corpuscles of the gray matter possess no distinctive differences,
such as may be appreciated by the microscope, while, on the
other hand, the peripheral termini appear under many different
forms, the peculiarity of ending being dependent in part upon
the type of tissue in which they are found, partly upon the
office they have to perform, and partly upon other causes that
are unknown to us. The nerve-centres are located in the brain,
spinal cord, and in the ganglia of the cerebro-spinal and sym-
pathetic system.
The methods of nerve- terminations that have been described
may be briefly enumerated here. They are by (1) peculiar
terminal bodies, (2) loops, (3) networks, (4) end bulbs, (5) proto-
plasmic bodies (cells), (6) free or pointed extremities.
Nerve-fibres. Of these there are three kinds that have
distinctive differences : 1. The myelinic or medullated fibres.
2. Fibres of RemaJc. 3. Ultimate fibrils. Intermediate forms,
such as have been described by various writers, under the names
of protoplasmic processes, primitive fasciculi or naked axis-
cylinders, varicose cylinders, etc., will be noticed in other con-
nections.
Myelinic fibres. These are also known as the medullated.
110 MANUAL OF HISTOLOGY.
To the naked eye they appear white and glistening, and are
the main constituents of the peripheric nerves, though they
occur in less number in the sympathetic and also in the brain
and cord. Each fibre is made up of three distinct parts : (a) a
central cylindrical cord, the axis-cylinder, about which is a (b)
coating of soft homogeneous fatty material, called myeline
(medulla, white substance of Schwann), forming for the axis-
cylinder a sort of tubular sheath, while exterior to both is a
delicate membrane or envelope (c\ the sJieath of ScTiwann or
primitive sheath. 1 These fibres run a parallel unbranching
course, except near their termini or origin, and are surrounded
by a connective-tissue coating of varying thickness. Their
diameter varies also according to their situation and the degree
of their tension or relaxation. In the nerve-trunks the average
diameter lies between -fa an ^ iio- millimetre. In the brain they
are described as having sometimes a diameter of -g-J-g- jnilli metre,
but it is difficult to determine the presence of a medulla in such
small fibres.
To study the properties of a myelinic nerve, we may take a
portion of the sciatic from a frog that has just been killed.
Having removed it with care and placed it in a drop of water
on a slide, we should separate the fibres carefully with needles,
taking care not to tear them. Then adjusting a covering
glass, it will be seen that from the broken end of the nerve a
soft substance is exuding (Fig. 43, 5) ; in a few minutes it is
pushed off in the form of drops of irregular shapes (Fig. 43, c).
This material is the myeline or medulla. It will be seen to re-
fract the light strongly, and show concentric markings. It will
also be seen that each fibre has a double contour and is divided
at tolerably regular intervals by transverse divisions, which are
now known as Ranmer* s nodes. (See Fig. 47.) Midway be-
tween each node we may perhaps see an oval body surrounded
by a broad expansion of protoplasm. In a few fibres we may
even see that a fine thread-like process is projecting from the
broken ends of the nerve-fibre the axis-cylinder (Fig. 43, d)
while the whole fibre is enclosed by a delicate tightly investing
membrane, the sheath of Schwann. Possibly we may also see the
1 A most unfortunate source of confusion among histologists has arisen from the
use of the word neurilemma, which by some is spoken of as synonymous with
Schwann 1 s sheath (Frey), and by others as the connective tissue which binds the
nerve-fibres together (Klein, Rutherford). We shall avoid the term altogether.
GENERAL HISTOLOGY OF THE NERVOUS SYSTEM. Ill
oblique or arrow markings (incisures of Schmidt) (Fig. 43, /),
which seem first to have been accurately described by Schmidt,
of New Orleans, later by Lantermann, of Cleveland, Shaw, and
others. The same appearances can be also obtained by the use
of iodized serum.
The double contour is not visible in all the myelinic nerves,
but is most marked where they show varicose swellings, a con-
dition that is due to a preponder-
ance of myeline at the enlarged
point. From this fact and anoth-
er, that the drops of myeline when
separated from the fibre show the
same double contour, it is argued
that the double marking in the
fibre is due to a refracting (double)
of the myeline, and has nothing to
do with the membranous sheath.
These varicosities just mentioned
are not to be confounded with the
bulgings of the ultimate fibrils, or
with the " necklace " appearances
seen in the course of the fibres of
Remak, both of which latter may
probably be regarded as artificial
productions, either from stretching
in the act of teasing or from the
imbibition of water. In the brain
of the calf they are frequently seen,
and they are said to be found in the
intracranial part of the olfactory,
optic, and acoustic nerves. The
fibres in which this change occurs
are usually quite small.
Staining in picro-carmine.
This reagent has been recommended by Ranvier. It is satis-
factorily prepared by Rutherford's process. 1 Taking precau-
tions not to injure the nerve in removing it, mount in the solu-
1 He takes 100 c.c. of a saturated solution of picric acid. Next he prepares an
ammoniacal solution of carmine by dissolving one gramme in a few c, c. of water,
with the aid of an excess of ammonia and heat. He then boils the picric acid solu-
tion on a sand-bath, and when boiling adds the carmine solution. The mixture is
Fio. 43. a, Myelinic fibre in a state of
" coagulation ; " &, myeline exuding from
the broken end of the fibre ; c, drops of mye-
line separated from the nerve-fibre ; rf, axis
cylinder ; <?, nucleus of Henle's sheath ; /,
arrow markings.
112
MANUAL OF HISTOLOGY.
tion. The nuclei will then be stained a brick-red, while the
sheath of Schwann, and, in fact, the whole nerve, will be stained
yellow. It is said that, if the axis-cylinder projects, it will be
stained a bright red, though twenty-four hours may be required
to effect the staining. In my hands picro-carmine has not
proved so successful a coloring agent as some others.
Staining witJi the nitrate of silver. The sciatic or any
peripheral nerve may be employed. Expose it without re-
moval in a frog that has just been killed. Then dry up all
fluid from about it, and pour on a solution of the nitrate (1 to
1,000). In this way the nerve-fibres will be made rigid. They
are then to be removed with a pair of delicate scissors, and
placed in a flat vessel containing a little more of the solution.
After a few minutes the nerve will look turbid, and then it
should be cut out and washed in distilled water, and exposed
to the sunlight. In a variable time (ten to fifteen minutes) the
turbid appearance will give way to a brown coloration. Exam-
ining a single funicu-
lus or bundle in gly-
cerine, it will be seen
that it has an endothe-
lial coating of one or
more layers (Fig. 44).
If another f uniculus
be separated with fine
needles, 1 the same care
being taken to spread
the fibres apart and not
tease, and so lacerate
them, it will be seen
that each fibre con-
tains a series of Latin
crosses at certain pretty regular intervals. The transverse bar
of the cross corresponds to the " annular constriction" seen
in Ranvier's node, while the axis-cylinder forms the longitudi-
nal bar. Close observation with high powers will show that
this latter is marked by transverse lines of a dark brown or
then evaporated to dryness, the residue dissolved in 100 c.c. of water, and filtered.
If the solution is not clear, he adds more ammonia, evaporates, and then dissolves as
before.
1 Milliners 7 are the best.
FIG. 44. Fnniculns or Nerve Bundle covered with Endothe-
lium (Epithelium). From the sciatic of the frog. Hartnack,
object. 4, oc. 2.
GENERAL HISTOLOGY OF THE NERVOUS SYSTEM. 113
black (Frommanrfs lines). It appears probable, as Ranvier
explains, that, owing to the break in the myeline, at the * c an-
nular constriction," the particles of silver gain an entrance to
the axis-cylinder at this the only unprotected spot. If the
action of the salt is long con-
tinued, the axis-cylinder is col-
ored for a somewhat longer
distance. The transverse bar
seems to be formed of two
conical segments set base to
base. The position of this bi-
conical segment usually cor-
responds in position with the
"annular constriction," but
i t won 1 rl n rmpa v fh a f fh PV m n v FlG ' 45< ~ a ' Ranvier ' s disk 5 & ' Frommann's lines ;
11 WOUld clppedT tlldt tliey nitty c , nucleus of interannular segment.
be separated, for, when the
tissue of the nerve has been put upon the stretch, the biconical
segment may be drawn away from the annular constriction.
(See Fig. 45.)
Now, as Schwann's sheath is understood to end at the an-
nular constriction, where it is cemented to the next adjoining
segment just as epithelial cells are joined together, the biconi-
cal disk may belong to the axis-cylinder exclusively, and
merely constitute a dividing line between its segments. Ac-
cording to Engelmann, the axis-cylinder is divided up into
portions corresponding with the interannular segments.
According to Bawitz, Schwann's sheath does not end at the nodes, but is
continuous with the sheath of the adjacent interannular segment.
Staining of the nerve in osmic acid semi-desiccation.
Osmic acid is one of the most valuable reagents for histological
work, and the method now to be described (a modification of
KanvierV) succeeds well. Take the frog's sciatic, or any other
peripheral nerve, carefully remove a portion with the surround-
ing tissue, keep the whole extended with pins, upon a flat bit
of cork, and then dip it into a vessel containing a 1 per cent,
watery solution of osmic acid. 2 The vessel is then to be exposed
to the light. The whole nerve will be more or less thoroughly
1 Lemons sur 1'Histologie du Systeme Nerveux, Paris, 1878.
a The solution should, of course, have been kept in a dark bottle away from the
light.
114
MANUAL OF HISTOLOGY.
6.-.
stained in a few hours. The external portions, however, will
be stained in a few minutes, and they may be removed by care-
ful separation with fine needles. To mount, take a glass slide
and slip it under the nerve-fibres, while the needle is employed
to carry them up on to a dry part of the slide
where they can be placed side by side. Then
remove the excess of water with bibulous paper,
and let the fibres get so dry that they adhere
to the slide. Place about them a ring of tis-
sue-paper, so that when the cover is adjusted
it will not press upon the fibres. Fix the cover
at different points with paraffine, then put a
drop of glycerine upon one side, and a drop of
water upon the other. The union of water and
glycerine should be allowed to go on for
twenty-four hours in a damp place. The con-
strictions and arrow-markings are usually well
seen. The nuclei also are occasionally to be
found in a niche of the myeline. These bodies,
however, are better seen in specimens that have
been a short time (fifteen or twenty minutes)
in osmic acid, and then in picro-carmine a few
hours. It still is a question among histologists
whether the arrow-markings are artificial or
not ; each of the sections lying between the
markings is called the cylindro-conical segment
(Hohlcylinder, Kuhnt). (See Fig. 43.)
Transverse sections of myelinic 'nerves.
Certain points are best seen by making trans-
verse sections. Prepare the sciatic of a frog or
any of the human peripheral nerves by im-
mersing a few days in a sherry-colored solution
of bichromate of potash, or in Mueller' s fluid, *
and then in 90 per cent, of alcohol, until the tis-
sue is hard enough to cut. Then it is to be
mounted in the microtome with wax and oil of
about its own consistence. Sections are to be made with the
razor; or it may be mounted in elder-pith in the following
way : bore out from the centre of the pith-cylinder a cylindri-
1 The well-known eye-fluid, of which the composition is : Bichromate of potash,
2 to 2 grammes ; sulphate of soda, 1 gramme ; distilled water, 100 grammes.
FIG. 46. Human
myelinic nerve : a, In-
terannular segment ; &,
Ranvier's node ; c. nu-
cleus of the interannu-
lar segment surround-
ed by granular proto-
plasm; rf, Henle's
sheath with nucleus.
GENERAL HISTOLOGY OF THE NERVOUS SYSTEM. 115
cal liole a little larger than the trunk of the nerve, then im-
merse the whole in water, and the pith will begin to swell. As
soon as it has firmly embraced the nerve, sections may be
made with the knife. Ammonia-carmine will stain the axis-
cylinder well, but the outline of the cut will appear irregular
rather than round. This appearance is doubtless artificial. In
my hands, borax-carmine l has proved much better than the
ammonia-carmine, as it diffuses very little, and much of the
excess may be removed by dilute acetic acid (about J per cent.),
in which the specimen should remain, from a few seconds to a
minute or two, until it has become bright to the eye. The fur-
ther steps in the process of making a permanent preparation
are the same as those for other specimens; i.e., it may be
mounted in glycerine and water, or clarified by clove-oil and
mounted in dammar varnish or Canada balsam.
Preparation ~by the bichromate of ammonia. Ranvier em-
ploys of this a 2 per cent, solution, allowing the specimen to
remain, with frequent changes of the
fluid, from two or three months to a
year. The sections are to be stained
in ammonia-carmine or picro-carmine.
and mounted in glycerine. It will
then be seen that immediately about
the axis-cylinder is a sheath. This is
called by Ranvier the sTieath of MautJi-
ner, from the author who described it.
(See Fig. 46, b.) Specimens prepared
in the ordinary way, by long immer- -* 8 *-*
sion in Mueller's fluid alone, or sub-
sequently in the chromic acid solution (gr. ij. 1 j.) and stained
with ammonia-carmine, occasionally show the same thing.
Sometimes histologists find that embedding in gum succeeds
best in securing these transverse sections of nerves. The diffi-
culty of the task is one of considerable moment. The method
is as follows : Take a fresh nerve, harden it in osmic acid (1 per
cent., if it is desirable to expedite the process, or T V per cent,
if it is not necessary to conclude the examination the same
day). Then, when the nerve is thoroughly blackened all through,
1 The powder is prepared by Eimer & Amend, of this city (205 to 211 Third
Avenue), according 1 to Arnold's formula. The strength required is> gr. xv. j.
distilled water.
116 MANUAL OF HISTOLOGY.
it is to be immersed in water for a few hours ; then in 90 per
cent, alcohol, and then in a weak solution of gum-arabic,
which fills the interstices between the bundles, and finally in
strong alcohol (95 per cent.), which hardens the gum sufficiently.
The sections, cut as thin as possible, should be placed on a
slide to remove the excess of alcohol, which may be done with
filter-paper. A drop of water is then to be added ; about
the cover put a few drops of carbolized water ; remove to a
damp place. At the end of twenty -four hours the gum will
have dissolved, and then the glycerine may be allowed to enter
slowly without displacing the elements (Ranvier).
In examining such cross-sections, the medullated nerves will
present various diameters, and the contour of the myelinic
sheath will vary in width and outline according as the cut
comes through the broadest part of the arrow-marking, or
through the thin overlapping parts. (See Fig. 43.) If the cut
chances to pass close to the annular constriction, no myeline
will of course be seen. For these reasons, the cross-sections of
such nerves, when stained with osmic acid, are very different.
Modern conceptions of myelinic nerves. The specimens
that have been studied according to the methods given will not
have shown any termination of the nerves, or any division,
either into trunks of any considerable size or into the fibrils of
which they are said to be composed. They do, however, as we
have already said, divide both near their origin and near their
termination. It is presumed that each fibril of which the axis-
cylinder is composed passes directly through from its point of
origin of the nerve-centres, to its final point of distribution,
without branching. It is difficult, however, with the instru-
ments in ordinary use, to see any distinct marks of fibrillation
in cross-sections of the axis- cylinder, and it is in them that we
should expect to see them best. The ideas pf Ranvier are well
worthy of consideration, as he has given more form and solid-
ity to our conception of the intimate structure of a myelinic
nerve-fibre than any previous writer. According to him, each
section of nerve between the annular constrictions represents
an ultimate morphological element. It is, in fact, a tubular
cell, whose proper external portion (the membrane of the cell,
according to common phraseology) is the sheath of Schwann,
while the myeline or medulla fills the interior, just as in adi-
pose tissue a globule of oil fills out and distends an ordinary
GENERAL HISTOLOGY OF THE NERVOUS SYSTEM. 117
connective- tissue corpuscle. Each of these bodies, which he
calls an interannular segment, begins and ends at the constric-
tion. It contains a single ovoid flattened nucleus, which fills
a niche in the myeline, and is surrounded by a broad, thin ex-
pansion of protoplasm (the body of the corpuscle). The axis-
cylinder has nothing to do with this body that we have de-
scribed, except that it pierces it. Instead of stopping short at
each constriction, it goes on indefinitely. As we have already
seen, the annular constriction and the biconical disk are not
always at the same point, which argues strongly for Ranvier's
views. The myelinic sheath probably protects the delicate
fibre from external injury, but whether it also insulates it, is
problematical. In the foetus all nerves are devoid of myeline.
Fibres of Remak. These are called by some the amyelinic
or non-medullated fibres, by others the pale, gray, or gelati-
nous fibres. The term Remak' s fibres has come into use re-
cently as the distinctive name for certain nerve-fibres abound-
ing in the sympathetic, as distinguished from others which
also contain no myeline, and are found in the cranial portions
of the optic, auditory, and olfactory nerves. Each fibre is
marked with oval nuclei at pretty short intervals, and has an
indistinct longitudinal striation, probably the evidence of fibrils
such as are believed to exist in the axis-cylinder. The nuclei
are imbedded in a homogeneous sheath. There being no breaks
in the continuity of the fibre, there can be no sheath of Schwann
in the sense that has been described. In diameter each fibre va-
ries between -fa and T i 7 millimetre. In 1838 Remak first called
attention to them, but his views were received with disfavor.
More recently, Max Schultze, Frey, Leydig, and Henle have
joined in representing them as long, cylindrical, continuous,
slightly striated, and dotted with nuclei.
The fibres of Remak are found in great abundance in all
the nerves of the organic system, but they also exist in all the
mixed nerves, varying with the kind of nerve and the animal.
They are not found in special nerves. The pneumogastric of the
cat is well adapted for the study of them, as the myelinic fibres
are present in considerable quantity, and make the mechanical
separation of the bundle easy. Associated with them, fibres are
often seen, that are shown in Fig. 48, c. They are delicate,
run a wavy course, and sometimes exhibit curious varicosities
(a), (necklace appearance). The nuclei are placed at about the
118
MANUAL OF HISTOLOGY.
same distances apart as in the other form of fibre already men-
tioned.
Preparation in osmic acid and pier o-car mine. Remove
the pneumogastric in the following way, from a cat that has
just been killed : Having exposed the nerve, slip under it in
situ a long narrow strip of cork, to which, pin down the nerve
with some adjacent tissue, all of which may be removed at
once and placed in a solution
of osmic acid (11,000) for
twenty -four hours ; the nerve
may then be separated from
its attachments and placed
in the picro-carmine solution
for still another twenty -four
hours. The excess of the col-
oring agent may be removed
by dipping for a few seconds
in acetic-acid solution (J- per
cent.), and then the nerve
may be placed in alcohol,
afterwards in water, and fin-
ally mounted in glycerine.
It will be seen that the nerve-
fibres are stained a reddish
yellow, while the nuclei are
brick-red. The picric-acid
yellow is apt, however, to
diffuse. Careful separation
of the fibres may show that
they branch, as shown in Fig.
48, A, B ; and yet this char-
acteristic, which Eanvier in-
sists upon, is by no means
easy to see in most of the fibres, in fact it requires much care-
ful work before it is apparent. The myelinic nerves will be dis-
tinguished by their greater average size, their dusky, granular
medulla, broken at points, and by the axis-cylinder, which,
if it does not project, may be seen winding spirally along be-
neath its medullary coat. In them, too, as a rule, each in-
terannular segment contains but one nucleus.
Preparation of Remak" 1 s fibres in hcematoxylin. One of
FIG. 48. Fibres of Remak. A, Pneumogastric
of the cat haeiuatoxylin specimen : a, nerve nu-
clei ; &, appearances of branching ; c, connective-
tissue sheath. B, Same. Picro-carmine specimen.
The branching in this case is more evident. C,
Same haematoxylin specimen. The necklace ap-
pearance is >shown at a.
GENERAL HISTOLOGY^ OF THE NERVOUS SYSTEM. 119
the most rapid and successful methods is by the use of hsema-
toxylin. The pneumogastric nerve of a cat is removed and
immediately placed in the hsematoxylin solution ; then, after
thorough staining, which may only take a few minutes, in
dilute acetic acid (^ per cent.), and finally mounted in gly-
cerine. In this way the nuclei will be stained a beautiful pur-
ple, while the fibres will be unaffected. The number of nuclei
and absence of medulla will serve to distinguish the fibres of
Eemak from the medullated. It is difficult by any method of
preparation to see that there are any precise limits to the lon-
gitudinal lines in the fibres, i.e., that the striation is due to
little, short, narrow rods, lying side by side (Ranvier). The
nitrate of silver demonstrates no transverse markings and no
constrictions or crosses. There is but little likelihood in these
specimens to mistake the fibres for connective- tissue bundles.
In the first place, the nuclei, and what cell-bodies happen to
be about them, of the one, are small, flattened, ovoid bodies
occurring at pretty regular intervals, while the connective-tis-
sue corpuscles are usually larger, longer, and, though they
may appear oat- shaped, when the side is turned to the observer,
are broad plates with irregular edges when seen flatwise. In
the second place, the fibres run their course in long, narrow
bundles, as no connective tissue does.
Ganglionio bodies. Of these there are three kinds : 1.
Those that are connected with the spinal and some cerebral
nerves. 2. Those found in the gray substance of the brain and
spinal cord. 3. Those in the ganglia of the sympathetic sys-
tem. These bodies are of such large size that they may often
be seen with the naked eye. In the human species they are
usually in close connection with the origin of the nerves, though
they also may be interspersed at points through the course of
the fibres or may be present near their points of distribution
(ganglia of AuerbacTi). Their immediate connection with the
nerve-fibre is made in the following ways : 1. A large process,
which does not at first appear to branch, passes off, and is
continuous with the axis- cylinder. 2. Fine branches are given
off from one or more corpuscles, and, uniting, contrive to form
a nerve-fibre (either a fibre of Eemak or a myelinic fibre). 3.
These branches after combination may pass through a gangli-
onic corpuscle, which then is called bipolar (Gerlach, Wal-
deyer). In the sympathetic system we have the unbranched
120 MANUAL OF HISTOLOGY.
process and the superficial or spiral fibre, which corresponds to