Part 2

[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.

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.

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.

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.)

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.

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.)

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

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.

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.

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.

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.

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.

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.

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.

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.

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.)