With the tweezers pick up the pupillary margin of the iris, and with the
fine-pointed scissors cut through the iris and the ciliary processes
(Fig. 31); separate both from the choroid by cutting close to the
posterior edge of the processes. (Fig. 32.) In doing that, cut partly
through the vitreous also, but be careful not to injure the peripheral
edge of the retina--ora serrata. After the iris has been separated from
the choroid, cut completely through the vitreous in such a way that the
lens will also be removed with the iris. (Fig. 33.)
[Illustration: Fig. 34--Showing how to force glass blowpipe (A) into
vitreous (B). (Page 80.)]
[Illustration: Fig. 35--Showing bulging out of vitreous caused by
blowing air through glass blowpipe. (Page 80.)]
[Illustration: Fig. 36--Showing the vitreous (A) removed.]
Holding the eye suspended by its optic nerve, force the glass blowpipe
through the vitreous until it all but touches the posterior part of
the retina (Fig. 34); blow gently at first, increasing the pressure
until the vitreous suddenly bulges outward. (Fig. 35.) If the iris has
been cut away close to the ora serrata, the vitreous will not only bulge
forward, but it will fall out. If, however, it does not detach itself at
once, insert the scalpel close to the choroid and with its flat side
press downward until a separation occurs. Do not let the vitreous drop
out too suddenly, because it may tear the retina. Let the vitreous
detach itself slowly by the force of its own weight, though it will be
well to hold some of its weight on the scalpel. (Fig. 36.)
[Illustration: Fig. 37--A. Showing retina folded upon itself by blowing
air at it through the glass blowpipe. (Page 83.)]
[Illustration: Fig. 38--A. Showing folded retina suspended from its
attachment, so sclerotic and choroid may be easily cut away. (Page 83.)]
After the vitreous has been removed, turn the eye upward, and by blowing
strongly through the blowpipe at the marginal edge of the retina, turn
the retina upon itself. Repeat this until the retina lies in a small
wrinkled lump at the “bottom” of the posterior part of the eye. (Fig.
37.) Invert the eye (Fig. 38) and cut away both the choroid and the
sclerotic close to the optic nerve. No care need be taken in doing this
until the scissors come close to the optic nerve. (Fig. 39.)
[Illustration: Fig. 39--Showing the sclerotic nearly all cut away.]
[Illustration: Fig. 40--Isolated retina, with optic nerve attached.]
After the choroid and the sclerotic have been cut away, drop the retina
into some water, and it will slowly unfold itself by “ballooning” out
into a perfect and beautiful specimen. (Fig. 40.) But, if it is desired
to study the specimen closely, it is better to suspend it in a jar or
bottle made of thin glass, and containing a 5 per cent. solution of
formaldehyde. Remember that the retina is a delicate membrane in any
state; the slightest rough handling may cause it to be torn, or
otherwise damaged. If the vessel, in which the specimen has been placed
and suspended, has enough preserving fluid to completely fill it, and it
is firmly stoppered, the whole thing may be inverted, and turned in any
direction, even abruptly, without fear of damaging the retina. This way
of keeping the retina will give opportunity to inspect and study the
inside as well as the outside of the membrane; the blood-vessels, and
other important parts easily recognized.
THE SAGITTAL OR VERTICAL SECTION OF THE EYE
Place an eye in a 5 per cent. solution of formaldehyde for about two
weeks. If the eye is kept in that solution longer than that time, the
lens is apt to become so hard that in cutting it the capsule and
suspensory ligament will be torn, and the lens will then become
detached; if for a shorter space of time, the lens and other tissues
will be so soft that all may be so badly torn or lacerated, that a
perfect specimen will not be possible.
It sometimes happens that in keeping a number of eyes together in a
vessel for the purpose of hardening them in the formaldehyde solution,
the corneas of some will be crushed in. For this dissection, select an
eye that has the cornea in perfect condition.
Remove all the outside tissues with the scissors, being particular to
have the region immediately surrounding the optic nerve perfectly clear
and clean. If the optic nerve is longer than 5 mm., cut it off to that
length.
[Illustration: Fig. 41--Showing the beginning of the cutting of the eye
for sagittal sections.]
[Illustration: Fig. 42--Showing method of cutting through the
crystalline lens.]
To cut the eye in two, use a safety-razor blade; never a scalpel. The
latter is too thick, too dull, and too clumsy a tool. Begin by cutting
through the optic nerve; dividing it as nearly as possible into halves.
(Fig. 41.) Continue cutting through the sclerotic and all underlying
tissues, stopping at the corneo-scleral junction, but do not, during
this procedure, even touch the lens. After the eye has been thus partly
separated into, as nearly as possible, two equal parts, lay it down upon
the cornea, and, holding the razor blade in the forefingers and thumbs
of both hands, cut the lens in two by forcing the blade down through it.
(Fig. 42.) Partly open the cut eye to allow one jaw of the large
scissors to enter, turn the eye over so the cornea will rest on that
jaw, and then cut through the cornea. (Fig. 43.)
[Illustration: Fig. 43--Showing method of cutting through the cornea and
completing the two sagittal sections.]
The two specimens may be kept indefinitely by placing them in a 3 per
cent. formaldehyde solution. It will be well to remove the lens from one
of the specimens, because it will give better opportunity to see the
anatomical relationships. Also, these specimens should be mounted, one
above the other, between two pieces of glass, before placing them in the
receptacle that is to hold them.
Much can be studied in such specimens. Moreover, they present to view
the various parts of the organ of vision in such an impressive way, that
one does not soon forget the wonderful appearance of the construction of
this, Nature’s perfect camera. (Fig. 44.)
[Illustration: Fig. 44--Sagittal section enlarged.]
If another eye is cut into two parts, additional interesting specimens
may be procured; for instance, one showing the presence of the second
coat only, the retina having been torn out. Another good specimen may be
made by removing all of the inner tissues, and leaving only the
sclerotic and cornea. This specimen will show that the first coat is
almost entirely a coat which affords strength and protection to the
parts that lie within.
THE PAPILLA, PUNCTA LACRIMALIA, AND NASAL DUCT
The only way to dissect the lacrimal apparatus, other ocular
accessories, and the extrinsic muscles, is to procure the head of some
animal, preferably a calf’s head, because of its size. Any butcher will
supply one for from forty to sixty cents. Have the lower jaw removed. It
will make a less bulky piece of material to handle.
Close to the inner canthus, on the inner side of each lid, will be found
a little rounded eminence--lacrimal papilla--in the centre of which is a
small opening--punctum lacrimalis. Both may be seen better on the lower
lid, if it is pulled down, and on the upper lid, if it is pulled up.
[Illustration: Fig. 45--Showing only a part of a calf’s head and the
knitting-needles inserted in the puncta. (Page 94.)]
[Illustration: Fig. 46.]
Be supplied with two knitting-needles. Take one, lubricate its full
length with a little vaseline, lard, oil, or any other lubricant. Insert
the needle into the punctum of the lower lid, and push it downward and
forward, aiming to come out in the nose a short distance from its end.
At first it may be a little difficult to get the needle started; if so,
just wiggle the needle, pushing it at the same time as directed, until
the nasal duct is found. Do not remove the needle. To insert the other
needle into the punctum of the upper lid is rather difficult; for that
reason the punctum of the lower lid was chosen, first. Grease the
needle, as was done to the first one, and, with a little patience and
careful manipulation, the canal opening and its course will soon be
found. The needle may then be pushed through until it meets the first
one. (Fig. 45.) From the puncta lacrimalia to the place of meeting of
the two needles, marks the course of the two canaliculi and their
junction before they merge and form the nasal duct. Leave the needles
where they are, and begin cutting away the skin. The needles will then
mark the course of each canal and the duct very plainly. With the small
scissors the canals and the duct may be loosened from the surrounding
tissues. Or, the scalpel may be used to lay open the canals, cutting
along over the top of the needles. (Fig. 46.)
The cilia, palpebræ, palpebral conjunctiva, ocular conjunctiva, and
other superficial ocular accessories may be examined without
dissection.
MEIBOMIAN GLANDS AND DUCTS
An examination of the eyelids will show the openings of the ducts of the
meibomian glands a short distance back of the cilia. Very fine pins or
needles that have been greased may be easily inserted for a short
distance into the ducts, and then a dissection made along the course of
the duct as outlined by the presence of the inserted pins or needles.
Another way to see the glands is to slice through the ducts, with the
scalpel or safety-razor blade, the entire width of either eyelid. This
will separate the glands into two parts and show their length, breadth
and structure.
ENUCLEATION OF THE ORBITAL CONTENTS
The eyes one procures from a butcher or a slaughter house will always
have the extrinsic tissues so badly cut and torn that identification of
the various parts and their relations is impossible. Therefore, it is
best to supply one’s self with the head of an animal, such as a sheep or
a calf, and dissect an eye with all its extrinsic tissues intact. For
this dissection, a hammer and a chisel are necessary in addition to the
tools needed for doing the previous dissections.
[Illustration: Fig. 47--Showing method of making the initial cuts in the
skin. (Page 97.)]
[Illustration: Fig. 48--Part of calf’s head, showing the first cut to be
made in the bones of the orbit. (Page 102.)]
[Illustration: Fig. 49--Showing all the cuts to be made through the
bones of the orbit. (Page 102.)]
Using the left orbit, begin the dissection by making an incision
directly over the supra-orbital ridge, extending from over the inner to
the outer canthus. At the middle of that line, make an incision, and cut
at right angles upward to the top of the head. Next make a cut below the
eye, extending from the outer to the inner canthus. (Fig. 47.) Loosen
the skin from the bone with the scalpel, and lay bare the skull
immediately over the orbit. Fold the flaps of the skin back and fasten
them down to the skull with pins or tacks so they will not interfere
with the work.
[Illustration: Fig. 50--Showing how to pry the cut bone loose. (Page
102.)]
[Illustration: Fig. 51--“In removing orbital contents dissect close to
the bone.”]
[Illustration: Fig. 52--Showing excavated orbit. (Page 105.)]
Using the hammer and the chisel, cut through the roof of the orbit at
the middle of the supra-orbital ridge, and continue upward for about two
and one-half inches. Do not strike hard blows, or the chisel may be
driven through the underlying tissues. Listen for the peculiar sound
that is heard when the bone has been completely penetrated; then remove
the chisel. Continue until the full distance of two and one-half inches
of bone has been separated. (Fig. 48.) Now, begin at the upper end,
and cut through the bone downward to the right for about two inches
toward the outer canthus. A similar line should be cut on the right of
the centre line toward the inner canthus. This will mark out two
irregular, triangular-shaped pieces of bone. (Fig. 49.) Remove the piece
on the right-hand side by prying it off. (Fig. 50.) The left-hand piece
should be pried loose and then carefully cut away with the scalpel, so
that the pulley through which the superior oblique muscle runs its
tendon, will not be injured. In removing the orbital contents, dissect
close to the bone (Fig. 51), so that the periosteum will also be
removed, and form a sort of sac or capsule in which will be contained
the eye with all its extrinsic tissues. If difficulty is experienced in
getting at the posterior parts of the orbit, it will be best to cut away
as much more of the obstructing bone as is necessary. In this way the
“capsule” containing the eye, its six muscles, the lacrimal gland, and
both eyelids, all _in situ_, will be removed. (Fig. 54.) As the orbital
entrance of the optic nerve is neared, care must be exercised not to cut
into this “capsule,” or sever any of the muscles. (Fig. 52 shows the
excavated orbit. Fig. 53 shows an anterior view of the enucleated eye.
Fig. 54 is a side view of the enucleated eye.)
[Illustration: Fig. 53--Anterior view of the enucleated eye.]
[Illustration: Fig. 54--Showing the enucleated eye, its muscles, and its
accessories, all in situ.]
OPHTHALMOSCOPIC EXAMINATION
By practising on an enucleated eye, one may gain considerable ability in
the use of the ophthalmoscope, and also learn to recognize the
blood-vessels and other important parts of the retina. To do this, the
eye to be examined must be very fresh, for only in this condition will
the cornea and lens be sufficiently clear to permit rays of light to
enter the inside of the eye.
However, since the pupil is oblong in shape, and often only a narrow
slit--but several millimetres in diameter--the field presented for
observation is a rather limited one. To increase the pupillary aperture,
take a pin, and force the point through the cornea about three or four
millimetres from the corneo-scleral junction, and at right angles to the
direction of the parallel edges of the pupil. After the pin has been
pushed through until it has reached to within a short distance (one
millimetre) of the edge of the iris, carefully pick up the iris by
raising the pin into a position perpendicular to the cornea, and force
the pin further down into the eye. The pupil will have been enlarged on
one side. Do the same thing on the opposite side, and at each extremity
of the pupil. (Fig. 55.)
[Illustration: Fig. 55--Showing one pin before the iris has been picked
up and pulled back, and three pins after the iris has been picked up and
pulled back.]
[Illustration: Fig. 56--Showing method of gathering up the extrinsic
tissues in order to get rid of the pucker in the cornea.]
The pupil will now have been made square, and so large that no
difficulty will be experienced in reflecting either light into the eye,
or in examining the inside of the eye. Care must be taken not to
lacerate the anterior surface of the lens when the iris is drawn back by
the pins.
Putting the pins into the cornea, and using them as levers with the
point of entrance in the cornea as a fulcrum, will pucker the cornea
considerably, and a good clear fundus cannot be obtained. This is easily
overcome. Simply gather up all the tissues surrounding the eye, force
them backward, and hold them firmly with the fingers of the left hand.
(Fig. 56.) The right hand is then free to handle the skiascope or
ophthalmoscope, so that the interior of the eye may be thoroughly
examined.
Another way to prepare an eye for ophthalmoscopic examination is as
follows: Go to a slaughter house and procure a beef eye from an animal
that has been killed but a few minutes previously. Placing the eye
immediately into an 8 per cent. solution of cocaine and leaving it there
for about an hour will dilate the pupil to such an extent that work with
the ophthalmoscope will be made very easy. This, as indicated, can be
done only with an eye that is very fresh.
[Illustration: Fig. 57--Showing window cut in sclerotic, choroid, and
retina.]
Still another way to see the interior is to cut out a piece of the
sclerotic about the size of a twenty-five-cent piece; then pinch up and
tear out the choroid and the retina under the opening made in the
sclerotic. (Fig. 57.) Hold the eye, the cornea forward, close to a
bright light, and the image of the light will be seen upon the retina.
The closer the light is to the eye, the greater the illumination will be
in the interior of the eye. If the opening or “window” is close enough
to the optic nerve, the optic papilla can be seen easily. And, if care
has been taken to have the opening made midway between the two branches
of the retinal artery, the entire course may be followed. The direction
of the retinal artery can be determined by ophthalmoscopic examination.
THE LACRIMAL DUCTS
To find the lacrimal ducts, cut across the outer and inner canthi of the
enucleated eye, pushing the eye forward and the lids backward. That will
expose the conjunctiva of both eyelids and eye, and also show the
conjunctival fornices. On the upper surface of the palpebral
conjunctiva, and near the outer canthus, will be seen, upon close
inspection, a number of minute openings, usually eight. These are the
openings of the lacrimal ducts. Pins or straw that have been lubricated
with vaseline, may be inserted and pushed into these openings for a
considerable distance, and the course of the ducts then can be traced
easily. (Fig. 58.)
[Illustration: Fig. 58--Showing how pins may be inserted in the lacrimal
ducts.]
THE LACRIMAL GLAND
The lacrimal gland is easily distinguished by its pink appearance. There
are two parts, inferior and superior. The gland lies directly over the
eye and near the outer angle of the orbit. In the enucleated eye, it
will be found to lie near the outer canthus and over the eye. The gland
may be easily dissected out of its position and then examined more
closely. A hand lens will show the racemose construction of the gland.
If the gland is cut in two, the racemose construction may be seen even
better.
THE CAPSULE OF TENON
To dissect the capsule of Tenon, it is necessary to carefully remove the
superficial fat and connective tissue. In text-books and illustrations,
the capsule is usually shown as a definite sac-like membrane of
considerable thickness, with all its parts well defined. The dissector
will soon find that the capsule is not discerned so easily. It will be
found to be the thin, semi-transparent, fibrous membrane that surrounds
each muscle, as well as the “posterior two-thirds of the eye,” and is
continuous anteriorly with the ocular conjunctiva. Portions may be
pinched up and inflated through an inserted blow-pipe. This will help to
merely demonstrate its location and parts. (Fig. 59.)
[Illustration: Fig. 59--Enlarged to show part of the Capsule of Tenon
blown up. (Page 116.)]
THE SUPERIOR OBLIQUE MUSCLE AND ITS PULLEY
After the lacrimal gland has been dissected away, a beginning will have
been made for cutting away the fat and the connective tissue. The first
thing to do then is to locate the superior oblique muscle. Try to keep
track of which part of the eye is the inner side. Having located the
inner side, feel along the top for a little hard eminence. That is the
pulley. Begin to dissect around the pulley, not through it, and then
follow the muscle along to its origin; do not separate the muscle from
its origin. When the superior oblique is completely freed, the action of
the muscle may be readily demonstrated by holding the “ring” or
tendinous pulley with the fingers of one hand, while the muscle is
pulled backward and forward with the other.
THE OTHER EXTRINSIC MUSCLES
With the dissection of Tenon’s capsule and the superior oblique muscle,
the work of isolating the other extrinsic muscles will have begun. This
work needs no directions except a warning to be careful not to injure
the pulley of the superior oblique, and to be careful not to cut away
the inferior oblique. The inferior oblique will be found to be near the
“pulley.” If the dissection is not carried too close to the origin of
the recti muscles, all the muscles may be kept in place.
If the eye has not been previously subjected to the hardening influence
of formaldehyde, it may be put into a 5 per cent. solution, and at the
end of ten or twelve hours the muscles will have become rigid. They can
then be better studied, and may be kept indefinitely. (Fig. 60.)
[Illustration: Fig. 60--Showing the tendinous pulley of the superior
oblique muscle and the extrinsic muscles.]
THE THREE TUNICS OF THE EYE, THE HYALOID AND ITS ATTACHMENTS
This dissection is a rather difficult one to make, and requires
patience.
[Illustration: Fig. 61--Cutting through the iris.]
Prepare an eye by placing it in a 5 per cent. solution of formaldehyde
for about ten days to two weeks. Remove all the outside tissues. Cut
away the cornea, as in the dissection for the choroid or the retina.
Loosen, as far back as possible, the sclerotic from the choroid. Remove
the sclerotic for about 10 mm. back of the equator of the eye. With the
tweezers pick up the pupillary edge of the iris. Using the small pointed
scissors, cut through the iris. (Fig. 61.) Lift either one of the cut
edges of the iris, and, with the sharp edge of the scalpel, gently
scrape the _processus zonuloe_ free from the ciliary processes, cutting
through the ciliary ring as the ciliary processes are detached from the
hyaloid (_processus zonuloe_). (Fig. 62.)
[Illustration: Fig. 62--Scraping the ciliary processes free. Showing,
also, the choroid cut around the ciliary ring.]
Great care must be taken not to thrust the point of the scissors into
the hyaloid, suspensory ligament, or vitreous, else the lens may become
detached.
After the iris with the processes has been removed, pinch up with the
tweezers a fold in the choroid. Make an incision with the fine-pointed
scissors, and begin removing the choroid to within about 5 mm. of the
cut end of the sclerotic. (Fig. 63.) Care must be taken not to penetrate
the underlying retina while making this part of the dissection.
[Illustration: Fig. 63--Cutting away the choroid.]
After part of the choroid has been removed, the specimen will show the
three coats of the eye in layer-like arrangement, the hyaloid and lens.
The lens may now be cut away, if the specimen is preferred without it.
Removing the lens before this time is unwise, because it acts as a
protection to the other tissues while the specimen is being handled
during the dissection.
This specimen will show to the best advantage if it is suspended in a
jar containing a 5 per cent. solution of formaldehyde. Figure 64 shows
the specimen.
[Illustration: Fig. 64--A. Optic nerve. B. Sclerotic. C. Choroid. D.
Retina. E. Hyaloid. F. Lens.]
INDEX
A
Accessories, ocular, 95
Alcohol as a preservative, 34
Alcohols, method of running through, 58, 59
Anterior chamber, 50, 51, 86, 90
half of eye, 49
half, removing vitreous from, 49
surface of lens, 55
Aperture, increasing pupillary, 106, 107, 109
Aqueous humor, 51
Arteria centralis retinæ, 43, 49
Arteries, ciliary, 72
course of retinal, 111
Artery of retina, central, 43, 49
hyaloid, 43
B
Beef eyes, 26, 27
Beginning of dissection of orbital contents, 97
Benzine, 59
Blood-vessels of retina, 106
Blowing through blowpipe, folding retina by, 81, 82
Blowpipe, folding retina by blowing through, 81, 82
glass, 35
using it to force out vitreous, 78-81
Body, hyaloid, 41
Boiling crystalline lens, 55, 56
Books, text, 23, 25, 46, 52
C
Canal, hyaloid, 43
of Petit, 35-37
Canaliculi, puncta, papilla, and nasal duct, locating, 92-95
Capsule of lens, 33, 50, 54
of lens, removing, 59
Capsule of Tenon, 115
of Tenon and conjunctiva, 116
Cedar oil, 60
Cells of processes, pigment, 49, 50
Central artery of retina, 43, 49
Chamber, anterior, 50, 51, 86, 90
posterior, 50, 51, 86, 90
Chemicals, 22, 23
Chisel and hammer, use of, 99-101