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].)
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.
Fig. 2—Making the first cut. (Page 27.)
Fig. 3—Showing how the point of the lower jaw of the scissors is to be kept away from the underlying tissues. (Page 27.)
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].)
Fig. 5—Picking up the choroid with the point of the scissors.
Fig. 6—A. Hyaloid, vitreous, and lens ready to drop out of the eyeball.
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.