Transcriber’s Notes
Italics text has been transcribed _between underscores_. Small
capitals have been replaced with ALL CAPITALS.
More Transcriber’s Notes may be found at the end of this text.
The eye--it cannot choose but see.
--_Wordsworth._
[Illustration: Brain, showing eyes with muscles removed, optic nerves,
and chiasm.]
TECHNIQUE
OF
EYE DISSECTIONS
BY
FREDERIC A. WOLL, Ph.D.,
_Associate Professor, Department of Hygiene, College of the
City of New York; Optometry Courses, Columbia University;
Member of New York State Board of Examiners in Optometry;
Honorary Member: American Optometric Association; State
Societies--Alabama, California, Connecticut, Kentucky, Maine,
Massachusetts, North Carolina, Rhode Island; Local Societies--Lehigh
Valley Society of Optometrists, Mahoning Valley Optometrists’
Society, and Optometrists’ Club of Brooklyn_.
_SECOND EDITION_
[Illustration]
NEW YORK
FREDERIC A. WOLL
1924
COPYRIGHT, 1914, BY
FREDERIC A. WOLL
_Printed in the United States of America_
First Edition, July, 1914
Second Edition, April, 1924
SCHLUETER PRINTING CO., NEW YORK
DEDICATION
THIS LITTLE BOOK IS DEDICATED WITH AFFECTION
AND ESTEEM TO MY FIRST TEACHER
IN OPTICS, MY FRIEND FOR YEARS
ANDREW JAY CROSS
WHOSE RESEARCH IN THE FIELD OF APPLIED
OPTICS HAS WON FOR HIM RECOGNITION
AND HONOR
CONTENTS
PAGE
Preface 13
Introduction 17
Removal of Hyaloid Membrane with Contents and Attachments Intact 25
Canal of Petit, The 35
Interior of the Eye, The 38
Posterior Half, The 40
Optic Nerve, The 47
Anterior Half, The 49
Iris, The 51
Cornea, The 52
Crystalline Lens, The 53
Choroid, The 62
Retina, The 74
Sagittal or Vertical Section of the Eye, The 86
Papilla, Puncta Lacrimalia, and Nasal Duct, The 92
Meibomian Glands and Ducts, The 96
Enucleation of the Orbital Contents, The 97
Ophthalmoscopic Examinations 106
Lacrimal Ducts, The 112
Lacrimal Gland, The 114
Capsule of Tenon, The 115
Superior Oblique Muscle and its Pulley 117
Other Extrinsic Muscles, The 118
Three Tunics of the Eye, the Hyaloid and its Attachments, The 120
ILLUSTRATIONS
PAGE
0. Brain showing eyes with muscles removed, optic nerves, and
chiasm _Frontispiece_
1. Glassware and tools 22
2. The first cut 28
3. How the point of the scissors is kept away from the underlying
tissues 29
4. Half of the sclerotic separated 30
5. Picking up the choroid 31
6. Emptying the eye of its contents 32
7. Isolated hyaloid, contents and attachments intact 33
8. Petit’s Canal 36
9. Cutting eye into anterior and posterior sections with
safety-razor blade 39
10. Posterior half showing retinal vessels and choroid 40
11. Showing network of vitreous 42
12. Tearing retina away from posterior half of eye 44
13. Posterior half of eye with retina removed 45
14. Excavated posterior half of eye 46
15. Split optic nerve 47
16. Ciliary processes and the lens 48
17. How to pull off vitreous 50
18. Processus Zonuloe 54
19. Onion-like layers of lens removed 56
20. Cross section of lens 57
21. Lenses showing the results of different kinds of treatment 60
22. Puncturing the cornea 63
23. Removing the cornea 64
24. How to separate the choroid from the sclerotic 65
25. Cutting away the separated sclerotic 67
26. Scraping the choroid free from the sclerotic 68
27. The isolated choroid 69
28. Inserting scalpel to loosen lens and cut through vitreous 70
29. Taking out lens and “core” of vitreous 71
30. Squeezing out remaining part of vitreous 72
31. Cutting through the iris 75
32. Cutting around the ciliary ring 76
33. Lens, iris, and part of vitreous removed 77
34. How to force blowpipe into the vitreous 78
35. Bulging out of vitreous caused by blowing air through glass
blowpipe 79
36. Showing vitreous removed 80
37. Folding the retina by blowing air at it through blowpipe 81
38. Suspended retina. Sclerotic ready to be cut away 82
39. Showing sclerotic nearly all cut away 83
40. Isolated retina 84
41. The beginning of the cutting of the eye for sagittal sections 87
42. Method of cutting through the crystalline lens 88
43. Cutting through cornea to complete the sagittal sections 89
44. Sagittal section enlarged 90
45. Part of calf’s head showing knitting needles inserted in
puncta 93
46. Course of knitting needles showing the course of the
canaliculi 94
47. Initial cuts to be made in the skin 98
48. First cut in bones of orbit 99
49. All the cuts to be made in bones of orbit 100
50. How to pry bone loose 101
51. Dissecting close to bones of orbit 102
52. Excavated orbit 103
53. Anterior view of enucleated eye 104
54. Side view of enucleated eye. All parts _in situ_ 104
55. Enlarging pupil for ophthalmoscopic inspection 107
56. How to get rid of the pucker in the cornea 108
57. Window cut in the eye 110
58. Pins inserted in lacrimal ducts 112
59. Capsule of Tenon blown up 115
60. Showing the extrinsic muscles of the eye 118
61. Cutting through the iris 120
62. Scraping ciliary processes free. Choroid cut around ciliary
ring 121
63. Cutting away choroid 122
64. Three tunics, hyaloid, and lens 123
PREFACE
The aim of this booklet is to present to the eye-specialist, the
teacher, the student, and others interested in the study of the anatomy
and physiology of the eye, some definite methods to follow in the
dissection of that organ.
Most dissections of the eye are not made with the same degree of care
and skill used in the dissections of other organs. In following the
usual method of dissecting eyes, much of the important detail is lost.
Often certain membranes are confounded with others, and wrongly
demonstrated. Furthermore, an eye is merely divided by some
demonstrators into an anterior and a posterior half, a very short time
is spent by the students scrutinizing each half; then the text-book is
turned to, and the anatomy is studied descriptively.
Not enough time has been given to thoroughly dissecting all parts of the
eye. As much time should be given as is necessary to bring out
prominently all its parts. Other organs of the body are more thoroughly
dissected, and, therefore, the student has better opportunity to gain a
clearer comprehension and better understanding of the anatomy and
physiology of those organs. Also, as much time should be given,
proportionately, to the learning of the technique of the dissection of
the eye as is given to the learning of the technique of dissecting other
organs of the body.
Many now make a direct specialty of ministering to those suffering from
errors of ocular refraction, ocular diseases, and ocular reflexes, and
for those specialists, principally, this book is written. It is to
fulfil its mission to them by acting as a guide and as a complement to
the descriptive matter in the text-book. It is sent forth in the hope
that it will tend to create more interest in the study of the practical
anatomy of the eye. It is written with a desire to stimulate the ability
to make careful and intelligent observation. It carries with it, as a
final end, an earnest wish that it may, in some small way, be the means
of opening up to the original researcher, a larger field for the
further study of the most important of the senses--the eyesight.
Most of the dissections explained in the following pages are original;
some, however, are only revisions of old methods.
This opportunity is taken to acknowledge the many helpful suggestions
that were made by Dr. Ivin Sickels, of the College of the City of New
York, and by the late Dr. Edward C. Spitzka, of New York. Thanks are due
Mr. E. F. Howes, of Messrs. Swift & Co., for furnishing the necessary
supply of beef eyes; to Messrs. Lee & Beach, photographers, of New York,
for their painstaking efforts in producing good photographs of the
actual dissections; and to Schlueter Printing Company, of New York, for
their many courtesies and interest in the production of the book.
FREDERIC A. WOLL.
_New York, July 21, 1914._
INTRODUCTION
In eye dissections it is unnecessary to have either a large equipment of
instruments or a special room. To have a laboratory at one’s disposal is
but a small added convenience. Not to have it, is no serious hindrance.
The work may be carried on and successfully done in one’s office or in
the home, as well as in class-room or laboratory. If it is true that the
atmosphere of a laboratory adds zeal to the efforts of a worker, but
there is no laboratory available, then reverse the order; let the zeal
of the worker add to the atmosphere of the place in which he is doing
his work.
Two things, among others to be mentioned later, are essential; a table
of convenient height, and a good light, natural or artificial. Both are
but modest needs. Compared with other dissections, there will be found
an absence of offensive odors. Neither are there any repulsive
sensations experienced. Such experiences are quite common when making
other kinds of dissections. This work is clean and attractive. Indeed,
one may even develop a rather keen sense of the æsthetic. Many of the
various parts of the eye, when separated and properly preserved, then
viewed and inspected, are bound to bring forth exclamations of
appreciation and wonderment. One can then better understand the
statement: “When Nature perfected the first eye she took a day off so
she could admire the result of her finest piece of handiwork.”
This does not imply that dissections of the eye tend to develop art
appreciation. Appreciation of the wonderful in Nature’s construction of
the special organs is not, however, to be relegated to a distant point.
Such appreciation is concomitant with the knowledge that comes from
having seen, handled, and examined the object studied.
The orchestra leader must have a good listening and hearing ear. This is
developed in him because he has to exercise constantly his power of
listening and hearing. The dissector who would become proficient in eye
dissections and in anatomical investigations must have a good seeing
eye and a dextrous hand. To acquire these two most valuable aids
necessary to carry on careful inquiry or research, it is essential to
practise using the eye and the hand. Combined with the expertness of
these two, must be the ability to continue one’s efforts in the face of
failure; to redouble one’s efforts to attain success despite the
shortcomings of eye or hand. This simply means practise and patience.
And the one who is without that wonderful virtue, patience, will never
stay long enough with his problem to gain either an observing eye or an
expert hand, or to achieve his end, and thereby reap the full and
pleasing results of his efforts. In order of importance, patience really
precedes dexterity, skill, and observation; and persistence of effort is
a factor not to be entirely outshone by any other virtue. With these
attributes, knowledge of the subject in hand naturally follows.
One reason why eye dissections are easily carried on is because material
can always be readily procured. Any butcher will furnish sheep, pig, or
beef eyes. Or, if one has the time to visit the manager of a
slaughter-house, and make known to him one’s needs, he will supply
enough eyes to carry through a host of interesting dissections and
experiments, and give sufficient material for careful, orderly, and
fruitful study. Perhaps in no other kinds of dissections will the
investigator find so much of interest, or have his efforts crowned with
such abundant and satisfactory results, as in the dissections of the
eye. But no one should try to study all parts of the eye with only one
specimen. To try to do so is an error, and a common one often committed
by both teachers and students. Specimens cost little or nothing, and it
is no more trouble to prepare a half-dozen eyes for dissection than one.
The cost of preparation, too, is but little more for a number of eyes
than it is for one, and may be no more in some instances. Besides,
having enough material on hand saves time in case of a failure. Also,
one can quickly repeat a dissection, and so procure any number of
desired specimens of specific parts, or do over again the same
dissection on another eye just for the purpose of practise, and thus add
to one’s dexterity. It is, therefore, strongly advocated to have plenty
of material on hand before beginning work. Economy here is not even
“penny wise.”
The tools, or instruments, needed are but few in number; an ordinary
scalpel, a pair of blunt tweezers, or forceps, as they are sometimes
called; a pair of sharp-pointed tweezers, a pair of small, sharp-pointed
scissors of about three to three and a half inches in length, and a pair
of large scissors, about four or five inches in length, having one jaw
sharp-pointed and one jaw blunt.
For glassware, any wide-mouthed jar or bottle, such as the ordinary
fruit or jelly jar, will do for preparing material. For clarifying
tissues, or for preserving and keeping them, small, wide-mouthed bottles
or vials should be used. And for temporary keeping, or for purposes of
“running through” various fluids, the regular Stender dishes are most
convenient. A glass graduate is almost indispensable if accuracy in
measuring fluids is desired. (Fig. 1.)
[Illustration: Fig. 1.]
The chemicals needed are few in number and small in quantity: Alcohols
in varying strength, which can be made by diluting a 95 per cent.
alcohol, and keeping an absolute alcohol on hand. A few ounces of
formaldehyde will make enough solutions of different strengths to be
sufficient for the preparation and keeping of many specimens. Of other
chemicals, such as xylol and cedar oil, only small quantities are
needed; enough to cover a specimen. Fifteen to twenty cents worth of
each will be an ample supply to keep on hand. All of these may be
procured at any large drug store, and are the only chemicals required
for doing the dissections as explained in this book.
Before further advance is made, it will be best to state that this work
deals only with methods for dissecting the various parts of an eye, and
is primarily intended to aid in the study of the anatomy and physiology
of the eye by being used in conjunction with such books as “Gray’s
Anatomy,” “A Text-Book of Physiology,” by Howell, “The Anatomy and the
Physiology of the Eye,” by Brown and Zoethout, and similar other works
of authority. However, if it is desired to acquire only specimens, then,
of course, no other works are necessary, and the matter contained herein
is sufficient to enable one to procure just what is wanted.
It is also wise to state here that since human eyes are hard to procure,
and not available in large quantities, one must resort to the use of
the eyes of animals, which are procurable in large quantities, and which
may be used without “feelings” in the matter. Though there is a
difference between the eyes of human beings and the eyes of other
animals, the difference is slight and of minor importance when compared
with the similarity of the more important parts.
TECHNIQUE OF EYE DISSECTIONS
REMOVAL OF THE HYALOID MEMBRANE WITH CONTENTS AND ATTACHMENTS
INTACT[1][2]
One of the easiest and most satisfactory dissections to attempt is the
isolating of the hyaloid membrane with its contents and its attachments.
The success one meets with in making this dissection will surely prove a
strong incentive for making all the rest. For these reasons this has
been placed first in this arrangement.
In eye dissections it is quite customary, in giving directions for
dissections, merely to mention the hyaloid membrane and its relations
with other parts of the eye. Rarely is there any attempt made to isolate
it. Often, too, the retina is mistaken for the hyaloid, and the retina
then wrongly demonstrated as being attached to the choroid. Of course,
it is impossible to separate the hyaloid from the vitreous; but a
dissection can be made which, when placed in a glass of some kind, will
show the hyaloid. If the following simple technique is carefully
observed, the membrane, with all its connections, can be easily
separated from certain other parts of the eye. Opportunity for thorough
study and observation will then be made extremely easy.
Procure the eye of either a sheep or a bullock. Instead of following the
usual procedure of hardening in any one of the several solutions used
for the purpose of toughening the ocular tissues, place the eye in a
cool place and permit it to collapse a trifle. Usually two or three days
is a sufficient length of time to accomplish the result.
Experiments have shown that if an eye is too fresh the ciliary
processes will not be easily detached from the hyaloid (zonular
processes), and if the eye has been in a preserving fluid, the same
result will follow. A sheep’s eye will make a better specimen even if it
is small, because the ciliary processes are more easily separated from
the zonular processes. If a bullock’s eye is used, it must be left in a
cool place a day or two longer than in the case of a sheep’s eye, in
order to permit a long enough time to elapse to allow disintegration of
the eye to take place sufficient to have the two processes separate
easily and cleanly.
[Illustration: Fig. 2--Making the first cut. (Page 27.)]
[Illustration: Fig. 3--Showing how the point of the lower jaw of the
scissors is to be kept away from the underlying tissues. (Page 27.)]
[Illustration: Fig. 4--Showing about half of the sclerotic separated.
(Page 29.)]
With a pair of dissecting forceps pinch up the sclerotic about 5 mm.
anterior to the equator. With a pair of small, fine-pointed scissors,
make an incision. (Fig. 2.) Next hold the eye in the left hand without
exerting any pressure. Insert the point of the scissors into the
incision which has been made, and cut. Be careful to keep the point of
the scissors close to the sclerotic or an untimely puncturing of the
choroid will occur. (Fig. 3.) Continue the cutting on a line parallel
to the equatorial meridian and about 5 mm. anterior to it until about
half the sclerotic has been separated. In cutting, always move the point
of the scissors forward with a slight oscillating lateral movement.
(Fig. 4.) While doing this, partly suspend the eyeball from the point of
the scissors. Doing these things will tend to loosen the choroid from
the sclerotic and prevent puncturing too soon the former mentioned
membrane. Now apply pressure in such a manner that the lips of the cut
sclerotic will gap. Into this put the point of the scissors and very
carefully pick up the choroid and the retina with the point of the
scissors and cut them. (Fig. 5.) If the choroid alone has been picked
up and separated, the retina will show milky white or yellowish white
underneath. The retina must then also be separated. Care must be taken
not to go deeper than the retina or the hyaloid may be damaged. Continue
the cutting of the choroid and the retina for a distance of about 20 mm.
Apply enough pressure occasionally so that the vitreous will be forced
upward and above the cut choroid and the retina. This will show whether
any strands of the two membranes have been left uncut. If the separation
is complete for the distance specified above, invert the eyeball,
squeeze and shake gently over some receptacle, such as a Stender dish,
three-fourths filled with a 2½ or 5 per cent. solution of formaldehyde,
and the hyaloid membrane containing the vitreous, its attachments,
suspensory ligament to the lens capsule, and lens, will drop out intact,
as when one empties the contents of an egg. (Fig. 6.)
[Illustration: Fig. 5.--Picking up the choroid with the point of the
scissors.]
[Illustration: Fig. 6--A. Hyaloid, vitreous, and lens ready to drop out
of the eyeball.]
[Illustration: Fig. 7--Photograph of dissected hyaloid membrane (A),
with pigmented indentations of the ciliary processes, the suspensory
ligament (B), and the crystalline lens in its capsule (C). (Page 34.)]
Many times a considerable amount of pigment from the second tunic will
remain attached to the _processus zonuloe_. This pigment may be easily
removed by scraping it off with the sharp edge of the scalpel or by
brushing it off with a soft, wet tooth-brush.
It is unwise to use alcohol as a preservative because it produces an
almost immediate opaqueness and hardness which spoils the specimen for
further study.
This description may give the impression that the dissection is a
lengthy one; however, it can be done by an expert in two or three
minutes; by a beginner in five or six minutes.
For purposes of demonstration or study the specimen should be placed in
a small bottle or a vial containing a 5 per cent. solution of
formaldehyde. It can then be examined with hand lens or microscope.
(Fig. 7.)
[1] Approved as an original article in _The Anatomical Record_,
September, 1912.
[2] This dissection, and several of the following, appeared in _The
Optical Journal and Review_, beginning with the issue of January 16,
1913.
THE CANAL OF PETIT
The canal of Petit is a “triangular space around the circumference of
the lens.” That it can be “inflated through a fine blowpipe inserted
through the suspensory ligament,” is the usual direction given. However,
the ordinary “fine blowpipe” is much too large and too dull to be
inserted through the suspensory ligament. Take a long medicine-dropper
(5 or 10 cents at a drug store), or a pipette, and heat it until it is
red hot over an alcohol lamp or a Bunsen burner; hold one end with one
hand and the other end with a pair of tweezers. As the glass becomes
white hot pull the tube apart. This will leave the places of separation
pointed and sharp-edged. Use the larger of the two pieces. Sometimes the
point or tapering end of the tube is too long and the bore too small.
All that is necessary is to first mark off with a file the length to be
broken off, and then that length may be snapped off, leaving a
sharp-edged, tapering point.
[Illustration: Fig. 8--A. Glass blowpipe. B. Petit’s canal.]
After having completed the first dissection (the hyaloid, contents and
attachments), and the specimen has been in the formaldehyde solution for
ten days or two weeks, it will have become hard and tough enough to
stand a considerable amount of rough handling. If the specimen has been
kept in a large receptacle, such, for instance, as a jar, remove it
with a spoon; if in a small jar or vial, empty out the fluid, then
slide the specimen out on whatever has been prepared to receive it. Turn
it so that the lens will be uppermost. Find the suspensory ligament in
the Zone of Zinn. Insert the pointed end of the glass tube, close to the
lens, and blow gently until the canal shows its sacculated construction
by filling with air, giving the appearance of a lot of little bubbles
surrounding the periphery of the lens. (Fig. 8.) It may be necessary to
move the blowpipe in and out in order to find the canal, all the while
blowing steadily through the tube.
THE INTERIOR OF THE EYE
For the study of the interior of the eye and its contents _in situ_
either a fresh or a hardened eye will do; a hardened eye is preferable.
In the dissection for isolating the hyaloid membrane, vitreous, lens,
and other parts, the anterior and posterior halves of the evacuated eye
may be separated entirely, and each half studied. However, the choroid
and the retina will be more or less mutilated, and the vitreous and
other parts will be removed. The absence of these parts will prevent one
from receiving a definite idea of their anatomical relationships.
Therefore, it is better to work with an entire and complete eye.
[Illustration: Fig. 9--Showing method of cutting eye into anterior and
posterior sections with safety-razor blade.]
Remove all the muscles and fatty tissues from the outside of the globe;
then cut it in half through the equator, thus dividing it into an
anterior and a posterior half. The cutting of the sclerotic, as well as
the underlying tissues and the vitreous, should be done with the large
scissors; using a knife or scalpel will tend to disturb the positions of
those tissues or so tear them that they will not be of much use for
purposes of study. An ordinary safety-razor blade makes an excellent
instrument for separating the eye into two halves, because it cuts
through the tissues without tearing them in any way. (Fig. 9.) The
rather dark colored, viscid fluid that escapes when the eye is halved
is the perichoroidal lymph, not the aqueous, as is sometimes stated.
THE POSTERIOR HALF
The posterior half is taken first because it is the simplest and easiest
of the two halves to dissect. In this half of the eye the retina may be
readily seen through the vitreous; the choroid and its apparent
iridescent colors through both vitreous and retina. (Fig. 10.) Remove
the vitreous by simply tilting this half of the eye, and with the finger
push out the vitreous.
[Illustration: Fig. 10--The retina, retinal vessels, and iridescent
choroid showing through the vitreous.]
Sometimes the vitreous will adhere very closely to the retina. This
occurs especially when the eye has been in formaldehyde for a long time.
In such a case the removal of the vitreous without injuring the retina
requires patience and care. The use of the scalpel and the scissors may
become necessary. Another very good way to remove the vitreous is to
take hold of the sclerotic, turn it so that the vitreous is downward,
and then shake gently until the vitreous separates itself from the
retina and, drops out. After the vitreous has been removed, notice its
glassy appearance; hence its name--hyaloid body. Try to pull it apart
with the fingers, and it will be noticed that it seems to be held
together by more or less of a network of fibres. (Fig. 11.)
[Illustration: Fig. 11--Showing how vitreous seems to be held together
by a network of fibres. (Page 41.)]
Whichever method for removing the vitreous is followed, the retina will
be left rather badly wrinkled and out of place. If the last-mentioned
method, which is really the best of the three described, is the one
adopted, the retina will be left in an entirely collapsed and folded
form. In any case, to straighten out the retina against the choroid,
immerse the whole posterior half in water, inside uppermost. The retina
will then slowly unfold itself and lie flat against the choroid. With
the tweezers remove the whole half from the water; tilting it slowly to
empty it of all the water, and, having done so, turn it down upon the
table rather forcibly in order to help it drain itself of all the water.
Notice the thinness of the retina, and, also, that the seeming
iridescence of the choroid shows through. The optic disc, which is the
point of entrance of the optic nerve, and the optic cup are easily
recognized, though neither will be seen as large as when viewed in the
living eye with an ophthalmoscope. The blood vessels of the retina, as
they ramify outward or forward, after their entrance through the optic
nerve through which they pass, are also very plainly seen. A closer
inspection will show, in the very centre of the “entrance” of the optic
nerve, a whitish, pointed vessel, about 1 or 2 mm. long. That is the
sloughed-off and atrophied end of the hyaloid artery, which, when the
eye was in an embryonic state, ran forward from the central artery of
the retina through the hyaloid canal to the posterior surface of the
lens. With the forceps pick up the peripheral edge of the retina, and,
by pulling gently upward, tear it away from its apparent place of
attachment to the “entrance” of the optic nerve. (Fig. 12.) When this
has been done, there will be seen some threads protruding from the
optic nerve. Filling the half with water will tend to separate these
strands, which are optic-nerve elements.
[Illustration: Fig. 12--Picking up the retina in order to tear it away
from the entrance of the optic nerve.]
[Illustration: Fig. 13--The lighter area is the field of iridescence of
the choroid.]
After the removal of the retina, the iridescence of the choroid
(_tapetum lucidum_) (Fig. 13) may be examined with a hand lens, or,
after its removal, a piece may be cut and placed under a microscope.
This iridescence is, of course, not present in the human eye.
(“Physiology of the Senses,” McKendrick & Snodgrass, page 101.)
[Illustration: Fig. 14--Excavated posterior half of the sclerotic.]
After the choroid is removed, which is accomplished in the same manner
that the retina is removed, the inner side of the sclerotic is laid bare
to view. The brownish color is mostly due to the presence of a small
amount of pigment in the cells of one of the inner layers, it is also
due, to a slight extent, to the staining influence of the perichoroidal
fluid. (Fig. 14.)
THE OPTIC NERVE
[Illustration: Fig. 15--Enlarged to show the entrance of the optic
nerve. (Page 48.)]
The excavated posterior half may be used now to show and to study the
construction of the optic nerve. In cutting the optic nerve away from
the sclerotic leave at least 5 mm. of the sclerotic attached. It will
make handling easier. With the thumb and forefinger of the left hand
hold the nerve in such a way on the table that it will be straightened
out lengthwise, and then, using the scalpel or a safety-razor blade,
the latter being preferable, cut the nerve in two longitudinally. (Fig.
15.) The cutting must be done with one movement, otherwise the nerve
will be hacked, and will not make a good specimen. This specimen will
show the way the nerve fibers are arranged. A cross section should be
cut from the optic nerve of another eye, and then the two sections
should be compared. The cross section will show the sheath of the nerve
a little better than will the longitudinal section.
[Illustration: Fig. 16--Showing ciliary processes and crystalline lens.]
In cutting the longitudinal section, one is sometimes so fortunate as to
cut through the central blood vessels of the retina. These vessels will
show up then as a rather thin dark streak about 5 or 6 mm. long.
THE ANTERIOR HALF
The anterior half will show the lens _in situ_, the ciliary processes,
the posterior aspects of the iris and the lens, the corona ciliaris, the
orbicularis ciliaris, and the ora serrata. (Fig. 16.) If the eye has
been cut in two too far forward of its equator, the ora serrata will not
be present. The ciliary processes and posterior aspect of the lens may
be seen to better advantage when the anterior half of the vitreous is
removed. This is done with the dull-pointed tweezers, by catching hold
of the vitreous at any part of its free or cut margin, and stripping it
off both the ciliary processes and the lens, using a prying, pulling
movement to do so. (Fig. 17.) The two layers of the pigment cells, pars
ciliaris retinae, which cover the inner surface of the processes, may
be removed by picking them away carefully with the tweezers. The
processes then will be seen to be a whitish color. The pupillary edge of
the iris rests upon the capsule of the lens, but the nearer the approach
is to the choroidal edge the farther the iris is from the lens; thus are
formed the anterior and the posterior chambers of the eye. The
dissection of the sagittal section of the eye, explained further on,
will show these two chambers in section. One will gain a much clearer
conception of their construction in that section than in the “anterior
half” specimen.
[Illustration: Fig. 17--Anterior half, showing how to pull off vitreous.
(Page 49.)]
Now, remove the lens, using the point of the scalpel to cut through the
suspensory ligament close to the lens. When this has been done there
will be seen in the anterior chamber a thin, watery liquid--the aqueous
humor.
The corona ciliaris and orbicularis ciliaris may be better seen and
studied if viewed through a hand lens.
THE IRIS
To see the iris, take hold of the cut edge of the choroid, and, gently
pulling, separate it from its attachment to the corneo-scleral junction.
The white ring on the anterior surface of this part of the second coat
of the eye is the ciliary ring. With a scissors, cut around this ciliary
ring at its outer edge. This specimen will show the anterior surface of
the iris, and on the posterior side it will show the close relationship
between the iris and the ciliary processes. A hand lens will help
greatly to bring out the very interesting fine points.
THE CORNEA
After the anterior portion has had everything removed from it there will
be left nothing but the first coat or tunic of the eye--the anterior
portion of the sclerotic and the cornea. The way the cornea seems to fit
into the sclerotic is not quite as one is led to believe when told that
it fits into the sclerotic much the same way in which a watch crystal
fits into a watch.[3] Holding this part of the eye up to a strong light
one will see that the sclerotic seems to overlap the cornea in the
vertical axis.
By using the tweezers the cornea may be split. Nothing in the way of
locating its layers can be recognized, however, unless a section is made
for microscopic examination. The epithelial may be scraped off when the
cornea is a trifle dry. This is the ocular epithelium reduced to a layer
of flattened cells.
THE CRYSTALLINE LENS
If the preceding dissections have been done, the crystalline lens will
already have received some notice. To study the lens properly one should
use an eye that has not been hardened and also an eye or the lens of an
eye that has been in a 5 per cent. solution of formaldehyde for about
two weeks.
The lens in the unhardened eye will prove too friable to permit much
handling. The dissection should be made, however, in order to give
opportunity to notice the crystalline clearness of the lens substance,
its great magnifying power, its attachments, its capsule, etc. For this
purpose it is necessary to proceed only as in the dissection for the
“hyaloid membrane, etc.” That is, use an eye that has been kept in a
cool place for several days, and then open it, and remove hyaloid,
vitreous, and lens intact, as in the first dissection taken up in this
book. To examine the specimen in detail, turn it so the lens will be
uppermost. (Fig. 18.)
To remove the lens it is necessary to separate the suspensory ligament,
using for this purpose the small-pointed scissors. The capsule may be
removed by picking it up on the periphery of the lens, and stripping it
off. It will peel off about the same way that the outer skin of a bean
or pea does.
[Illustration: Fig. 18--Enlarged to show the processus zonuloe. (Page
53.)]
The tri-radiate lines on the posterior and the anterior surfaces of the
lens will not be as clearly discernible as in the lens coming from the
hardened eye. Close inspection and the use of a hand lens will help
bring them out more clearly.
Now, with the point of the scalpel try to separate the outer layers
(cortex) from the harder inner layers (nucleus). This will not prove
very successful but is suggested for the purpose of comparison when the
same thing is done to the hardened lens.
It will be found that the lens after having been in the formaldehyde
solution is no longer crystal like, but more or less translucent. When
viewed from either the anterior side or the posterior side, the
tri-radiate lines on each surface will be seen to begin at the poles of
the lens and radiate outward toward the lens equator. Holding the lens
up to a strong light will show that though the lines on either surface
form angles of 120 degrees, the angles formed by the lines on one side
with the lines on the other side are 60 degrees. On the anterior surface
of the lens the vertical line extends upward from the pole; on the
posterior surface downward from the pole.
To study the laminated structure of the lens, it is best to boil the
lens. The best way to do that is to drop the lens from either a hardened
or unhardened eye into boiling water. Let it boil in the water for about
two and a half to three minutes. Longer than that time will cause the
lens to be put out of shape, and make it so fragile that it can no
longer be handled without having it fall apart. If the lens comes from
an unhardened eye it might be best to boil it not more than about two
minutes.
[Illustration: Fig. 19--Showing the way the onion-like layers of the
lens may be peeled off.]
Insert the point of the scalpel carefully at one of the poles, and lift
gently in the direction of one of the radiating lines. This will tend to
raise one of the concentric layers, which can be easily peeled off.
Repeat this in the direction of the other two radiating lines.
Examining, with a hand lens, the exposed surfaces and the layers, as
they are taken off, will show the arrangement of the lens fibres, and
will also show plainly their directions. (Fig. 19.) To get another view
of the onion-like layers of the lens, cut through it with a safety-razor
blade, either longitudinally or equatorially. (Fig. 20.) The better way
is to have enough lenses to make one of each kind. Never try to work
with only one piece of material. If the lens is first stained with
chromic acid the layers may be seen better, or, a simpler way is to drop
the lens, before cutting it in two, into a carmine solution; red ink
slightly diluted, will do.
[Illustration: Fig. 20--Section through lens showing its concentric
layers.]
A lens that has been boiled and partly dissected may be placed in a 5
per cent. formaldehyde solution, and kept indefinitely. The lens fibres,
concentric layers, and lens laminae in such a specimen will always be
interesting.
A lens that has lost its transparency because of hardening in
formaldehyde or boiling may be made clear and nearly transparent again
in the following way: First: Place the lens in a 50 per cent. alcohol
for several hours. Second: Remove the lens, and let it drain on a piece
of blotting-paper; then place it in a 75 per cent. alcohol. Third:
Remove the lens, as before, then place it in an 85 per cent. alcohol.
The lens may be left in this alcohol from ten to twelve hours, after
which length of time it should be removed and drained. Fourth: Place the
lens in an absolute alcohol, and leave it there for ten or twelve hours.
Several hours longer will not injure the lens, nor interfere with the
success of the work. Fifth: Remove the lens from the absolute alcohol.
Place it upon a piece of blotting-paper, moving it to another place on
the blotting-paper whenever the paper around the lens seems to have
taken up as much moisture as it can hold. Be sure that the lens has
given up nearly all, if not all, moisture. “Running through the
alcohols,” as this process is called, is for the purpose of dehydrating
the tissue. It will be on the side of safety to let the lens lie exposed
on the blotting-paper for an hour. Sometimes, if the capsule has not
been removed, a small quantity of alcohol will remain between the lens
and the inner surface of the capsule. This must be removed. It may be
done by either puncturing the capsule with a pin or needle, and
squeezing out the fluid, or by removing the capsule entirely. The latter
is preferable.
Now drop the lens into xylol. Benzine will answer, though it will not
produce quite so clear a lens as the xylol does. At the end of 24 or 36
hours the softer cortex will show quite clear, while the harder nucleus
will be still cloudy. At the end of a week the whole lens, if it is a
small one--pig, calf, sheep--will have become quite clear and
transparent; if from a beef eye it will take longer. It sometimes takes
nearly two weeks. In the case of a boiled lens it will take much longer
to clear; it may take a month.
Cedar oil may also be used for the purpose of clarifying or “clearing”
the lens. Harden in the usual way, run through the alcohols, and then
place in cedar oil. The oil, however, will stain the lens a yellowish
brown, and the lens will not be as transparent and clear as when xylol
is used.
[Illustration: Fig. 21.
A. Lens hardened in formaldehyde.
B. Lens hardened in formaldehyde, run through the alcohols, and cleared
in xylol.
C. Lens hardened in formaldehyde, run through the alcohols, and cleared
in cedar oil.
D. Boiled lens.]
The longer a lens is left in either of these two clarifying fluids the
harder and smaller it will become. At the end of a month or six weeks
the lens will have become so hard that it can no longer be cut through
with a knife. If it is desired to halve it, a scroll saw will be found
to be the best thing to use for this purpose. (Fig. 21.)
[3] “Anatomy and Physiology of the Eye,” Brown & Zoethout.
THE CHOROID
Select an eye that has had a long part of the optic nerve left on it and
place it into a 5 per cent. solution of formaldehyde. Leave it in that
solution for from two to three weeks. That period of time in the fluid
will be sufficient to permit the choroid to become sufficiently
toughened and hardened. Leaving it in the solution longer than that
length of time will not injure the eye in any way.
[Illustration: Fig. 22--Showing how to puncture the cornea. (Page 62.)]
[Illustration: Fig. 23--Removing the cornea. (Page 63.)]
[Illustration: Fig. 24--Showing method of inserting the scalpel to
separate the choroid from the sclerotic.]
After having removed the eye from the formaldehyde, wash it for a few
moments in running water. This will remove the preserving and hardening
fluid from the surface, and will save the hands a little from the
effects of the fluid. Next remove all the muscles and fatty tissues from
the sclerotic. After that has been done, puncture the cornea with the
pointed jaw of the scissors about 2 mm. from the corneo-scleral
junction. (Fig. 22.) Then proceed to cut the cornea away, being careful
not to lacerate the choroid or the iris. (Fig. 23.) The escaping aqueous
humor will flow over the eye and make it very slippery, and, therefore,
difficult to hold. Dip the eye in water, wash it, and then take it out
and thoroughly dry it with a cloth. This procedure is absolutely
necessary, and, if omitted, will surely result in the dropping of the
eye about the time the work on the specimen is nearly finished. Insert
the scalpel between the peripheral edge of the exposed iris and what is
left of the cornea. With the back edge of the scalpel, gently loosen the
choroid from the inner side of the corneo-scleral junction to which
part it is not securely attached. (Fig. 24.) This requires only ordinary
care, and but little skill other than that necessary to always keep the
scalpel close to the inner surface of the sclerotic. When the
choroid-iris edge has been detached from the inner side of the
corneo-scleral junction, the weight of the contents of the second tunic
will cause it to sag and give opportunity to easily separate, with the
back edge of the scalpel, the choroid from the sclerotic for about a
distance of from 8 to 10 mm.
It has been the method in the past to force water through a blowpipe
between the sclerotic and the choroid, in order to separate the
attachments. It has also been the method to work under water when
wishing to expose or isolate either the choroid or the retina. It is
unnecessary to do either of these two things.
When the sclerotic has been loosened from the choroid for about 10 mm.
back from its cut edge around the eye, carefully cut the loosened part
away. (Fig. 25.) Then loosen the choroid as far back as to within 1 cm.
of the optic nerve. Cut the separated sclerotic away. It will be well to
state here that during this dissection the specimen should not be lifted
from the table. Keep the eye resting on the table all the time, and
never lift it by holding it suspended from the optic nerve. Loosening
the choroid from the sclerotic up to this point is a very easy matter;
ordinary precaution is all that is necessary to prevent puncturing the
choroid with the scalpel, just be sure to remember to keep the point of
the scalpel close to the sclerotic.
[Illustration: Fig. 25--Cutting away the sclerotic after it has been
loosened from the choroid, as shown in Fig. 24.]
[Illustration: Fig. 26.--Showing how to scrape the choroid free from the
sclerotic near the optic nerve.]
To remove the remaining part of the first coat is a little more
difficult, and needs a little more care. Hold the optic nerve in the
left hand, and pull it so that the sclerotic will pull away from the
choroid. Then, using the cutting edge of the scalpel, scrape the choroid
loose from the sclerotic close up to the entrance of the optic nerve.
(Fig. 26.) Do not separate the optic nerve from the choroid. Cut away
the remainder of the sclerotic close up to the optic nerve and the
choroid will be free. (Fig. 27.)
[Illustration: Fig. 27--Showing the choroid, the optic nerve still
attached, the ciliary ring, and the ciliary nerves.]
[Illustration: Fig. 28--Showing method of inserting scalpel in order to
loosen the lens and cut through the vitreous.]
To get a perfect specimen and completely isolated choroid, it must be
emptied of its contents. Insert the scalpel between the lens and the
iris, force it on through, and in such a manner as to keep the scalpel
close to the ciliary processes. (Fig. 28.) Cut the vitreous around the
processes. Push the scalpel further into the vitreous, and cut out the
central part of it, just as one would cut out the core of an apple.
(Fig. 29.) Remove the scalpel, pick out the lens and the cut centre of
the vitreous with the broad-point tweezers, holding the choroid a trifle
suspended by the optic nerve. The remaining part of the vitreous may be
broken down by cutting with the scalpel, and by squeezing and crushing
with the fingers of both hands. (Fig. 30.) The choroid will be tough
enough to stand this treatment provided the pupil is left clear and open
to prevent inter-choroidal pressure. After the vitreous has been removed
the choroid will be left in a greatly collapsed condition. Dropping it
into water and letting it fill up will make it resume its original
shape immediately. The retina does not always come out with the
vitreous. In such a case, the tweezers may be used to pick out the
retina when the choroid is in a collapsed condition.
[Illustration: Fig. 29--Taking out the lens and “core” of the vitreous.]
[Illustration: Fig. 30--Showing how to squeeze out the remaining part of
the vitreous (A) from the choroid. (Page 71.)]
This specimen will show the vena vorticosa, the ciliary nerves, and
their way of ramifying, and the long ciliary arteries, which run
opposite each other and which may be recognized by their rather
colorless, tubular appearance. The evacuated choroid makes an excellent
specimen and one easily examined. Place it in a 3 per cent. solution of
formaldehyde, and then examine with a skiascope, an ophthalmoscope, or
by “oblique illumination.”
This dissection is wholly original, and may be done in about five
minutes. The old technique for doing it required at least an hour of
time with the possibility of procuring one perfect specimen in every six
or seven. The technique as given here will make it possible to do the
work in not longer than five or six minutes for the beginner, and about
four minutes for the expert.
THE RETINA
Isolating the retina from the other tissues requires considerable
patience and dexterity. When the retina has been removed and placed in a
special receptacle, it will be found that the specimen is well worth the
little amount of time spent in making it. Previous techniques, even the
writer’s own, sometimes took nearly two hours to do, and rarely was the
retina isolated without puncturing or tearing it; perfect specimens were
almost impossible. The following method will assure one of success in
nearly every instance. Failures are almost impossible. Punctures,
perforations, tears, etc., are rare. The beginner should isolate the
retina in about six to seven minutes; the expert in about four and a
half to five minutes.
Select an eye with a long optic nerve, and prepare it for this
dissection by placing it in a 10 per cent. solution of formaldehyde for
about ten to fourteen days, but no longer. If it is left in the
hardening fluid longer than that length of time, it will interfere with
the easy removal of the vitreous.
[Illustration: Fig. 31--Cutting through the iris. (Page 77.)]
[Illustration: Fig. 32--Showing how to cut around the ciliary ring.
(Page 77.)]
The first part of this dissection is the same as the beginning of the
dissection for the isolation of the choroid. Remove all the outside
tissues first, and then the cornea, and about 10 mm. of the sclerotic,
as described in the preceding dissection. (See Figs. 22, 23, 24, and
25.) That will lay bare the iris and a few millimetres of the choroid.
[Illustration: Fig. 33--Lens, iris, and part of vitreous removed. (Page
78.)]