently
discovered that the round and the ellipsoidal granules are
about equal in number, and that the former have, as a general
thing, two transverse strise, whilst the latter have three. They
have a breadth of 0.001 mm., and the distance between them, is



64 THE ANATOMY AND HISTOLOGY

0.15mm. These striae disappear in a few hours after death.
The longest period that Bitter could discern them was seventeen
hours after dissolution. These striae are found only in the mam-
malia. A few hours after death the borders of the striae become
less distinct, and finally they disappear, leaving only a small
point, thus giving to the granule its dotted or granulated
aspect. These striae can be retained several days, by a weak
solution of chromic acid, or by diluted alcohol. All efforts
made, hitherto, to determine the difference between these striae
and the rest of the granule-body have failed. Each grain is,
likely, composed of two different substances, superimposed in
layers. The outer granules, having the depressions on which
the rods and cones rest, and which are firmly connected with
the fibre of Muller (as also the innermost granules, w T hich have
also a firm connection with the same filaments), do not seem to
possess the transverse striae.

These granules are contained within the fibre of Muller ;
that is, the axial fibres, or central filament, as it leaves the rods
and cones, and proceeds a certain distance, expands, and em-
braces with its walls from two to five of the granules. In the
central parts of the retina the granules are more abundant than
near the periphery. The fibre of Muller consists of a very
delicate membrane, which incloses these granules.

These fibres traverse the mter-granular layer perpendicularly
in their course, where they are interlaced by bundles of cellu-
lar fibres. In this lamina two of the fibres of Muller are some-
times seen to run into each other to form one filament. The
innermost layer of the granular lamina is the thinnest of the
three granular layers, and, until Ritter's investigations, its
cells were considered the same in character as the granular
bodies of the external layer. They are cells (Fig. 21) of
0.01 mm. in diameter, and are round and polyhedrian in form.
In their fresh state they are entirely transparent, and have
the appearance of small vesicles. "Within the cell there is a
finely granulated substance, and a large, round nucleus, with
distinct outlines, containing a nucleolus of 0.006 mm. diameter.



OF THE HUMAN EYE.



65



From the angles of each cell a process is given off each cell
sending off two or three one precisely in the internal pole of



FIG. 20.




Not copied from observation, but
intended merely to show the con-
nection of the retinal layers.

1. Rod, with the axial fibre.

2. Rod granule.

3. Fibre of Miiller, inclosing

granules of the granular
layer.

4. Intermediate granular layer.

5. Granule cell.

6. Fibrous layer.

7. Ganglion cell.

8. Optic nerve fibre.



the cell, and the others from its external surface. The inter-
nal fibre enters the fibrous layer, and the external penetrates



FIG. 21.





Cells of the granular layer with filaments. (Hitter.)

the inter-granular layer. These fibres are generally denomi-
nated the radiary fibres. Eitter prefers naming them the fibres

5



66 THE ANATOMY AND HISTOLOGY

of Miiller, being nerve-filaments, and they must be distin-
guished from the connective tissue system of fibres, which
will soon be described.

Two fibres originate from each cell externally, or, when
only one is given off, it soon divides into branches, variable in
number, which proceed toward the granules. The internal
fibre, after a short perpendicular direction, runs in various
directions before reaching its destination in a ganglion-cell.

The Layer of Ganglion-Cells. The multipolar nerve-cells, con-
stituting this lamina, have the same character as those of the
brain, and have a diameter of //r .004 to //r .016. They are very
finely granulated, and possess a nucleus of O r// .003 to //r .005,
with a distinct nucleolus. They form an unequal lamina, in
the centre of the retina being composed of eight layers of cells,
the number diminishing until near the ora serrata, where they
no longer form a continuous layer, but occur quite isolated.
Under the influence of hardening substances the granulated
contents may be removed from the cellular membrane, which
is found to be a vitreous membrane, very delicate, which
shows itself in ruptured cells, and in pieces prepared by tear-
ing, in the form of small isolated scales. It is rarely that a
cell contains two nuclei. The size of a cell will point out the
portion of the retina to which it belongs. The smaller cells
occupy the centre of the retina, and the larger are found in
the periphery. This disposition of the ganglion-cells is ob-
served in all animals ; the retinal ganglion-cells have a direct
relation to the size of the ganglion-cells of the brain. The
same relation exists between the length and the breadth of
the fibres of the retina and the ganglionic fibres of the brain,
which are identical in the same animal.

Externally, the ganglion-cells give off processes or fibres, in
variable number, from two to twenty-five, according to Ritter.
Internally only one filament is given off, which soon becomes
continuous with an optic nerve fibre. Those given off exter-
nally plunge into the fibrous lamina. The cells in the centre
of the retina send off less filaments than those near the peri-



OF THE HUMAN EYE.



67



phery. The smallest processes have a breadth, of 0.002mm.,
whilst the largest have a thickness half the size of the cell.
These processes originate in the cell, and for some distance in
the course of the fibre the granulated substance that fills the
cells can be detected. These divide into a certain number of
branches, in their course to the granule-cells, and in the fibrous
layer they form a brush by divergent ramifications, which
sometimes cross like the connective tissue fibres, with the
distinction, however, that they never anastomose like the
latter. These bifurcations are less common in the central



cz &



FIG. 22.

cz'ct> a a eta




Sketch showing the connection between the various nervous elements of the retina, as
taught by Ritter. #, a, a, a. Cones, d, d, d, d, d. Rods. b. Rod and cone granules,
c. Granules, d. Inner layer of granules, e. Progress of the fibres of Miiller through
the intermediate granular layer, many uniting in one granule-cell,/, g. Further prog-
ress inward of the fibres of Miiller through the fibrous lamina, many uniting in one
ganglion-cell, //. *', i. Optic nerve fibres. (Strict anatomical correctness is not claimed
for this sketch, being merely illustrative.)



68 THE ANATOMY AND HISTOLOGY

parts of the retina than near the periphery, which accounts
for the fact that the fibrous lamina is more striated perpen-
dicular to its direction than near the ora serrata. The fibrous
lamina is made up of two systems of fibres ; the one consisting
of cellular connective fibres, and the other is formed by the ex-
ternal prolongations of the ganglion-cells. These two systems
of fibres interlace, crossing each other in such a manner as to
form a complicated network, that can only be unravelled by
the skilful microscopist, and then only by untiring patience.

On the internal surface of each ganglion-cell but one pro-
longation is given off, consisting of a pale fibre of 0.0025 mm.
breadth, which, soon after leaving the cell, dilates into varicose
expansions with considerable regularity. They are now rec-
ognized as optic nerve fibres, running the same course, and
having the same characteristics. This then traces a nervous
connection between the rods and cones arid the fibres of the
optic nerve.

It will be perceived that this is quite a modification of the
radiary fibre system of Miiller, as
taught in text-books heretofore. Bit-
ter first successfully traced the true
character of the fibres of Miiller in
the retina of the whale (Balcena mysti-
the retina cetus). He has certainly made consid-
of man. Magnified 300 diame- erable progress in pursuing the work

ters. (From Bitter.) ?

begun by the great Hemrrch Muller,

and the anatomy of the retina begins to emerge from the un-
satisfactory cloudiness that previously enveloped it.

The Lamina of Optic Nerve fibres. This forms the innermost
layer of the nervous elements of the retina, and lies on the
membrana limitans. The optic nerve, from its commissure to
the eye, has a structure somewhat similar to an ordinary nerve
(see description of this nerve), and its varicose, dark-bordered
fibres are surrounded by an ordinary neurilemma. The nerve
loses its sheath in the sclerotica, and on the inner surface of
the latter the neurilemma of the fibres also terminates, where




OFTHEHUMANEYE. 69

it is connected with the lamina cribrosa, so that in their further
progress within the eye, the nerve-tubules are divested of their
connective tissue surroundings (Kolliker). They are pale nerve-
fibres, and have a diameter of O r// .0005 to O r// .001, which, from
want of granules, are strongly refractive, and have, at least in
the dead eye, spindle-shaped varicosities.

They unite in laterally compressed fasciculi of various sizes,
which anastomose mostly by sharp angles, leaving, posteriorly,
small, and nearer the periphery, larger interspaces, which are
filled up by cellular tissue, which forms a considerable portion
of this lamina. These fibres are identical with the pale fibres
of the brain (Kolliker, Bitter). They radiate in all directions
from the papilla nervi optici, and form a nervous expansion,
which extends as far as the ora serrata, but is not a continuous
layer near the latter point, the fibres being only found at inter-
vals. The thickness of the layer of optic fibres is 0"'.090 close
to the entrance of the optic nerve; more anteriorly, 0'".028 to
0'".036; quite in front, 0'".002 ; at the bottom' of the eye,
0'".036 ; two lines external to the yellow spot, 0'".006 to 0'".008
(Kolliker). This rapid diminution of the optic nerve fibre layer
arises from the fact that the fibres are lost in the ganglion-
cells, or, more properly speaking, in a histological sense, they
originate in those cells. It is then easily understood why the
lamina rapidly diminishes in localities where there are many
cells agglomerated. For this reason, the lamina almost disap-
pears in the central part of the retina. At the macula lutea
only a small part of the optic fibres proceed directly to the
inner end ; much the larger portion which are destined for the
lateral parts of the spot, describe a series of curves, which take
sweeps as they advance forward. At the yellow spot itself,
these fibres lose themselves in its deeper portion, among the
ganglion-cells, so that here there is no superficial layer of optic
fibres ; the nerve-fibres of this spot are, probably, the processes
of the ganglion-cells (Kolliker).

The membrana limitans retince, on the inner side of the ex-
pansion of the optic fibres, has a thickness 0' r/ .0005, and ex-



70 THE ANATOMY AND HISTOLOGY

tends forward as far as the ora serrata ; and some histologists
assert that its connective fibres extend further forward on the
inner surface of the membrana limitans uvece, under the name
of pars ciliares retince. Heinrich Miiller, Kolliker, and Pilz,
consider it to belong, in structure, to the vitreous membranes.
Max Schultze declares it is composed of cellular tissue, and
that it is formed by the expansion of the radiary fibres. More
recently, Carl Bitter, by his investigations on the retina of the
whale, has demonstrated that it consists of cellular tissue.

The cellular tissue of the retina, says Hitter, belongs to a variety
of connective tissue which Virchow discovered in the brain, and
named neuroglia. In the mammalia, the utmost extent to which
the retinal cellular tissue can be isolated is a fibre-cell, i. <?.,
a fusiform cell with two elongated extremities, and which con-
tains a rounded nucleus. This nucleus measures 0.005mm., is
slightly granulated, and sometimes has a rounded nucleolus. In
rare cases, the cell contains some granules along the circumfer-
ence of the nucleus. These cells are united with each other by
their elongated extremities, which sometimes expand like fine
ribbons, and at other times, on the contrary, they assume the
form of thick cords. A certain distance from the cells these
prolongations bifurcate, without being diminished, and a very
fine network is formed, and it is uncertain whether it springs
from the cells, or whether it is independent of them. These
cells vary in size, the smaller being found in the central parts
of the retina, and the larger toward the periphery. Their size
also varies according to the layer in which they are found.
They also vary in configuration, according to the layer in which
they exist. In regarding Figures 24 and 25, it is difficult to
distinguish that they are of the same character. Their in-
vestigation is most easy in the region of the ora serrata, as in
that region the cellular tissue largely predominates over the
nervous.

The membrana limitans is exclusively composed of cellular tis-
sue, and hence it is appropriate to begin a description of the con-
nective tissue from this point. It is a thin membrane, of not



OF THE HUMAN EYE. 71

more than 0.002 mm. in thickness, except in the region of the
ora serrata, where it is thicker. It is a continuous membrane,
and is in contact internally with the hyaloid membrane. On





Cells of the connective tissue of the hu- Vitreous metamorphosis of the cells of

man retina. Magnified 300 diameters. connective tissue of the human retina.
(From Bitter.) Magnified 300 diameters. (From Ritter.)



the outer surface it is rugous, and gives origin to numerous
compact fibres, which pass into the retina, and have received
the name of limitary fibres. This membrane is transparent, and
possesses two parallel surfaces, the external of which is inter-
rupted by the fibres. It is distinguished from the vitreous

FIG. 26.




Fibres of the membrana limitans, traced as far as the fibrous layer in the retina of
a man. Magnified 300 diameters. (From, Ritter.)

membrane, to which it adheres, by a peculiar rigidity, by striae,
which extend irregularly from one surface to the other, and by
the presence of some disseminated nuclei. The aspect of rigidity
which this membrane possesses is caused by the deep coloration
of its outlines. Its striae and its nuclei distinguish its charac-
ter. These transverse striae traverse the membrane in various
angles, yet they run in pairs, which assume a parallel direction.
On the internal surface they are confined to the limitary mem-
brane, whilst externally they are confounded with fibres ema-
nating therefrom, which connect the limitary membrane to the
other parts of the cellular tissue of the retina. The nuclei dis-
seminated through this limitary membrane are always found



72 THE ANATOMY AND HISTOLOGY

between two parallel striae. The nuclei of the fibres which are
connected with the limitary membrane are not, generally, di-
rectly attached to this membrane, but occupy fibres at some
distance from it. The size of the cells of the limitary membrane
is somewhat uniform, diminishing but little in the central
parts of the retina, and enlarge in but small proportion to-
ward the periphery. The length of the cells is, however, vari-
able, being short at the centre and the periphery of the retina,
whilst at intermediate points they are found considerably
longer. It is but seldom that an entire cell is contained within
the thickness of the limitary membrane. The greatest im-
portance is to be attached to its investigation, in consequence
of the fibres given off from it. For a long time these fibres
have been described as being attached to the limitary mem-
brane by an expanded triangular base, after having commenced
by a slender origin, as seen in Fig. 27, from Kolliker,- the ex-
pansion at &, representing the termination of a fibre
of Miiller on the membrana limitans. Ritter is quite
certain that this is not correct. By the fibres of the
membrana limitans is understood only those fibres
which can be traced to the- fibrous lamina in which
they decidedly change in character. These fibres
are composed of filiform cells, which communicate
with each other by their filiform prolongations. In
the central parts oT the retina these fibres have a
perpendicular course from the membrana limitans to
the fibrous layer; they are not united with each
other, and preserve throughout a breadth uniformly
of 0.002mm., which is identical in thickness with the
fibres of the limitary membrane. In the vicinity of
the ora serrata, where the nerve-fibres and the gan-
glion-cells are almost wanting, the size of these fibres increases
to two or three times the dimensions above given. They run
in various directions, forming various angles, and a network of
large meshes. In the most narrow interspaces are found, at
intervals, a few nerve-fibres, and in the largest meshes ganglion-



OF THE HUMAN EYE. 73


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