Part 8

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.

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.

After that has been done, turn the eye so the iris will be uppermost. 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.)

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.)

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.)

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.

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.)

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.)

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.

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.

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.

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.)

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.