THE POSTERIOR HALF

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 thelarge 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 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.

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.)

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 thethree 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 removethe 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 thelens. 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 protrudingfrom the optic nerve. Filling the half with water will tend to separate these strands, which are optic-nerve elements.

Fig. 12—Picking up the retina in order to tear it away from the entrance of the optic nerve.

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 humaneye. (“Physiology of the Senses,” McKendrick & Snodgrass, page 101.)

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.)

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-razorblade, 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 alittle better than will the longitudinal section.

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 will show the lensin 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 coverthe 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 furtheron, 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.

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.

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 ciliaryprocesses. A hand lens will help greatly to bring out the very interesting fine points.

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.

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 separatethe 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.

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 theharder 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 tothree 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.

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 surfacesand 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.

Fig. 20—Section through lens showing its concentric layers.

A lens that has been boiled and partly dissectedmay 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 takenup 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 lensit 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.

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.

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 sixweeks 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.

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.

Fig. 22—Showing how to puncture the cornea. (Page 62.)

Fig. 23—Removing the cornea. (Page 63.)

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, beingcareful 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 aboutthe 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 towhich 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-scleraljunction, 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.

Fig. 25—Cutting away the sclerotic after it has been loosened from the choroid, as shown inFig. 24.

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 notseparate 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.)

Fig. 27—Showing the choroid, the optic nerve still attached, the ciliary ring, and the ciliary nerves.

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 lensand 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, andby 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 intowater 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.

Fig. 29—Taking out the lens and “core” of the vitreous.

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.

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 forabout 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.

Fig. 31—Cutting through the iris. (Page 77.)

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. (SeeFigs. 22,23,24, and25.) That will lay bare the iris and a few millimetres of the choroid.

Fig. 33—Lens, iris, and part of vitreous removed. (Page 78.)

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 vitreousin such a way that the lens will also be removed with the iris. (Fig. 33.)

Fig. 34—Showing how to force glass blowpipe (A) into vitreous (B). (Page 80.)

Fig. 35—Showing bulging out of vitreous caused by blowing air through glass blowpipe. (Page 80.)

Fig. 36—Showing the vitreous (A) removed.

Holding the eye suspended by its optic nerve, force the glass blowpipe through the vitreous until it all but touches the posteriorpart 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 downwarduntil 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.)

Fig. 37—A. Showing retina folded upon itself by blowing air at it through the glass blowpipe. (Page 83.)

Fig. 38—A. Showing folded retina suspended from its attachment, so sclerotic and choroid may be easily cut away. (Page 83.)

After the vitreous has been removed, turn the eye upward, and by blowing strongly through the blowpipe at the marginal edge of the retina, turn the retina upon itself. Repeat this until the retina lies in a small wrinkled lump at the “bottom” of the posterior partof 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.)

Fig. 39—Showing the sclerotic nearly all cut away.

Fig. 40—Isolated retina, with optic nerve attached.

After the choroid and the sclerotic have been cut away, drop the retina into somewater, 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 whichthe 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.

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 perfectlyclear and clean. If the optic nerve is longer than 5 mm., cut it off to that length.

Fig. 41—Showing the beginning of the cutting of the eye for sagittal sections.

Fig. 42—Showing method of cutting through the crystalline lens.

To cut the eye in two, use a safety-razor blade; never a scalpel. The latter is too thick, too dull, and too clumsy a tool. Begin by cutting through the optic nerve; dividing it as nearly as possible into halves. (Fig. 41.) Continue cutting through the sclerotic and all underlying tissues, stopping at the corneo-scleral junction, but do not, during this procedure,even touch the lens. After the eye has been thus partly separated into, as nearly as possible, two equal parts, lay it down upon the cornea, and, holding the razor blade in the forefingers and thumbs of both hands, cut the lens in two by forcing the blade down through it. (Fig. 42.) Partly open the cut eye to allow one jaw of the large scissors to enter, turn the eye over so the cornea will rest on thatjaw, and then cut through the cornea. (Fig. 43.)

Fig. 43—Showing method of cutting through the cornea and completing the two sagittal sections.

The two specimens may be kept indefinitely by placing them in a 3 per cent. formaldehyde solution. It will be well to remove the lens from one of the specimens, because it will give better opportunity to see the anatomical relationships. Also, these specimens should be mounted, one above the other, between two pieces of glass, before placing them in the receptacle that is to hold them.

Much can be studied in such specimens. Moreover, they present to view the various parts of the organ of vision in such an impressive way, that one does not soon forget the wonderful appearance of the construction of this, Nature’s perfect camera. (Fig. 44.)

Fig. 44—Sagittal section enlarged.

If another eye is cut into two parts, additionalinteresting 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 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.

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